4.6: Text Section

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The code is placed in the "section .text" section. There must be a space after the word 'section'. The instructions are specified one per line and each must be a valid instruction with the appropriate required operands.

The text section will include some headers or labels that define the initial program entry point. For example, assuming a basic program using the standard system linker, the following declarations must be included.

     global _start
     _start:

No special label or directives are required to terminate the program. However, a system service should be used to inform the operating system that the program should be terminated and the resources, such as memory, recovered and re-utilized. Refer to the example program in the following section.

A very simple assembly language program is presented to demonstrate the appropriate program formatting.

; Simple example demonstrating basic program format and layout.

; Ed Jorgensen
; July 18, 2014

; ************************************************************
;  Some basic data declarations

section   .data

; -----
;  Define constants

EXIT_SUCCESS    equ     0     ; successful operation
SYS_exit        equ     60    ; call code for terminate

; -----
;  Byte (8-bit) variable declarations

bVar1        db        17
bVar2        db        9
bResult      db        0

; -----
;  Word (16-bit) variable declarations

wVar1        dw        17000
wVar2        dw        9000
wResult      dw        0

; -----
;  Double-word (32-bit) variable declarations

dVar1           dd        17000000
dVar2           dd        9000000
dResult         dd        0

; -----
;  quadword (64-bit) variable declarations

qVar1        dq      170000000
qVar2        dq      90000000
qResult      dq      0

; ************************************************************
;  Code Section

section     .text
global _start
_start:

;  Performs a series of very basic addition operations
;  to demonstrate basic program format.

; ----------
;  Byte example
;   bResult = bVar1 + bVar2

    mov     al, byte [bVar1]
    add     al, byte [bVar2]
    mov     byte [bResult], al

; ----------
;  Word example
;   wResult = wVar1 + wVar2

    mov     ax, word [wVar1]
    add     ax, word [wVar2]
    mov     word [wResult], ax

; ----------
;  Double-word example
;   dResult = dVar1 + dVar2

    mov     eax, dword [dVar1]
    add     eax, dword [dVar2]
    mov     dword [dResult], eax

; ----------
;  Quadword example
;   qResult = qVar1 + qVar2

    mov     rax, qword [qVar1]
    add     rax, qword [qVar2]
    mov     qword [qResult], rax

; ************************************************************
;  Done, terminate program.

last:
    mov     rax, SYS_exit       ; Call code for exit
    mov     rdi, EXIT_SUCCESS   ; Exit program with success
    syscall

This example program will be referenced and further explained in the following chapters.