What is the function of an assembler

Introduction of Assembler

Assembler is a program for converting instructions written in low-level assembly code into relocatable machine code and generating along information for the loader.

It generates instructions by evaluating the mnemonics (symbols) in operation field and find the value of symbol and literals to produce machine code. Now, if assembler do all this work in one scan then it is called single pass assembler, otherwise if it does in multiple scans then called multiple pass assembler. Here assembler divide these tasks in two passes:

Firstly, We will take a small assembly language program to understand the working in their respective passes. Assembly language statement format:

Let’s take a look on how this program is working:

Working of Pass-1: Define Symbol and literal table with their addresses.
Note: Literal address is specified by LTORG or END.

Step-1: START 200 (here no symbol or literal is found so both table would be empty)

Step-2: MOVER R1, =’3′ 200 ( =’3′ is a literal so literal table is made)

Step-3: MOVEM R1, X 201
X is a symbol referred prior to its declaration so it is stored in symbol table with blank address field.

Step-4: L1 MOVER R2, =’2′ 202
L1 is a label and =’2′ is a literal so store them in respective tables

Step-5: LTORG 203
Assign address to first literal specified by LC value, i.e., 203

Step-6: X DS 1 204
It is a data declaration statement i.e X is assigned data space of 1. But X is a symbol which was referred earlier in step 3 and defined in step 6.This condition is called Forward Reference Problem where variable is referred prior to its declaration and can be solved by back-patching. So now assembler will assign X the address specified by LC value of current step.

Step-7: END 205
Program finishes execution and remaining literal will get address specified by LC value of END instruction. Here is the complete symbol and literal table made by pass 1 of assembler.

Now tables generated by pass 1 along with their LC value will go to pass-2 of assembler for further processing of pseudo-opcodes and machine op-codes.

Working of Pass-2:
Pass-2 of assembler generates machine code by converting symbolic machine-opcodes into their respective bit configuration(machine understandable form). It stores all machine-opcodes in MOT table (op-code table) with symbolic code, their length and their bit configuration. It will also process pseudo-ops and will store them in POT table(pseudo-op table).

How does an assembler work?

I am looking for a brief description of the use of an assembler in producing machine code.

So I know that assembly is a 1:1 translation of machine code. But I am getting confused about object code and linkers and how they place into it.

I don’t need a complex answer just a simple one will do fine

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Both an assembler and a compiler translate source files into object files.

Object files are effectively an intermediate step before the final executable output (generated by the linker).

The linker takes the specified object files and libraries (which are packages of object files) and resolves relocation (or ‘fixup’) records.

These relocation records are made when the compiler/assembler doesn’t know the address of a function or variable used in the source code, and generates a reference for it by name, which can be resolved by the linker.

But, a relocation record is made for the 4 bytes of the address :

The offset is ‘1’ and the type is ‘R_X86_64_PC32’ which tells the linker to resolve this reference, and put the resolved address into the specified offset.

When and how to use an assembler. Assembly programming basics

The basics of programming in assembly, the design of the processor, registers, memory, instruction, and use of assembly language within C++ and Delphi.

1. Introduction to assembly

Assembly language, a low-level programming language which allows you to use all the features of a computer processor is nowadays somewhat forgotten by “modern” developers.

The main reason for this is that writing in assembly is not the simplest of tasks, and is very time-consuming (testing code, finding bugs etc.).

However, in some situations assembly may be an ideal solution. An example is any kind of algorithm where speed is essential, such as in cryptographic (i.e. encryption) algorithms.

Despite incredible advancements in compilers in recent years, algorithms such as Blowfish, Rijndael, Idea written in assembly and “manually” optimised show significant speed advantages over their counterparts written e.g. in C++ and compiled at the maximum optimisation level.

In addition to cryptography, assembly is also often used by game developers. The best example may be the game QUAKE 2. After the publication of its source code, it turned out that all the algorithms that require speed were written in assembly.

So let’s get started. To be clear, I should add that in this article I will focus on assembly for x86 processors, and its use in a Windows environment.

2. Fundamentals of assembly

If you have never written in assembly, before you can even create the simplest program, you must first learn several fundamentals like the CPU registers, instructions, and the stack.

From the programmer’s perspective, a standard processor (I will use the Intel Pentium MMX as an example, as it is all I’ve got 🙂 has a large range of instructions ranging from 8 to 16 to 32-bit x86 instructions, as well as floating point and MMX instructions.

2.1. CPU registers

2.2. General purpose registers

When writing a program, or inline assembly code under Windows, you can use all the general purpose registers, but using the special registers ESP and EBP can interfere with the operation of the program. For example, if you reset the ESP register to zero within a function, the program will most likely crash later (e.g. if the program tries to return from the function).

2.3. The stack

The stack is an area of memory reserved for the needs of the program. These include passing parameters to functions (as 32-bit values), temporary data storage, and all local variables. When the program starts, the ESP register (stack pointer) points to the end of the stack. When data is stored on the stack, the ESP register is decremented, and the data is then stored in the memory location which ESP points to. To store data on the stack, the push instruction is used, for instance:

2.4. Limitations in Windows

If you have written assembly programs under MS/DOS, where there were no limitations, you will need to be aware that there are some differences under Windows. As I said earlier, in assembly we can use all the instructions that the CPU supports, however some instructions are not permitted by the operating system, in our case Windows. For instance, if we use I/O port instructions, the compiler will not give an error, but the program will most likely crash if these instructions are executed under Windows.

Instructions which can cause the program to be terminated include the above-mentioned I/O port instructions, as well as instructions that refer to interrupts, segment registers and control registers.

3. Using assembly language

To take advantage of the benefits of assembly, you must first check whether your development tools allow its use. Products such as Borland Delphi, Builder, Watcom C++ or Microsoft Visual C++ allow you to use (compile) assembly code; Visual Basic is the only popular RAD package which does not allow writing code in assembly. These products support the use of assembly code in two ways. The first is called inline assembly, where the assembly code is inserted into the regular code written in e.g. C++. The second method is linking modules (i.e. separate files) written in assembly with modules written e.g. in Delphi or C++.

3.1. Inline assembly

Before you start writing assembly code, you must check how to write it, because there are two types of syntax for assembly code. The first type is called “intel syntax”, and is used in products such as Delphi, Builder, MSVC, Borland TASM, Microsoft MASM (assembly compilers). This syntax is now the standard and is used in 90% of sources. The second type is called “at&t syntax”, and is used e.g. in C compilers, such as GCC (Linux platform), DJGPP and LCC.

Inline assembly is the easiest way to write asm code. When writing assembly code in Delphi or Builder, it must be enclosed between the asm keyword marking the beginning of the assembly code, and the end; keyword after the code. For example:

Writing inline assembly in MSVC only really differs in how the assembly code is introduced to the compiler:

3.2. Using variables in assembly

Writing in assembly, you have access to all global variables, and if the code is in a procedure, it also has access to the local variables and parameters of the procedure/function, so its capabilities are practically the same as normal code. An example of the use of global and local variables:

The example for MSVC is not much different from that of Delphi:

You can write entire functions in assembly language. When doing this, there are a few things to keep in mind. If the function returns a value, we must ensure that the returned value is stored in the EAX register before leaving the function. A simple example:

We already know that functions written in assembly must place the return value in the EAX register, but what about the other registers?

