Procedures are a small part of a larger project such that assembly code is usually larger in size. In this, to understand and manage the code properly, the entire code is divided according to different tasks. Only a group of small instructions made according to these different tasks are called Procedures.
What are procedures in assembly language?
To make assembly code more modular, readable and smaller in size, we use procedures. Processes are very similar to tasks, as each process performs a specific task.
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
proc_name:
procedure body
...
ret The procedure is called from another function using the CALL instruction. The CALL instruction must have the name of the called procedure as an argument as shown below -
CALL proc_name The called process returns control to the calling process 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 −
section .text
global _start ;must be declared for using gcc
_start: ;tell linker entry point
mov ecx,'4'
sub ecx, '0'
mov edx, '5'
sub edx, '0'
call sum ;call sum procedure
mov [res], eax
mov ecx, msg
mov edx, len
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
mov ecx, res
mov edx, 1
mov ebx, 1 ;file descriptor (stdout)
mov eax, 4 ;system call number (sys_write)
int 0x80 ;call kernel
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
sum:
mov eax, ecx
add eax, edx
add eax, '0'
ret
section .data
msg db "The sum is:", 0xA,0xD
len equ $- msg
segment .bss
res resb 1 When the above code is compiled and executed, it produces the following result −
The sum is: 9 Stacks Data Structure - Example
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 −
PUSH operand
POP address/registerThe 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 −
; Save the AX and BX registers in the stack
PUSH AX
PUSH BX
; Use the registers for other purpose
MOV AX, VALUE1
MOV BX, VALUE2
...
MOV VALUE1, AX
MOV VALUE2, BX
; Restore the original values
POP BX
POP AX Example
The following program displays the entire ASCII character set. The main program calls a procedure named display, which displays the ASCII character set.
section .text
global _start ;must be declared for using gcc
_start: ;tell linker entry point
call display
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
display:
mov ecx, 256
next:
push ecx
mov eax, 4
mov ebx, 1
mov ecx, achar
mov edx, 1
int 80h
pop ecx
mov dx, [achar]
cmp byte [achar], 0dh
inc byte [achar]
loop next
ret
section .data
achar db '0' When the above code is compiled and executed, it produces the following result
0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}
...
...
