by Jesmin Akther | Jun 4, 2021 | Assembly Language
Assembly language program practice
In the previous article we have share many basic resources and other compile program with Assembly language. Do and keep practice with assembly helps to gather better knowledge on the language.
Example
section .data ;This statement for Data segment
userMsg db 'Please enter a number: ' ; This is for the user to enter a number
lenUserMsg equ $-userMsg ; This is the length of the message
dispMsg db 'You have enter: '
lenDispMsg equ $-dispMsg
section .bss ;This is Uninitialized data
num resb 5
section .text ;This Code Segment
global _start
_start: ;This is for User prompt
mov eax, 4
mov ebx, 1
mov ecx, userMsg
mov edx, lenUserMsg
int 80h ;Read and store the user input
mov eax, 3
mov ebx, 2
mov ecx, num
mov edx, 5 ;5 bytes (numeric, 1 for sign) of that information
int 80h ;Output the message 'The enter number is: '
mov eax, 4
mov ebx, 1
mov ecx, dispMsg
mov edx, lenDispMsg
int 80h ;Output the number enter
mov eax, 4
mov ebx, 1
mov ecx, num
mov edx, 5
int 80h ; Exit code
mov eax, 1
mov ebx, 0
int 80h
OUTPUT:
Please enter a number:
1234
You have enter:1234
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. Generally, in assembly language an instruction needs two operands, the first operand is the destination, where data in a register or memory location and the second operand is the source. Source contains either the data to be delivered that is immediate addressing or the address that means in register / memory of the data. The source data remains unchanged after the operation.
The three basic modes of addressing are:
- Register addressing
- Immediate addressing
- Memory addressing
Register Addressing:
A register which contains the operand. In this addressing mode this is the first concern. In order to depending upon the instruction, the register may be the first operand, the second operand or both.
For example,
MOV DX, TAX_RATE ; This is the Register in first operand
MOV COUNT, CX ; This is the Register in second operand
MOV EAX, EBX ; This is Both the operands are in registers
As handing out data between registers does not comprise memory, it provides fastest processing of data.
Immediate Addressing:
In an immediate operand, it has a constant value or an expression. Therefore, 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.
For example,
BYTE_VALUE DB 150 ; This is A byte value is defined
WORD_VALUE DW 300 ; This statement A word value is defined
ADD BYTE_VALUE, 65 ; This statement An immediate operand 65 is added
MOV AX, 45H ; This statement is Immediate constant 45H is transferred to AX
Direct Memory Addressing:
This is used, when operands are specified in memory addressing mode. In direct access to main memory, usually to the data segment, it is needed. This is a way of addressing where results in slower processing of data. In order to locate the precise location of data in memory, we need the segment start address, which is typically found in the DS register and an offset value. Hence, this offset value is also called effective address. Again, In direct addressing mode, the offset value is definite directly as part of the instruction, 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.
For example,
ADD BYTE_VALUE, DL ; This statement is Adds the register in the memory location
MOV BX, WORD_VALUE ; This statement is Operand from the memory is added to register
Direct-Offset Addressing
This addressing mode uses the arithmetic operators to modify an address. For instance, following definitions define tables of data;
A_BYTE_TABLE DB 14, 15, 22, 45 ; This statement is Tables of bytes
A_WORD_TABLE DW 134, 345, 564, 123 ; This statement is Tables of words
The below operations access data from the tables in the memory into registers:
MOV CL, A_BYTE_TABLE [2] ; This is used to Gets the 3rd element of the BYTE_TABLE
MOV CL, A_BYTE_TABLE + 2 ; This is used, Gets the 3rd element of the BYTE_TABLE
MOV CX, A_WORD_TABLE[3] ; This is used, Gets the 4th element of the WORD_TABLE
MOV CX, A_WORD_TABLE + 3 ; This is used , Gets the 4th element of the WORD_TABLE
Indirect Memory Addressing
This addressing mode operates the computer’s capability of Segment like Offset addressing. Normally, 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 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.
MINE_TABLE TIMES 10 DW 0 ; This is used to Allocates 10 words (2 bytes) each initialized to 0
MOV EBX, [MY_TABLE] ; This is used to Effective Address of MINE_TABLE in EBX
MOV [EBX], 110 ; MINE _TABLE[0] = 110
ADD EBX, 2 ; EBX = EBX +2
MOV [EBX], 123 ; MINE _TABLE[1] = 123
The MOV Instruction
MOV instruction that is used for moving data from one storage space to another. The MOV instruction takes two operands.