When writing code in assembly that uses the stack, special attention should be paid to ensuring that the stack pointer ESP is always restored. E.g. if the procedure or function stores something on the stack, then this item must be removed before exiting the function. This time we’ll look at an example in MSVC:

3.3. Calling functions from assembly

The correct calling convention for functions in our own programs (as opposed to WinApi) often depends on the options with which the program was compiled. In Delphi the default convention is “register”, while for most programs written in C, the default is “cdecl”.

WinApi functions (Windows system functions) use the mechanism stdcall, where function parameters are first stored on the stack, and then the function is called. After the function returns, there is no need to adjust the stack (remove the previously saved parameters), since the called function does it for us. Interestingly, a few WinApi functions do not use the stdcall convention, but instead use cdecl, that is, the parameters are stored on the stack, then the function is called, but afterwards the stack must be cleaned up manually. An example of such a function is the wsprintfA function from the Windows system library user32.dll (whose counterpart in the C standard library is sprintf ). The cdecl was probably chosen because these functions do not have a fixed number of parameters:

4. MMX instructions

MMX is the name of an extension to the Pentium series of processors, introduced by Intel. The name is said to be an abbreviation of “MultiMedia eXtensions”, but Intel denies this, and has never explained the issue. The MMX extension to the Pentium line of processors includes a set of new instructions (57, to be exact), and 8 additional 64-bit registers.

MMX registers are shared with the FPU registers. This means that you cannot mix FPU (Floating Point Unit) instructions with MMX unit instructions otherwise the contents of the registers will be corrupted. MMX instructions can operate on data in SIMD fashion (Single Instruction Multiple Data). This means that one operation can be performed simultaneously on many data items, which is not possible using standard x86 instruction.

MMX instructions are ideal for processing multimedia data, e.g. video, graphics, sound. For example, programs such as DivX or Winamp make intensive use of MMX code. Currently, most processors produced by Intel, AMD and Cyrix possess MMX support.

Although MMX has for quite a few years been practically standard, HLL compilers generally do not generate MMX code (except specialised compilers like VectorC). It seems that the natural solution is to program MMX in assembly.

Writing procedures using MMX can sometimes get a 100% speed increase compared to the original code. This is possible because of the aforementioned SIMD mode. Imagine a situation where we have two tables of 8 bytes, and we want to add corresponding bytes from both tables to each other. In C++ we would do it this way:

There’s no problem with this, but the operation of adding bytes will be repeated 8 times. Let’s look at how this can be done much more efficiently by using MMX:

In total, just one instruction is executed instead of 8 additions. Neat, isn’t it? And more importantly, efficient. Here a few examples of graphical functions:

5. When to use assembly

As I mentioned at the beginning of the article, assembly is used mainly where speed is important. When writing an algorithm, we should sometimes stop and ask ourselves whether our program could be enhanced, if at some critical points (for instance in loops, etc.), we were to employ, say, MMX.

Imagine that you just wrote an mp3 encoder, and a competitor did the same, but you used hand-written MMX code which is three times faster than the competition. Which product will users choose, when they can complete a task in 10 minutes instead of 30? The answer is obvious.

Besides being ideal for writing algorithms that require speed, assembly is also used to write particular programs such as EXE-compressors. I’ll bet that most people will think of programs like UPX or Aspack, which are used to compress executables. Put simply, if you write a program which occupies let’s say 700 kB, when compressed by UPX its size will decrease to approx. 300 kB, but the program will still be in the form of an EXE file, and will be just as functional as before compression. This is achieved by using assembly to write a loader for the code. This is a fragment of code that is stored in the EXE file (almost like a virus), and when you start such a program, the loader decompresses the remainder of the EXE file and allows it to run. Writing a loader in a HLL, whether it be C++, Delphi or even Power Basic is virtually impossible.

It can be said that assembly programming is only useful for speed and unusual applications, but this is not entirely true. Writing in assembly language can be more than just inline routines and a few procedures here and there. Entire programs can be written in assembly language! Sometimes I hear people say that it is impossible; that you can’t write large applications in assembly from scratch. Often these are people who have only dabbled in assembly for a few hours. If you are a competent programmer, there is nothing stopping you from building professional applications in assembly language. Writing programs in assembly gives us full control over them. Everything is up to us, the program is executed according to our will, and we are not at the mercy of the compiler.

These days, writing in assembly is reasonably simple and convenient. A lot of people around the world are beginning to see the magic of this language. People are creating many projects; you can find a whole bunch of sample tutorials and source code, thanks to which many challenges have ceased to be problems. Writing entire applications in assembly also has the advantage that a project with 5MB of source code will be compiled to an executable of approximately 90kB. Compare an application written in Delphi 6, containing 1 window, which takes approx. 300kb compiled, to a program written in assembly language which does exactly the same thing, and works on every Windows release from 95 to XP, with just a 4kb executable. Why the big difference? It’s simple: the compiler adds a lot of unnecessary things, “just in case”. Why isn’t this made more efficient? We should ask the companies who make compilers.

Despite the fact that assembly can be used for many useful things, it is also used to write malicious programs, such as viruses, ransomware, or exploits, but in the words of Winnie the Pooh, that is a story for another day.

6. Summary

These examples represent only a small range of what is possible with assembly. There is a lot to discover, just as much for me as there is for you, because contrary to what they say, assembly is not dead, it is constantly changing, evolving, giving us possibilities which do not exist in any high-level language. The terms we hear in the press: SSE, SSE2, 3DNow, are not fiction. Everything is out there. We just have to reach for it.

For my part, writing assembly language gives me a feeling of freedom, which I never found when writing in any other language. I hope that your journey into assembly doesn’t end with this article!

7. References

About the Author

Bartosz Wójcik — is an author of PELock software protection and licensing system for programmers.

What is Assembly Language?

By Priya Pedamkar

Introduction to Assembly Language

Assembly Language is a low-level programming language. It helps in understanding the programming language to machine code. In computers, there is an assembler that helps in converting the assembly code into machine code executable. Assembly language is designed to understand the instruction and provide it to machine language for further processing. It mainly depends on the architecture of the system, whether it is the operating system or computer architecture.

Assembly Language mainly consists of mnemonic processor instructions or data and other statements or instructions. It is produced with the help of compiling the high-level language source code like C, C++. Assembly Language helps in fine-tuning the program.

Web development, programming languages, Software testing & others

Why is Assembly Language Useful?

Assembly language helps programmers to write human-readable code that is almost similar to machine language. Machine language is difficult to understand and read as it is just a series of numbers. Assembly language helps in providing full control of what tasks a computer is performing.

Example:

Find the below steps to print “Hello world” in Windows

Why should you learn Assembly Language?

