Page 58 - ARM 64 Bit Assembly Language

42 Chapter 2

.balign[lw] abs-expr, abs-expr, abs-expr
These directives adjust the location counter to a particular storage boundary. The first
expression is the byte-multiple for the alignment request. For example .balign 16
will insert fill bytes until the location counter is an even multiple of 16. If the location
counter is already a multiple of 16, then no fill bytes will be created. The second expres-
sion gives the fill value to be stored in the fill bytes. It (and the comma) may be omitted.
If it is omitted, then the fill value is assumed to be zero. The third expression is also op-
tional. If it is present, it is the maximum number of bytes that should be skipped by this
alignment directive.
The .balignw and .balignl directives are variants of the .balign directive. The
.balignw directive treats the fill pattern as a two byte word value, and .balignl treats
the fill pattern as a four byte longword value. For example, “.balignw 4,0x368d”
will align to a multiple of four bytes. 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 pro-
cessor).
.skip size, fill
.space size, fill
Sometimes it is desirable to allocate a large area of memory and initialize it all to the
same value. This can be accomplished by using these directives. These directives emit
size bytes, each of value fill.Both size and fill are absolute expressions. If the
comma and fill are omitted, fill is assumed to be zero. For the ARM processor, the
.space and .skip directives are equivalent. This directive is very useful for declaring
large arrays in the .bss section.
Listing 2.5 shows how the code in Listing 2.4 can be improved by adding an alignment direc-
tive at line 6. The directive causes the assembler to emit two zero bytes between the end of
the ch variable and the beginning of the ary variable. These extra “padding” bytes cause the
following word data to be word aligned, thereby improving performance when the word data
is accessed. It is good practice to always put an appropriate alignment directive when the size
of the data on the next line is larger than the size of the data on the current line. For example,
if the previous line declares string, byte, half-word, or word data, and the current line declares
double-word data, then an .align 3 directive should be placed between them. Likewise, if
the current line declares word data and the previous line declares string, byte, or half-word
data, then an .align 2 directive should be placed between them. For a line declaring half-
word data following a line declaring string or byte data, an .align 1 directive should be
placed between them.

2.3.4 Setting and manipulating symbols

The assembler provides support for setting and manipulating symbols which can then be used
in other places within the program. The labels that can be assigned to assembly statements