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Chapter 8. Assembler Directives

8.4.

.ascii "string"

. . .

.ascii

expects zero or more string literals (Section 4.6.1.1 Strings) separated by commas. It assem 

bles each string (with no automatic trailing zero byte) into consecutive addresses.

8.5.

.asciz "string"

. . .

.asciz

is just like

.ascii

, but each string is followed by a zero byte. The "z" in

.asciz

stands for

"zero".

8.6.

.balign[wl] abs expr, abs expr, abs expr

Pad the location counter (in the current subsection) to a particular storage boundary. The first expres 

sion (which must be absolute) is the alignment request in bytes. For example

.balign 8

advances

the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no

change is needed.

The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and

the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on some

systems, if the section is marked as containing code and the fill value is omitted, the space is filled

with no op instructions.

The third expression is also absolute, and is also optional. If it is present, it is the maximum number of

bytes that should be skipped by this alignment directive. If doing the alignment would require skipping

more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill

value (the second argument) entirely by simply using two commas after the required alignment; this

can be useful if you want the alignment to be filled with no op instructions when appropriate.

The

.balignw

and

.balignl

directives are variants of the

.balign

directive. The

.balignw

di 

rective treats the fill pattern as a two byte word value. The

.balignl

directives treats the fill pattern

as a four byte longword value. For example,

.balignw 4,0x368d

will align to a multiple of 4. If it

skips two bytes, they will be filled in with the value 0x368d (the exact placement of the bytes depends

upon the endianness of the processor). If it skips 1 or 3 bytes, the fill value is undefined.

8.7.

.byte expressions

.byte

expects zero or more expressions, separated by commas. Each expression is assembled into

the next byte.

8.8.

.comm symbol, length

.comm

declares a common symbol named

symbol

. When linking, a common symbol in one object

file may be merged with a defined or common symbol of the same name in another object file. If

ld

does not see a definition for the symbol just one or more common symbols then it will allocate

length

bytes of uninitialized memory.

length

must be an absolute expression. If

ld

sees multiple

common symbols with the same name, and they do not all have the same size, it will allocate space

using the largest size.

When using ELF, the

.comm

directive takes an optional third argument. This is the desired alignment

of the symbol, specified as a byte boundary (for example, an alignment of 16 means that the least

significant 4 bits of the address should be zero). The alignment must be an absolute expression, and

it must be a power of two. If

ld

allocates uninitialized memory for the common symbol, it will use

the alignment when placing the symbol. If no alignment is specified,

as

will set the alignment to the

largest power of two less than or equal to the size of the symbol, up to a maximum of 16.