The learning of assembly language is still important for programmers. It helps in taking complete control over the system and its resources. By learning assembly language, the programmer can write the code to access registers and retrieve the memory address of pointers and values. It mainly helps in speed optimization that increases efficiency and performance.

Assembly language learning helps in understanding the processor and memory functions. If the programmer is writing any program that needs to be a compiler, that means the programmer should have a complete understanding of the processor. Assembly language helps in understanding the work of processors and memory. It is cryptic and symbolic language.

Assembly Language helps in contacting the hardware directly. This language is mainly based on computer architecture, and it recognizes a certain type of processor and its different for different CPUs. Assembly language refers to transparency compared to other high-level languages. It has a small number of operations, but it is helpful in understanding the algorithms and other flow of controls. It makes the code less complex and easy debugging as well.

Features

The features of the assembly language are mentioned below:

Assemblers

The assemblers are used to translate the assembly language into machine language. There are two types of assembler are:

Advantages and Disadvantages

Mentioned are some advantages and disadvantages:

Advantages

Below are the advantages:

Disadvantages

Below mentioned are the disadvantages:

Conclusion

Assembly language is very important for understanding the computer architecture and programs for the programmers. The programmers mainly used many other programming languages for application development and software, but assembly language is also important. It helps programmers to achieve a lot if they implement the assembly language. Assemblies contain a lot of metadata that is version number, localization details, and other product details. It is an important part and provided to the user after digitally signed.

If an individual wants to know how the system works and the processor as well, then assembly language is the one that solves the purpose. It helps in all aspects, from understanding the algorithm of the program to the processor working and registering the registers of the computer. It depends on individual choice with which language to continue.

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What is Assembly Language?

Each personal computer has a microprocessor that manages the computer’s arithmetical, logical, and control activities.

Each family of processors has its own set of instructions for handling various operations such as getting input from keyboard, displaying information on screen and performing various other jobs. These set of instructions are called ‘machine language instructions’.

A processor understands only machine language instructions, which are strings of 1’s and 0’s. However, machine language is too obscure and complex for using in software development. So, the low-level assembly language is designed for a specific family of processors that represents various instructions in symbolic code and a more understandable form.

Advantages of Assembly Language

Having an understanding of assembly language makes one aware of −

Other advantages of using assembly language are −

It requires less memory and execution time;

It allows hardware-specific complex jobs in an easier way;

It is suitable for time-critical jobs;

It is most suitable for writing interrupt service routines and other memory resident programs.

Basic Features of PC Hardware

The main internal hardware of a PC consists of processor, memory, and registers. Registers are processor components that hold data and address. To execute a program, the system copies it from the external device into the internal memory. The processor executes the program instructions.

The fundamental unit of computer storage is a bit; it could be ON (1) or OFF (0) and a group of 8 related bits makes a byte on most of the modern computers.

So, the parity bit is used to make the number of bits in a byte odd. If the parity is even, the system assumes that there had been a parity error (though rare), which might have been caused due to hardware fault or electrical disturbance.

The processor supports the following data sizes −

Binary Number System

Every number system uses positional notation, i.e., each position in which a digit is written has a different positional value. Each position is power of the base, which is 2 for binary number system, and these powers begin at 0 and increase by 1.

The following table shows the positional values for an 8-bit binary number, where all bits are set ON.

The value of a binary number is based on the presence of 1 bits and their positional value. So, the value of a given binary number is −

1 + 2 + 4 + 8 +16 + 32 + 64 + 128 = 255

Hexadecimal Number System

Hexadecimal number system uses base 16. The digits in this system range from 0 to 15. By convention, the letters A through F is used to represent the hexadecimal digits corresponding to decimal values 10 through 15.

Hexadecimal numbers in computing is used for abbreviating lengthy binary representations. Basically, hexadecimal number system represents a binary data by dividing each byte in half and expressing the value of each half-byte. The following table provides the decimal, binary, and hexadecimal equivalents −

To convert a binary number to its hexadecimal equivalent, break it into groups of 4 consecutive groups each, starting from the right, and write those groups over the corresponding digits of the hexadecimal number.

To convert a hexadecimal number to binary, just write each hexadecimal digit into its 4-digit binary equivalent.

Binary Arithmetic

The following table illustrates four simple rules for binary addition −

Rules (iii) and (iv) show a carry of a 1-bit into the next left position.

A negative binary value is expressed in two’s complement notation. According to this rule, to convert a binary number to its negative value is to reverse its bit values and add 1.

To subtract one value from another, convert the number being subtracted to two’s complement format and add the numbers.

Subtract 42 from 53

Overflow of the last 1 bit is lost.

Addressing Data in Memory

The process through which the processor controls the execution of instructions is referred as the fetch-decode-execute cycle or the execution cycle. It consists of three continuous steps −

The processor may access one or more bytes of memory at a time. Let us consider a hexadecimal number 0725H. This number will require two bytes of memory. The high-order byte or most significant byte is 07 and the low-order byte is 25.

The processor stores data in reverse-byte sequence, i.e., a low-order byte is stored in a low memory address and a high-order byte in high memory address. So, if the processor brings the value 0725H from register to memory, it will transfer 25 first to the lower memory address and 07 to the next memory address.

x: memory address

When the processor gets the numeric data from memory to register, it again reverses the bytes. There are two kinds of memory addresses −

Local Environment Setup

Assembly language is dependent upon the instruction set and the architecture of the processor. In this tutorial, we focus on Intel-32 processors like Pentium. To follow this tutorial, you will need −

There are many good assembler programs, such as −

We will use the NASM assembler, as it is −

Installing NASM

If you select «Development Tools» while installing Linux, you may get NASM installed along with the Linux operating system and you do not need to download and install it separately. For checking whether you already have NASM installed, take the following steps −

Open a Linux terminal.

Type whereis nasm and press ENTER.

If it is already installed, then a line like, nasm: /usr/bin/nasm appears. Otherwise, you will see just nasm:, then you need to install NASM.

To install NASM, take the following steps −

Check The netwide assembler (NASM) website for the latest version.

cd to nasm-X.XX and type ./configure. This shell script will find the best C compiler to use and set up Makefiles accordingly.

Type make to build the nasm and ndisasm binaries.

Type make install to install nasm and ndisasm in /usr/local/bin and to install the man pages.

This should install NASM on your system. Alternatively, you can use an RPM distribution for the Fedora Linux. This version is simpler to install, just double-click the RPM file.

An assembly program can be divided into three sections −

The data section,

The bss section, and

The text section.

The data Section

The data section is used for declaring initialized data or constants. This data does not change at runtime. You can declare various constant values, file names, or buffer size, etc., in this section.

The syntax for declaring data section is −

The bss Section

The bss section is used for declaring variables. The syntax for declaring bss section is −

The text section

The text section is used for keeping the actual code. This section must begin with the declaration global _start, which tells the kernel where the program execution begins.

