What are procedures in assembly language?
Programmers worldwide commonly use functions to make code written in high-level languages, such as C++ and Python, less repetitive, more modular, and readable. Assembly code is usually larger in size and more complex than code written in most high-level languages.
To make assembly code more modular, readable, and shorter in size, we use procedures. Procedures are very similar to functions, as each procedure performs a particular task.
Syntax
To define a procedure, the following syntax is used:
procedure_name:
first_inst
second_inst
...
...
...
last_inst
ret
As shown above, each procedure has a unique name and must end with ret to exit gracefully.
To call a procedure, we use the call instruction. The call instruction takes in one operand, which is the name of the procedure that the programmer wishes to call.
call print
In the code snippet above, print is the name of our procedure.
Example
The following program demonstrates how a procedure may be used to print or output data on the console.
Instead of calling all the instructions required to print a string, we can define the relevant instructions in a procedure. In this way, we could call multiple instructions by simply calling that procedure via the call instruction. As visible, using a procedure to output data onto the console takes much fewer lines of code and has made our code modular.
Similarly, procedures can be used to perform other standard operations, such as taking input from the user or writing to any other FILE stream.
section .text global _start _start: mov ecx, msg0 ;ecx contains the message needs to be printed. msg0 stored in ecx mov edx, len0 ;edx contains the length of the message to be printed. len0 stored in edx call print ;print procedure called mov ecx, msg1 ;same as above for msg1 mov edx, len1 call print mov ecx, msg2 ;same as above for msg2 mov edx, len2 call print mov eax, 1 ;system call number for the exit system call saved in eax int 0x80 ; ;kernel interrupt to shift from user mode to kernel mode to execute the system call print: mov ebx,1 ; ;ebx contains the file descriptor of the file descriptor (1 for stdout) mov eax,4 ; ;eax contains the system call number. 4 is the system call number for the write system call int 0x80 ; ;kernel interrupts to execute the write system call in kernel mode ret ;returns to the address where the print procedure was last called section .data msg0 db "Welcome to Educative!", 0x0A ;defines message to be printed. 0xA is \n in hex. len0 equ $ - msg0 ;stores length of the message msg1 db "We are glad to have you here.", 0x0A ;defines message to be printed. 0xA is \n in hex. len1 equ $ - msg1 ;stores length of the message msg2 db "You are almost an expert at assembly language now!", 0x0A ;defines message to be printed. 0xA is \n in hex. len2 equ $ - msg2 ;stores length of the message segment .bss
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