Page 1 of 1
what means this insctruction?
Posted: Sat Jul 10, 2004 2:51 pm
by koni
what means this instruction, as example:
"leal (%eax,-1), %ecx", compiled with gcc
thanx, koni
Re:what means this insctruction?
Posted: Sat Jul 10, 2004 3:06 pm
by Candy
koni wrote:
what means this instruction, as example:
"leal (%eax,-1), %ecx", compiled with gcc
thanx, koni
That's the LEA instruction in GCC writing moving eax with some weird op to ecx. I don't know what the -1 is doing there, but I guess it's similar to [eax-1] in intel syntax, so that'd be moving eax minus one to ecx.
If it's eax*-1 I don't know how they do that, but that'd be moving the inverse of eax to ecx.
Re:what means this insctruction?
Posted: Sat Jul 10, 2004 11:11 pm
by chris
I'm not that great with AT&T but aren't all immediate values supposed to be prefixed with "$"? And I thought (,) was for two registers? I think [eax-1] is $-1(%eax) in AT&T syntax.
Re:what means this insctruction?
Posted: Sun Jul 11, 2004 7:40 am
by Candy
chris wrote:
I think [eax-1] is $-1(%eax) in AT&T syntax.
Thought so too, but encoding -1 is impossible (you can only do 1,2,4,8) so I guessed it was some weird way to say eax-1. Any other ideas?
Re:what means this insctruction?
Posted: Sun Jul 11, 2004 10:00 am
by LordKaT
It might help if we knew what architecture this was on? GCC has been ported to a ton of different processors
But, yeah, it looks like it's moving EAX-1 into ECX.
Re:what means this insctruction?
Posted: Sun Jul 11, 2004 11:57 am
by Schol-R-LEA
As is explained in the
OS Developer's FAQ
(among other places), in AT&T syntax, the operand before the parentheses is a segment displacement, and is added to the final result of the indexes. From the
GAS User Manual:
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 have 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 must specify its size (byte, word, or long) with an opcode suffix (`b', `w', or `l', respectively).
Re:what means this insctruction?
Posted: Mon Jul 12, 2004 12:12 am
by Candy
Schol-R-LEA wrote:
As is explained in the
OS Developer's FAQ
(among other places), in AT&T syntax, the operand before the parentheses is a segment displacement, and is added to the final result of the indexes.
The problem is that we don't understand the (%eax, -1) notation. How can you use a direct index (which is impossible to encode, so I assumed it was an addend or the scale, scale is also impossible to encode, so it came down to the addend for me) of -1?