The syntax for declaring text section is −

Comments

Assembly language comment begins with a semicolon (;). It may contain any printable character including blank. It can appear on a line by itself, like −

or, on the same line along with an instruction, like −

Assembly Language Statements

Assembly language programs consist of three types of statements −

The executable instructions or simply instructions tell the processor what to do. Each instruction consists of an operation code (opcode). Each executable instruction generates one machine language instruction.

The assembler directives or pseudo-ops tell the assembler about the various aspects of the assembly process. These are non-executable and do not generate machine language instructions.

Macros are basically a text substitution mechanism.

Syntax of Assembly Language Statements

Assembly language statements are entered one statement per line. Each statement follows the following format −

The fields in the square brackets are optional. A basic instruction has two parts, the first one is the name of the instruction (or the mnemonic), which is to be executed, and the second are the operands or the parameters of the command.

Following are some examples of typical assembly language statements −

The Hello World Program in Assembly

The following assembly language code displays the string ‘Hello World’ on the screen −

When the above code is compiled and executed, it produces the following result −

Compiling and Linking an Assembly Program in NASM

Make sure you have set the path of nasm and ld binaries in your PATH environment variable. Now, take the following steps for compiling and linking the above program −

Type the above code using a text editor and save it as hello.asm.

Make sure that you are in the same directory as where you saved hello.asm.

If there is any error, you will be prompted about that at this stage. Otherwise, an object file of your program named hello.o will be created.

Execute the program by typing ./hello

If you have done everything correctly, it will display ‘Hello, world!’ on the screen.

We have already discussed the three sections of an assembly program. These sections represent various memory segments as well.

Interestingly, if you replace the section keyword with segment, you will get the same result. Try the following code −

When the above code is compiled and executed, it produces the following result −

Memory Segments

A segmented memory model divides the system memory into groups of independent segments referenced by pointers located in the segment registers. Each segment is used to contain a specific type of data. One segment is used to contain instruction codes, another segment stores the data elements, and a third segment keeps the program stack.

In the light of the above discussion, we can specify various memory segments as −

Code segment − It is represented by .text section. This defines an area in memory that stores the instruction codes. This is also a fixed area.

Stack − This segment contains data values passed to functions and procedures within the program.

Processor operations mostly involve processing data. This data can be stored in memory and accessed from thereon. However, reading data from and storing data into memory slows down the processor, as it involves complicated processes of sending the data request across the control bus and into the memory storage unit and getting the data through the same channel.

To speed up the processor operations, the processor includes some internal memory storage locations, called registers.

The registers store data elements for processing without having to access the memory. A limited number of registers are built into the processor chip.

Processor Registers

There are ten 32-bit and six 16-bit processor registers in IA-32 architecture. The registers are grouped into three categories −

The general registers are further divided into the following groups −

Data Registers

Four 32-bit data registers are used for arithmetic, logical, and other operations. These 32-bit registers can be used in three ways −

As complete 32-bit data registers: EAX, EBX, ECX, EDX.

Lower halves of the 32-bit registers can be used as four 16-bit data registers: AX, BX, CX and DX.

Lower and higher halves of the above-mentioned four 16-bit registers can be used as eight 8-bit data registers: AH, AL, BH, BL, CH, CL, DH, and DL.

Some of these data registers have specific use in arithmetical operations.

AX is the primary accumulator; it is used in input/output and most arithmetic instructions. For example, in multiplication operation, one operand is stored in EAX or AX or AL register according to the size of the operand.

BX is known as the base register, as it could be used in indexed addressing.

CX is known as the count register, as the ECX, CX registers store the loop count in iterative operations.

DX is known as the data register. It is also used in input/output operations. It is also used with AX register along with DX for multiply and divide operations involving large values.

Pointer Registers

The pointer registers are 32-bit EIP, ESP, and EBP registers and corresponding 16-bit right portions IP, SP, and BP. There are three categories of pointer registers −

Instruction Pointer (IP) − The 16-bit IP register stores the offset address of the next instruction to be executed. IP in association with the CS register (as CS:IP) gives the complete address of the current instruction in the code segment.

Stack Pointer (SP) − The 16-bit SP register provides the offset value within the program stack. SP in association with the SS register (SS:SP) refers to be current position of data or address within the program stack.

Base Pointer (BP) − The 16-bit BP register mainly helps in referencing the parameter variables passed to a subroutine. The address in SS register is combined with the offset in BP to get the location of the parameter. BP can also be combined with DI and SI as base register for special addressing.

Index Registers

The 32-bit index registers, ESI and EDI, and their 16-bit rightmost portions. SI and DI, are used for indexed addressing and sometimes used in addition and subtraction. There are two sets of index pointers −

Source Index (SI) − It is used as source index for string operations.

Destination Index (DI) − It is used as destination index for string operations.

Control Registers

The 32-bit instruction pointer register and the 32-bit flags register combined are considered as the control registers.

Many instructions involve comparisons and mathematical calculations and change the status of the flags and some other conditional instructions test the value of these status flags to take the control flow to other location.

The common flag bits are:

Overflow Flag (OF) − It indicates the overflow of a high-order bit (leftmost bit) of data after a signed arithmetic operation.

Direction Flag (DF) − It determines left or right direction for moving or comparing string data. When the DF value is 0, the string operation takes left-to-right direction and when the value is set to 1, the string operation takes right-to-left direction.

Interrupt Flag (IF) − It determines whether the external interrupts like keyboard entry, etc., are to be ignored or processed. It disables the external interrupt when the value is 0 and enables interrupts when set to 1.

Trap Flag (TF) − It allows setting the operation of the processor in single-step mode. The DEBUG program we used sets the trap flag, so we could step through the execution one instruction at a time.

Sign Flag (SF) − It shows the sign of the result of an arithmetic operation. This flag is set according to the sign of a data item following the arithmetic operation. The sign is indicated by the high-order of leftmost bit. A positive result clears the value of SF to 0 and negative result sets it to 1.

Zero Flag (ZF) − It indicates the result of an arithmetic or comparison operation. A nonzero result clears the zero flag to 0, and a zero result sets it to 1.

Auxiliary Carry Flag (AF) − It contains the carry from bit 3 to bit 4 following an arithmetic operation; used for specialized arithmetic. The AF is set when a 1-byte arithmetic operation causes a carry from bit 3 into bit 4.

Parity Flag (PF) − It indicates the total number of 1-bits in the result obtained from an arithmetic operation. An even number of 1-bits clears the parity flag to 0 and an odd number of 1-bits sets the parity flag to 1.

Carry Flag (CF) − It contains the carry of 0 or 1 from a high-order bit (leftmost) after an arithmetic operation. It also stores the contents of last bit of a shift or rotate operation.

The following table indicates the position of flag bits in the 16-bit Flags register:

Segment Registers

Segments are specific areas defined in a program for containing data, code and stack. There are three main segments −

Code Segment − It contains all the instructions to be executed. A 16-bit Code Segment register or CS register stores the starting address of the code segment.

Data Segment − It contains data, constants and work areas. A 16-bit Data Segment register or DS register stores the starting address of the data segment.