The syntax of the MOV instruction is −
The MOV instruction may have one of the following five forms, example statements are given below:
MOV register, register
MOV register, immediate
MOV memory, immediate
MOV register, memory
MOV memory, register
Here, Both of the operands in MOV operation should be of same size so the value of source operand remains unchanged. The MOV instruction reasons ambiguity at times. For example, look at the statements:
MOV EBX, [MINE_TABLE] ; Effective Address of MINE_TABLE in EBX
MOV [EBX], 110 ; MY_TABLE[0] = 110
It is not clear. Here, either you want to move a byte equivalent or you want a word equivalent of the number 110. In such cases, it is wise to use a type specifier. Here is a table shows some of the common type of specifiers:
Type Specifier Bytes addressed
BYTE 1
WORD 2
DWORD 4
QWORD 8
TBYTE 10
Example
There is a program which illustrates some of the concepts discussed in the post. It stores a name ‘ATM SAMSUZZAMAN’ in the data section of the memory, then changes its value to another name ‘Humayon Faridi’ programmatically and displays both the names.
section .text
global _start ;This statement must be declared for linker (ld)
_start: ; This tell linker entry point
;writing the name 'ATM SAMSUZZAMAN'
mov edx,9 ;This is for message length
mov ecx, name ; This is for message to write
mov ebx,1 ; This is for a file descriptor (stdout)
mov eax,4 ; This is system call number (sys_write)
int 0x80 ; This call kernel
mov [name], dword ' Humayon Faridi ' ; Changed the name to Humayon Faridi ;writing the name ‘Humayon Faridi’
mov edx,8 ;This for message length
mov ecx,name ;This is for a message to write
mov ebx,1 ;This is the file descriptor (stdout)
mov eax,4 ;This is the system call number (sys_write)
int 0x80 ;this call kernel
mov eax,1 ;This is system call number (sys_exit)
int 0x80 ;This call kernel
section .data
name db 'ATM SAMSUZZAMAN'
When the above code is compiled and executed, it produces the following result as:
ATM SAMSUZZAMAN Humayon Faridi
by Jesmin Akther | Jun 4, 2021 | Assembly Language
Memory Arrangement or Memory Segments
A segmented memory model splits the system memory into clusters or set of autonomous segments. Each independent segments referenced by pointers located in the segment registers. This is used to contain a specific type of data. One segment is used to hold instruction codes, another segment stores the data elements, and a third segment preserves the program stack. Though there are various memory segments such as
This segment is represented by .data section and the .bss. The .data section is used to declare the memory section, where data elements are stored for the program. A section cannot be extended after the data elements are declared, and it remainders static all over the program.
This segment .bss section is also a static or not change memory section. This section comprises buffers for data to be declared later in the program. This buffer memory is zero-filled.
This is represented by .text section. This defines an area in memory that stores the instruction codes. This is also static or unchangeable or a fixed area.
This segment that is stack contains data values passed to functions and procedures within the program. In order to speed up the processor operations, the processor includes some internal memory storage locations, called registers. Registers store data elements for processing without taking to access the memory. A partial 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, for example
- General registers,
- Control registers,
- Segment registers.
The general registers are further divided into the following categorise:
- Data registers,
- Pointer registers, and
- Index registers.
- Data Registers
There are four 32-bit data registers which are used for arithmetic, logical, and other operations. These 32-bit registers can be used in three ways, such as complete 32-bit data registers: EAX, EBX, ECX, EDX. Here, 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.
Data Registers
Some of these data registers have specific use in arithmetical operations.
AX is the primary accumulator where this is used in input/output and most arithmetic instructions. For instance , 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. This register 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 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.
Pointer Registers
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.
Index Registers
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): This flag is indicating the overflow of a high-order bit (leftmost bit) of data after a signed arithmetic operation.
Direction Flag (DF): This flag is determining 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): This flag is governs 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) ): This flag is 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) ): This flag is 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) ): This flag is 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): This flag is containing 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): This flag is indicating 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): This flag is containing 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.
Segment Registers
Segments are specific areas defined in a program for containing data, code and stack. There are three main segments:
- Code Segment: This flag is containing 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: This flag is 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: This flag is containing 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.
Apart from the DS, CS and SS registers, there are other extra segment registers – ES (extra segment), FS and GS, which provide additional segments for storing data. These are combines the segment address in the segment register with the offset value of the location. Look at the following simple program to understand the use of registers.
Example
The use of registers in assembly programming. This program displays 7 stars on the screen with a message:
section .text
global _start ; This is must be declared for linker (gcc)
_start: ; This tell linker entry point
mov edx,len ; This is a message length
mov ecx,msg ; This is a message to write
mov ebx,1 ; This is a file descriptor (stdout)
mov eax,4 ; This is system call number (sys_write)
int 0x80 ; This is call kernel
mov edx,7 ; This is message length
mov ecx,s2 ; This is message to write
mov ebx,1 ; This is a file descriptor (stdout)
mov eax,4 ; This is system call number (sys_write)
int 0x80 ; This is call kernel
mov eax,1 ; This is system call number (sys_exit)
int 0x80 ; This is call kernel
section .data
msg db 'Displaying 7 stars',0xa ;a message
len equ $ - msg ;length of message
s2 times 7 db '*'
OUTPUT
Displaying 7 stars
*******
by Jesmin Akther | May 31, 2021 | Assembly Language
Environment Setup
Assembly language is dependent upon the instruction set and the architecture of the processor. There are many good assembler programs, for example
- NASM: It is an operating system independent assembler. One of the two widely used Linux assemblers and the other GNU
- The GNU assembler (GAS): The syntax differs significantly in many ways from
- MASM (Microsoft Assembler): MASM syntax is a standard. So, almost always understood by other x86 assemblers TASM, CHASM, A386, etc. The syntax has some significant defects that makes coding error hence many of them are rectified in NASM.
