as: i386-Memory
9.15.7 Memory References
------------------------
An Intel syntax indirect memory reference of the form
SECTION:[BASE + INDEX*SCALE + DISP]
is translated into the AT&T syntax
SECTION:DISP(BASE, INDEX, SCALE)
where BASE and INDEX are the optional 32-bit base and index registers,
DISP is the optional displacement, and SCALE, taking the values 1, 2, 4,
and 8, multiplies INDEX to calculate the address of the operand. If no
SCALE is specified, SCALE is taken to be 1. SECTION specifies the
optional section register for the memory operand, and may override the
default section register (see a 80386 manual for section register
defaults). Note that section overrides in AT&T syntax _must_ be
preceded by a '%'. If you specify a section override which coincides
with the default section register, 'as' does _not_ output any section
register override prefixes to assemble the given instruction. Thus,
section overrides can be specified to emphasize which section register
is used for a given memory operand.
Here are some examples of Intel and AT&T style memory references:
AT&T: '-4(%ebp)', Intel: '[ebp - 4]'
BASE is '%ebp'; DISP is '-4'. SECTION is missing, and the default
section is used ('%ss' for addressing with '%ebp' as the base
register). INDEX, SCALE are both missing.
AT&T: 'foo(,%eax,4)', Intel: '[foo + eax*4]'
INDEX is '%eax' (scaled by a SCALE 4); DISP is 'foo'. All other
fields are missing. The section register here defaults to '%ds'.
AT&T: 'foo(,1)'; Intel '[foo]'
This uses the value pointed to by 'foo' as a memory operand. Note
that BASE and INDEX are both missing, but there is only _one_ ','.
This is a syntactic exception.
AT&T: '%gs:foo'; Intel 'gs:foo'
This selects the contents of the variable 'foo' with section
register SECTION being '%gs'.
Absolute (as opposed to PC relative) call and jump operands must be
prefixed with '*'. If no '*' is specified, 'as' always chooses PC
relative addressing for jump/call labels.
Any instruction that has a memory operand, but no register operand,
_must_ specify its size (byte, word, long, or quadruple) with an
instruction mnemonic suffix ('b', 'w', 'l' or 'q', respectively).
The x86-64 architecture adds an RIP (instruction pointer relative)
addressing. This addressing mode is specified by using 'rip' as a base
register. Only constant offsets are valid. For example:
AT&T: '1234(%rip)', Intel: '[rip + 1234]'
Points to the address 1234 bytes past the end of the current
instruction.
AT&T: 'symbol(%rip)', Intel: '[rip + symbol]'
Points to the 'symbol' in RIP relative way, this is shorter than
the default absolute addressing.
Other addressing modes remain unchanged in x86-64 architecture,
except registers used are 64-bit instead of 32-bit.
Info Catalog
as: i386-Prefixes
as: i386-Dependent
as: i386-Jumps