Stack Segment − It contains data and return addresses of procedures or subroutines. It is implemented as a ‘stack’ data structure. The Stack Segment register or SS register stores the starting address of the stack.

In assembly programming, a program needs to access the memory locations. All memory locations within a segment are relative to the starting address of the segment. A segment begins in an address evenly divisible by 16 or hexadecimal 10. So, the rightmost hex digit in all such memory addresses is 0, which is not generally stored in the segment registers.

The segment registers stores the starting addresses of a segment. To get the exact location of data or instruction within a segment, an offset value (or displacement) is required. To reference any memory location in a segment, the processor combines the segment address in the segment register with the offset value of the location.

Example

Look at the following simple program to understand the use of registers in assembly programming. This program displays 9 stars on the screen along with a simple message −

When the above code is compiled and executed, it produces the following result −

System calls are APIs for the interface between the user space and the kernel space. We have already used the system calls. sys_write and sys_exit, for writing into the screen and exiting from the program, respectively.

Linux System Calls

You can make use of Linux system calls in your assembly programs. You need to take the following steps for using Linux system calls in your program −

There are six registers that store the arguments of the system call used. These are the EBX, ECX, EDX, ESI, EDI, and EBP. These registers take the consecutive arguments, starting with the EBX register. If there are more than six arguments, then the memory location of the first argument is stored in the EBX register.

The following code snippet shows the use of the system call sys_exit −

The following code snippet shows the use of the system call sys_write −

All the syscalls are listed in /usr/include/asm/unistd.h, together with their numbers (the value to put in EAX before you call int 80h).

The following table shows some of the system calls used in this tutorial −

Example

The following example reads a number from the keyboard and displays it on the screen −

When the above code is compiled and executed, it produces the following result −

Most assembly language instructions require operands to be processed. An operand address provides the location, where the data to be processed is stored. Some instructions do not require an operand, whereas some other instructions may require one, two, or three operands.

When an instruction requires two operands, the first operand is generally the destination, which contains data in a register or memory location and the second operand is the source. Source contains either the data to be delivered (immediate addressing) or the address (in register or memory) of the data. Generally, the source data remains unaltered after the operation.

The three basic modes of addressing are −

Register Addressing

In this addressing mode, a register contains the operand. Depending upon the instruction, the register may be the first operand, the second operand or both.

As processing data between registers does not involve memory, it provides fastest processing of data.

Immediate Addressing

An immediate operand has a constant value or an expression. When an instruction with two operands uses immediate addressing, the first operand may be a register or memory location, and the second operand is an immediate constant. The first operand defines the length of the data.

Direct Memory Addressing

When operands are specified in memory addressing mode, direct access to main memory, usually to the data segment, is required. This way of addressing results in slower processing of data. To locate the exact location of data in memory, we need the segment start address, which is typically found in the DS register and an offset value. This offset value is also called effective address.

In direct addressing mode, the offset value is specified directly as part of the instruction, usually indicated by the variable name. The assembler calculates the offset value and maintains a symbol table, which stores the offset values of all the variables used in the program.

In direct memory addressing, one of the operands refers to a memory location and the other operand references a register.

Direct-Offset Addressing

This addressing mode uses the arithmetic operators to modify an address. For example, look at the following definitions that define tables of data −

The following operations access data from the tables in the memory into registers −

Indirect Memory Addressing

This addressing mode utilizes the computer’s ability of Segment:Offset addressing. Generally, the base registers EBX, EBP (or BX, BP) and the index registers (DI, SI), coded within square brackets for memory references, are used for this purpose.

Indirect addressing is generally used for variables containing several elements like, arrays. Starting address of the array is stored in, say, the EBX register.

The following code snippet shows how to access different elements of the variable.

The MOV Instruction

We have already used the MOV instruction that is used for moving data from one storage space to another. The MOV instruction takes two operands.

Syntax

The syntax of the MOV instruction is −

The MOV instruction may have one of the following five forms −

Please note that −

The MOV instruction causes ambiguity at times. For example, look at the statements −

It is not clear whether you want to move a byte equivalent or word equivalent of the number 110. In such cases, it is wise to use a type specifier.

Following table shows some of the common type specifiers −

Example

The following program illustrates some of the concepts discussed above. It stores a name ‘Zara Ali’ in the data section of the memory, then changes its value to another name ‘Nuha Ali’ programmatically and displays both the names.

When the above code is compiled and executed, it produces the following result −

NASM provides various define directives for reserving storage space for variables. The define assembler directive is used for allocation of storage space. It can be used to reserve as well as initialize one or more bytes.

Allocating Storage Space for Initialized Data

The syntax for storage allocation statement for initialized data is −

Where, variable-name is the identifier for each storage space. The assembler associates an offset value for each variable name defined in the data segment.

There are five basic forms of the define directive −

Following are some examples of using define directives −

Please note that −

Each byte of character is stored as its ASCII value in hexadecimal.

Each decimal value is automatically converted to its 16-bit binary equivalent and stored as a hexadecimal number.

Processor uses the little-endian byte ordering.

Negative numbers are converted to its 2’s complement representation.

Short and long floating-point numbers are represented using 32 or 64 bits, respectively.

The following program shows the use of define directive −

When the above code is compiled and executed, it produces the following result −

Allocating Storage Space for Uninitialized Data

The reserve directives are used for reserving space for uninitialized data. The reserve directives take a single operand that specifies the number of units of space to be reserved. Each define directive has a related reserve directive.

There are five basic forms of the reserve directive −

Multiple Definitions

You can have multiple data definition statements in a program. For example −

The assembler allocates contiguous memory for multiple variable definitions.

Multiple Initializations

The TIMES directive allows multiple initializations to the same value. For example, an array named marks of size 9 can be defined and initialized to zero using the following statement −

The TIMES directive is useful in defining arrays and tables. The following program displays 9 asterisks on the screen −

When the above code is compiled and executed, it produces the following result −

There are several directives provided by NASM that define constants. We have already used the EQU directive in previous chapters. We will particularly discuss three directives −

The EQU Directive

The EQU directive is used for defining constants. The syntax of the EQU directive is as follows −

You can then use this constant value in your code, like −

The operand of an EQU statement can be an expression −

Above code segment would define AREA as 200.

Example

The following example illustrates the use of the EQU directive −

When the above code is compiled and executed, it produces the following result −

The %assign Directive

The %assign directive can be used to define numeric constants like the EQU directive. This directive allows redefinition. For example, you may define the constant TOTAL as −

Later in the code, you can redefine it as −

This directive is case-sensitive.

The %define Directive

The %define directive allows defining both numeric and string constants. This directive is similar to the #define in C. For example, you may define the constant PTR as −

The above code replaces PTR by [EBP+4].

This directive also allows redefinition and it is case-sensitive.

The INC Instruction

The INC instruction is used for incrementing an operand by one. It works on a single operand that can be either in a register or in memory.

Syntax

The INC instruction has the following syntax −

The operand destination could be an 8-bit, 16-bit or 32-bit operand.