- Borland Turbo Assembler (TASM)
Installing NASM
It could be used on both Linux and Windows, can download from various web sources. All are well documented. While installing Linux, if “Development Tools” is chacked, NASM installed along with the Linux operating system. For checking have NASM installed, take the following steps
- Open a Linux terminal.
- Type where is nasm and press ENTER.
We use a online compiler of assymbly language in this blog. Go to this link – https://www.jdoodle.com/compile-assembler-nasm-online/
Basic of NASM assembler
Character Set: Letters a..z; A..Z; ()
Digits: 0.9
Special Chars: ? _ @ $ . ~
- NASM is case-sensitive with respect to labels and variables
- It is not case-sensitive with respect to keywords, mnemonics, register names, directives, etc.
- Special Characters.
Write a basic assembly program
Generally, an assembly program can be divided into three sections, such as
- The data section,
- The bss section, and
- The text section.
The data Section
The section is used for declaring data or constants which are not modify at runtime. Various constant values, file names, or buffer size, etc. are declare in this section.
The syntax for declaring data section is “section.data”
The bss Section
The section is used for declaring variables. The syntax for declaring bss section is “section.bss”
The text section
This section must be begun with the declaration global _start, which tells the kernel where the program execution begins. The section is used for care the actual code.
The syntax for declaring text section is
section.text
global _start
_start:
Comments
AL comment begins with a semicolon (;). It may contain any printable character including blank. It can appear on a line by itself, like below
; This program displays a message on screen
or, on the same line along with an instruction, like
add eax, ebx ; this statement state as adds ebx to eax
Assembly Language Statements
Assembly language programs consist of three types of statements, for example
- Executable instructions or instructions:
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.
- Assembler directives or pseudo-ops
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
In AL macros are basically a text substitution mechanism.
Syntax of Assembly Language(AL) Statements
Assembly language statements are entered one statement per line. Each statement follows the following format:
[label] mnemonic [operands] [;comment]
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. Here the fields in the square brackets are optional.
Following are some examples of typical assembly language statements :
INC COUNT ; this statement state as Increment the memory variable COUNT
MOV TOTAL, 48 ; this statement state as Transfer the value 48 in the ; memory variable TOTAL
ADD AH, BH ; this statement state as Add the content of the BH register into the AH register
AND MASK1, 128 ; this statement state as Perform AND operation on the variable MASK1 and 128
ADD MARKS, 10 ; this statement state as Add 10 to the variable MARKS
MOV AL, 10 ; this statement state as Transfer the value 10 to the AL register
The Hello World Program in Assembly Language
The following assembly language code displays the string ‘Hello World’ on the screen −
section .text
global _start ;This is must be declared for linker (ld)
_start: ; this statement tells linker entry point
mov edx,len ; this state as message length
mov ecx,msg ; this statement state as message to write
mov ebx,1 ; this statement state as file descriptor (stdout)
mov eax,4 ; this statement state as system call number (sys_write)
int 0x80 ;call kernel
mov eax,1 ; this statement state as system call number (sys_exit)
int 0x80 ;call kernel
section .data
msg db 'Hello, world!', 0xa ;string to be printed
len equ $ - msg ;length of the string
OUTPUT:
Hello, world!
Now Compiling and Linking an AL 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.
- To assemble the program, type nasm -f elf 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.
- To link the object file and create an executable file named hello, type ld -m elf_i386 -s -o hello hello.o
- Execute the program by typing ./hello
- If you have done everything correctly, it will display ‘Hello, world!’ on the screen.
if replace the section keyword with segment by follow code:
segment .text
global _start ;This is must be declared for linker (ld)
_start: ; this statement tells linker entry point
mov edx,len ; this state as message length
mov ecx,msg ; this statement state as message to write
mov ebx,1 ; this statement state as file descriptor (stdout)
mov eax,4 ; this statement state as system call number (sys_write)
int 0x80 ;call kernel
mov eax,1 ; this statement state as system call number (sys_exit)
int 0x80 ;call kernel
segment .data
msg db 'Hello, world!', 0xa ;string to be printed
len equ $ - msg ;length of the string
OUTPUT
Hello, world!
