Re: Problem with "mov" in GNU GAS

On Mar 26, 6:49 am, "Rod Pemberton" <spamt...@xxxxxxxxxx> wrote:
"junkoi" <spamt...@xxxxxxxxxx> wrote in message

news:94d18289-183c-4a52-9770-48fa230d2943@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



Hello everybody,

I am writing a simple ASM code with GNU GAS. I am having some problems
with mov command. Please could anybody tell me why 2 lines (*) and
(**) are not equivalent?? (currently my code works as expected with
(**), but not with (*)

(My program is in 16bit mode)

---
var:
       .long 0xf1234

.code16gcc
movl var, %ecx                                // (*)
movl $0xf1234, %ecx                      // (**)

I don't think it's critical, but I'm assuming the var is in a ".data"
section.

Yes, they are functionally equivalent.  "movl var,ecx" will mov the value at
var's address into ecx, i.e., 0xf1234.  If you wanted to mov the address of
var into ecx [like another responder claimed you were doing...], that'd be
"movl $var,ecx".

Are you linking the .o file?  If not, a zero (0) value placeholder for the
memory address will be in the .o code for "movl var, %ecx".  The placeholder
is not filled in with the address of var until linked using "ld".  Then, the
code should be functionally equivalent.  Is this the problem?  Typically, ld
is used like this, after assembled with AS or GCC:

  ld --oformat binary --Ttext 0x7C00 -o mybootcode.bin mybootcode.o

Although very unlikely, you might need to adjust the address size or
segment:

  addr32 movl var, %ecx
  movl %es:var, %ecx

You can use these for mixed code:

.code16gcc
.code16
.code32
.code64
.data
.text
.att_syntax
.intel_syntax
data16  - operand size override
data32  - operand size override
addr16  - address size override
addr32  - address size override

Let us know...

Rod Pemberton

Thank you, Rod. Your post gave me some hints, and I solved my
problem.

Actually I want to move the value of "var" to %ecx, not its address.
And "var" is in .code section, not .data. So I fixed my code with:

movl %cs:var, %ecx // i.e movl $0xf1234, %ecx

and it works now, as expected.

Thanks a lot, folks!

Best,
J

.

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