Example

The DEC Instruction

The DEC instruction is used for decrementing an operand by one. It works on a single operand that can be either in a register or in memory.

Syntax

The DEC instruction has the following syntax −

The operand destination could be an 8-bit, 16-bit or 32-bit operand.

Example

The ADD and SUB Instructions

The ADD and SUB instructions are used for performing simple addition/subtraction of binary data in byte, word and doubleword size, i.e., for adding or subtracting 8-bit, 16-bit or 32-bit operands, respectively.

Syntax

The ADD and SUB instructions have the following syntax −

The ADD/SUB instruction can take place between −

However, like other instructions, memory-to-memory operations are not possible using ADD/SUB instructions. An ADD or SUB operation sets or clears the overflow and carry flags.

Example

The following example will ask two digits from the user, store the digits in the EAX and EBX register, respectively, add the values, store the result in a memory location ‘res‘ and finally display the result.

When the above code is compiled and executed, it produces the following result −

The program with hardcoded variables −

When the above code is compiled and executed, it produces the following result −

The MUL/IMUL Instruction

There are two instructions for multiplying binary data. The MUL (Multiply) instruction handles unsigned data and the IMUL (Integer Multiply) handles signed data. Both instructions affect the Carry and Overflow flag.

Syntax

The syntax for the MUL/IMUL instructions is as follows −

Multiplicand in both cases will be in an accumulator, depending upon the size of the multiplicand and the multiplier and the generated product is also stored in two registers depending upon the size of the operands. Following section explains MUL instructions with three different cases −

When two bytes are multiplied −

The multiplicand is in the AL register, and the multiplier is a byte in the memory or in another register. The product is in AX. High-order 8 bits of the product is stored in AH and the low-order 8 bits are stored in AL.

When two one-word values are multiplied −

The multiplicand should be in the AX register, and the multiplier is a word in memory or another register. For example, for an instruction like MUL DX, you must store the multiplier in DX and the multiplicand in AX.

The resultant product is a doubleword, which will need two registers. The high-order (leftmost) portion gets stored in DX and the lower-order (rightmost) portion gets stored in AX.

When two doubleword values are multiplied −

When two doubleword values are multiplied, the multiplicand should be in EAX and the multiplier is a doubleword value stored in memory or in another register. The product generated is stored in the EDX:EAX registers, i.e., the high order 32 bits gets stored in the EDX register and the low order 32-bits are stored in the EAX register.

Example

Example

The following example multiplies 3 with 2, and displays the result −

When the above code is compiled and executed, it produces the following result −

The DIV/IDIV Instructions

The DIV (Divide) instruction is used for unsigned data and the IDIV (Integer Divide) is used for signed data.

Syntax

The format for the DIV/IDIV instruction −

The dividend is in an accumulator. Both the instructions can work with 8-bit, 16-bit or 32-bit operands. The operation affects all six status flags. Following section explains three cases of division with different operand size −

When the divisor is 1 byte −

The dividend is assumed to be in the AX register (16 bits). After division, the quotient goes to the AL register and the remainder goes to the AH register.

When the divisor is 1 word −

The dividend is assumed to be 32 bits long and in the DX:AX registers. The high-order 16 bits are in DX and the low-order 16 bits are in AX. After division, the 16-bit quotient goes to the AX register and the 16-bit remainder goes to the DX register.

When the divisor is doubleword −

The dividend is assumed to be 64 bits long and in the EDX:EAX registers. The high-order 32 bits are in EDX and the low-order 32 bits are in EAX. After division, the 32-bit quotient goes to the EAX register and the 32-bit remainder goes to the EDX register.

Example

The following example divides 8 with 2. The dividend 8 is stored in the 16-bit AX register and the divisor 2 is stored in the 8-bit BL register.

When the above code is compiled and executed, it produces the following result −

The processor instruction set provides the instructions AND, OR, XOR, TEST, and NOT Boolean logic, which tests, sets, and clears the bits according to the need of the program.

The format for these instructions −

The first operand in all the cases could be either in register or in memory. The second operand could be either in register/memory or an immediate (constant) value. However, memory-to-memory operations are not possible. These instructions compare or match bits of the operands and set the CF, OF, PF, SF and ZF flags.

The AND Instruction

The AND instruction is used for supporting logical expressions by performing bitwise AND operation. The bitwise AND operation returns 1, if the matching bits from both the operands are 1, otherwise it returns 0. For example −

The AND operation can be used for clearing one or more bits. For example, say the BL register contains 0011 1010. If you need to clear the high-order bits to zero, you AND it with 0FH.

Let’s take up another example. If you want to check whether a given number is odd or even, a simple test would be to check the least significant bit of the number. If this is 1, the number is odd, else the number is even.

Assuming the number is in AL register, we can write −

The following program illustrates this −

Example

When the above code is compiled and executed, it produces the following result −

Change the value in the ax register with an odd digit, like −

The program would display:

Similarly to clear the entire register you can AND it with 00H.

The OR Instruction

The OR instruction is used for supporting logical expression by performing bitwise OR operation. The bitwise OR operator returns 1, if the matching bits from either or both operands are one. It returns 0, if both the bits are zero.

The OR operation can be used for setting one or more bits. For example, let us assume the AL register contains 0011 1010, you need to set the four low-order bits, you can OR it with a value 0000 1111, i.e., FH.

Example

The following example demonstrates the OR instruction. Let us store the value 5 and 3 in the AL and the BL registers, respectively, then the instruction,

should store 7 in the AL register −

When the above code is compiled and executed, it produces the following result −

The XOR Instruction

The XOR instruction implements the bitwise XOR operation. The XOR operation sets the resultant bit to 1, if and only if the bits from the operands are different. If the bits from the operands are same (both 0 or both 1), the resultant bit is cleared to 0.

XORing an operand with itself changes the operand to 0. This is used to clear a register.

The TEST Instruction

The TEST instruction works same as the AND operation, but unlike AND instruction, it does not change the first operand. So, if we need to check whether a number in a register is even or odd, we can also do this using the TEST instruction without changing the original number.

The NOT Instruction

The NOT instruction implements the bitwise NOT operation. NOT operation reverses the bits in an operand. The operand could be either in a register or in the memory.

Conditional execution in assembly language is accomplished by several looping and branching instructions. These instructions can change the flow of control in a program. Conditional execution is observed in two scenarios −

This is performed by the JMP instruction. Conditional execution often involves a transfer of control to the address of an instruction that does not follow the currently executing instruction. Transfer of control may be forward, to execute a new set of instructions or backward, to re-execute the same steps.

This is performed by a set of jump instructions j depending upon the condition. The conditional instructions transfer the control by breaking the sequential flow and they do it by changing the offset value in IP.

Let us discuss the CMP instruction before discussing the conditional instructions.

CMP Instruction

The CMP instruction compares two operands. It is generally used in conditional execution. This instruction basically subtracts one operand from the other for comparing whether the operands are equal or not. It does not disturb the destination or source operands. It is used along with the conditional jump instruction for decision making.

