Page 59 - ARM 64 Bit Assembly Language

GNU assembly syntax 43

Listing 2.5 A listing with properly aligned data.

ARM GAS variable2.S page 1
line addr value code
1 .data
2 0000 00000000 i: .word 0
3 0004 01000000 j: .word 1
4 0008 48656C6C fmt: .asciz "Hello\n"
4 6F0A00
5 000f 414200 ch: .byte ’A’,’B’,0
6 0012 0000 .align 2
7 0014 00000000 ary: .word 0,1,2,3,4
7 01000000
7 02000000
7 03000000
7 04000000

and directives are one type of symbol. The programmer can also declare other symbols and
use them throughout the program. Such symbols may not have an actual storage location in
memory, but they are included in the assembler’s symbol table, and can be used anywhere
that their value is required. The most common use for defined symbols is to allow numeri-
cal constants to be declared in one place and easily changed. The .equ directive allows the
programmer to use a label instead of a number throughout the program. This contributes to
readability, and has the benefit that the constant value can then be easily changed every place
that it is used, just by changing the definition of the symbol. The most important directives
related to symbols are:

.equ symbol, expression
.set symbol, expression
This directive sets the value of symbol to expression. It is similar to the C language
#define directive.
.equiv symbol, expression
The .equiv directive is like .equ and .set, except that the assembler will signal an
error if the symbol is already defined.
.global symbol
.globl symbol
This directive makes the symbol visible to the linker. If symbol is defined within a file,
and this directive is used to make it global, then it will be available to any file that is