Syntax

CMP compares two numeric data fields. The destination operand could be either in register or in memory. The source operand could be a constant (immediate) data, register or memory.

Example

CMP is often used for comparing whether a counter value has reached the number of times a loop needs to be run. Consider the following typical condition −

Unconditional Jump

As mentioned earlier, this is performed by the JMP instruction. Conditional execution often involves a transfer of control to the address of an instruction that does not follow the currently executing instruction. Transfer of control may be forward, to execute a new set of instructions or backward, to re-execute the same steps.

Syntax

The JMP instruction provides a label name where the flow of control is transferred immediately. The syntax of the JMP instruction is −

Example

The following code snippet illustrates the JMP instruction −

Conditional Jump

If some specified condition is satisfied in conditional jump, the control flow is transferred to a target instruction. There are numerous conditional jump instructions depending upon the condition and data.

Following are the conditional jump instructions used on signed data used for arithmetic operations −

Following are the conditional jump instructions used on unsigned data used for logical operations −

The following conditional jump instructions have special uses and check the value of flags −

The syntax for the J set of instructions −

Example

The following program displays the largest of three variables. The variables are double-digit variables. The three variables num1, num2 and num3 have values 47, 22 and 31, respectively −

When the above code is compiled and executed, it produces the following result −

The JMP instruction can be used for implementing loops. For example, the following code snippet can be used for executing the loop-body 10 times.

The processor instruction set, however, includes a group of loop instructions for implementing iteration. The basic LOOP instruction has the following syntax −

Where, label is the target label that identifies the target instruction as in the jump instructions. The LOOP instruction assumes that the ECX register contains the loop count. When the loop instruction is executed, the ECX register is decremented and the control jumps to the target label, until the ECX register value, i.e., the counter reaches the value zero.

The above code snippet could be written as −

Example

The following program prints the number 1 to 9 on the screen −

When the above code is compiled and executed, it produces the following result −

Numerical data is generally represented in binary system. Arithmetic instructions operate on binary data. When numbers are displayed on screen or entered from keyboard, they are in ASCII form.

So far, we have converted this input data in ASCII form to binary for arithmetic calculations and converted the result back to binary. The following code shows this −

When the above code is compiled and executed, it produces the following result −

Such conversions, however, have an overhead, and assembly language programming allows processing numbers in a more efficient way, in the binary form. Decimal numbers can be represented in two forms −

ASCII Representation

In ASCII representation, decimal numbers are stored as string of ASCII characters. For example, the decimal value 1234 is stored as −

Where, 31H is ASCII value for 1, 32H is ASCII value for 2, and so on. There are four instructions for processing numbers in ASCII representation −

AAA − ASCII Adjust After Addition

AAS − ASCII Adjust After Subtraction

AAM − ASCII Adjust After Multiplication

AAD − ASCII Adjust Before Division

These instructions do not take any operands and assume the required operand to be in the AL register.

The following example uses the AAS instruction to demonstrate the concept −

When the above code is compiled and executed, it produces the following result −

BCD Representation

There are two types of BCD representation −

In unpacked BCD representation, each byte stores the binary equivalent of a decimal digit. For example, the number 1234 is stored as −

There are two instructions for processing these numbers −

AAM − ASCII Adjust After Multiplication

AAD − ASCII Adjust Before Division

The four ASCII adjust instructions, AAA, AAS, AAM, and AAD, can also be used with unpacked BCD representation. In packed BCD representation, each digit is stored using four bits. Two decimal digits are packed into a byte. For example, the number 1234 is stored as −

There are two instructions for processing these numbers −

DAA − Decimal Adjust After Addition

DAS − decimal Adjust After Subtraction

There is no support for multiplication and division in packed BCD representation.

Example

The following program adds up two 5-digit decimal numbers and displays the sum. It uses the above concepts −

When the above code is compiled and executed, it produces the following result −

We have already used variable length strings in our previous examples. The variable length strings can have as many characters as required. Generally, we specify the length of the string by either of the two ways −

$ points to the byte after the last character of the string variable msg. Therefore, $-msg gives the length of the string. We can also write

Alternatively, you can store strings with a trailing sentinel character to delimit a string instead of storing the string length explicitly. The sentinel character should be a special character that does not appear within a string.

String Instructions

Each string instruction may require a source operand, a destination operand or both. For 32-bit segments, string instructions use ESI and EDI registers to point to the source and destination operands, respectively.

For 16-bit segments, however, the SI and the DI registers are used to point to the source and destination, respectively.

There are five basic instructions for processing strings. They are −

MOVS − This instruction moves 1 Byte, Word or Doubleword of data from memory location to another.

LODS − This instruction loads from memory. If the operand is of one byte, it is loaded into the AL register, if the operand is one word, it is loaded into the AX register and a doubleword is loaded into the EAX register.

STOS − This instruction stores data from register (AL, AX, or EAX) to memory.

CMPS − This instruction compares two data items in memory. Data could be of a byte size, word or doubleword.

SCAS − This instruction compares the contents of a register (AL, AX or EAX) with the contents of an item in memory.

Each of the above instruction has a byte, word, and doubleword version, and string instructions can be repeated by using a repetition prefix.

These instructions use the ES:DI and DS:SI pair of registers, where DI and SI registers contain valid offset addresses that refers to bytes stored in memory. SI is normally associated with DS (data segment) and DI is always associated with ES (extra segment).

The DS:SI (or ESI) and ES:DI (or EDI) registers point to the source and destination operands, respectively. The source operand is assumed to be at DS:SI (or ESI) and the destination operand at ES:DI (or EDI) in memory.

For 16-bit addresses, the SI and DI registers are used, and for 32-bit addresses, the ESI and EDI registers are used.

The following table provides various versions of string instructions and the assumed space of the operands.

Repetition Prefixes

The Direction Flag (DF) determines the direction of the operation.

The REP prefix also has the following variations:

REP: It is the unconditional repeat. It repeats the operation until CX is zero.

REPE or REPZ: It is conditional repeat. It repeats the operation while the zero flag indicates equal/zero. It stops when the ZF indicates not equal/zero or when CX is zero.

REPNE or REPNZ: It is also conditional repeat. It repeats the operation while the zero flag indicates not equal/zero. It stops when the ZF indicates equal/zero or when CX is decremented to zero.

We have already discussed that the data definition directives to the assembler are used for allocating storage for variables. The variable could also be initialized with some specific value. The initialized value could be specified in hexadecimal, decimal or binary form.

For example, we can define a word variable ‘months’ in either of the following way −

The data definition directives can also be used for defining a one-dimensional array. Let us define a one-dimensional array of numbers.

The above definition declares an array of six words each initialized with the numbers 34, 45, 56, 67, 75, 89. This allocates 2×6 = 12 bytes of consecutive memory space. The symbolic address of the first number will be NUMBERS and that of the second number will be NUMBERS + 2 and so on.

Let us take up another example. You can define an array named inventory of size 8, and initialize all the values with zero, as −

Which can be abbreviated as −

The TIMES directive can also be used for multiple initializations to the same value. Using TIMES, the INVENTORY array can be defined as:

Example

The following example demonstrates the above concepts by defining a 3-element array x, which stores three values: 2, 3 and 4. It adds the values in the array and displays the sum 9 −

When the above code is compiled and executed, it produces the following result −

Procedures or subroutines are very important in assembly language, as the assembly language programs tend to be large in size. Procedures are identified by a name. Following this name, the body of the procedure is described which performs a well-defined job. End of the procedure is indicated by a return statement.

Syntax

Following is the syntax to define a procedure −

The procedure is called from another function by using the CALL instruction. The CALL instruction should have the name of the called procedure as an argument as shown below −

The called procedure returns the control to the calling procedure by using the RET instruction.

Example

Let us write a very simple procedure named sum that adds the variables stored in the ECX and EDX register and returns the sum in the EAX register −

When the above code is compiled and executed, it produces the following result −

Stacks Data Structure

A stack is an array-like data structure in the memory in which data can be stored and removed from a location called the ‘top’ of the stack. The data that needs to be stored is ‘pushed’ into the stack and data to be retrieved is ‘popped’ out from the stack. Stack is a LIFO data structure, i.e., the data stored first is retrieved last.

Assembly language provides two instructions for stack operations: PUSH and POP. These instructions have syntaxes like −

The memory space reserved in the stack segment is used for implementing stack. The registers SS and ESP (or SP) are used for implementing the stack. The top of the stack, which points to the last data item inserted into the stack is pointed to by the SS:ESP register, where the SS register points to the beginning of the stack segment and the SP (or ESP) gives the offset into the stack segment.

The stack implementation has the following characteristics −

Only words or doublewords could be saved into the stack, not a byte.

The stack grows in the reverse direction, i.e., toward the lower memory address

The top of the stack points to the last item inserted in the stack; it points to the lower byte of the last word inserted.

As we discussed about storing the values of the registers in the stack before using them for some use; it can be done in following way −

Example

The following program displays the entire ASCII character set. The main program calls a procedure named display, which displays the ASCII character set.

When the above code is compiled and executed, it produces the following result −

A recursive procedure is one that calls itself. There are two kind of recursion: direct and indirect. In direct recursion, the procedure calls itself and in indirect recursion, the first procedure calls a second procedure, which in turn calls the first procedure.

Recursion could be observed in numerous mathematical algorithms. For example, consider the case of calculating the factorial of a number. Factorial of a number is given by the equation −

For example: factorial of 5 is 1 x 2 x 3 x 4 x 5 = 5 x factorial of 4 and this can be a good example of showing a recursive procedure. Every recursive algorithm must have an ending condition, i.e., the recursive calling of the program should be stopped when a condition is fulfilled. In the case of factorial algorithm, the end condition is reached when n is 0.

The following program shows how factorial n is implemented in assembly language. To keep the program simple, we will calculate factorial 3.

When the above code is compiled and executed, it produces the following result −

Writing a macro is another way of ensuring modular programming in assembly language.

A macro is a sequence of instructions, assigned by a name and could be used anywhere in the program.

In NASM, macros are defined with %macro and %endmacro directives.

The macro begins with the %macro directive and ends with the %endmacro directive.

The Syntax for macro definition −

Where, number_of_params specifies the number parameters, macro_name specifies the name of the macro.

The macro is invoked by using the macro name along with the necessary parameters. When you need to use some sequence of instructions many times in a program, you can put those instructions in a macro and use it instead of writing the instructions all the time.

For example, a very common need for programs is to write a string of characters in the screen. For displaying a string of characters, you need the following sequence of instructions −

In the above example of displaying a character string, the registers EAX, EBX, ECX and EDX have been used by the INT 80H function call. So, each time you need to display on screen, you need to save these registers on the stack, invoke INT 80H and then restore the original value of the registers from the stack. So, it could be useful to write two macros for saving and restoring data.

We have observed that, some instructions like IMUL, IDIV, INT, etc., need some of the information to be stored in some particular registers and even return values in some specific register(s). If the program was already using those registers for keeping important data, then the existing data from these registers should be saved in the stack and restored after the instruction is executed.

Example

Following example shows defining and using macros −

When the above code is compiled and executed, it produces the following result −

The system considers any input or output data as stream of bytes. There are three standard file streams −

File Descriptor

A file descriptor is a 16-bit integer assigned to a file as a file id. When a new file is created or an existing file is opened, the file descriptor is used for accessing the file.

File Pointer

A file pointer specifies the location for a subsequent read/write operation in the file in terms of bytes. Each file is considered as a sequence of bytes. Each open file is associated with a file pointer that specifies an offset in bytes, relative to the beginning of the file. When a file is opened, the file pointer is set to zero.

File Handling System Calls

The following table briefly describes the system calls related to file handling −

The steps required for using the system calls are same, as we discussed earlier −

Creating and Opening a File

For creating and opening a file, perform the following tasks −

The system call returns the file descriptor of the created file in the EAX register, in case of error, the error code is in the EAX register.

Opening an Existing File

For opening an existing file, perform the following tasks −

The system call returns the file descriptor of the created file in the EAX register, in case of error, the error code is in the EAX register.

Among the file access modes, most commonly used are: read-only (0), write-only (1), and read-write (2).

Reading from a File

For reading from a file, perform the following tasks −

Put the system call sys_read() number 3, in the EAX register.

Put the file descriptor in the EBX register.

Put the pointer to the input buffer in the ECX register.

Put the buffer size, i.e., the number of bytes to read, in the EDX register.

The system call returns the number of bytes read in the EAX register, in case of error, the error code is in the EAX register.

Writing to a File

For writing to a file, perform the following tasks −

Put the system call sys_write() number 4, in the EAX register.

Put the file descriptor in the EBX register.

Put the pointer to the output buffer in the ECX register.

Put the buffer size, i.e., the number of bytes to write, in the EDX register.

The system call returns the actual number of bytes written in the EAX register, in case of error, the error code is in the EAX register.

Closing a File

For closing a file, perform the following tasks −

The system call returns, in case of error, the error code in the EAX register.

Updating a File

For updating a file, perform the following tasks −

The reference position could be:

The system call returns, in case of error, the error code in the EAX register.

Example

The following program creates and opens a file named myfile.txt, and writes a text ‘Welcome to Tutorials Point’ in this file. Next, the program reads from the file and stores the data into a buffer named info. Lastly, it displays the text as stored in info.

When the above code is compiled and executed, it produces the following result −

The sys_brk() system call is provided by the kernel, to allocate memory without the need of moving it later. This call allocates memory right behind the application image in the memory. This system function allows you to set the highest available address in the data section.

This system call takes one parameter, which is the highest memory address needed to be set. This value is stored in the EBX register.

Example

The following program allocates 16kb of memory using the sys_brk() system call −

When the above code is compiled and executed, it produces the following result −