Re: clobbered registers
- From: "Rod Pemberton" <spamtrap@xxxxxxxxxx>
- Date: Thu, 27 Apr 2006 20:23:01 -0400
"luke" <spamtrap@xxxxxxxxxx> wrote in message
news:1146140295.876099.121750@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi everybody,
I'm trying to develop a little operating system,
but I can't resolve the following issue.
I have defined a macro which purpose is to set the base address of a
segment:
#define set_base(addr,base) \
__asm__("movw %%dx,%0\n\t" \
"rorl $16,%%edx\n\t" \
"movb %%dl,%1\n\t" \
"movb %%dh,%2" \
::"m" (*((addr)+2)), \
"m" (*((addr)+4)), \
"m" (*((addr)+7)), \
"d" (base) \
)
The problem is that when I use it in the following way:
set_base(ldt[1], code_base);
set_base(ldt[2], data_base);
the gcc compiler doesn't reload the edx register so I get a wrong
result.
The code I get after disassemby is:
6133: 89 f2 mov %esi,%edx
6135: 66 89 93 52 01 00 00 mov %dx,0x152(%ebx)
613c: c1 ca 10 ror $0x10,%edx
613f: 88 93 54 01 00 00 mov %dl,0x154(%ebx)
6145: 88 b3 57 01 00 00 mov %dh,0x157(%ebx)
614b: 66 89 93 5a 01 00 00 mov %dx,0x15a(%ebx)
6152: c1 ca 10 ror $0x10,%edx
6155: 88 93 5c 01 00 00 mov %dl,0x15c(%ebx)
615b: 88 b3 5f 01 00 00 mov %dh,0x15f(%ebx)
as you can notice there is no reload of the register edx between the
two calls.
Some hints I can give you are:
1) I use gcc 3.4.4 with the following options:
-Wall -O -fstrength-reduce -fomit-frame-pointer -finline-functions
2) If I put some dummy code between the two calls all is working
correctly. For example:
set_base(ldt[1], code_base);
printk("dummy\n");
set_base(ldt[2], data_base);
3) I have tryed to add edx in the "clobbered registers list" but the
compiler complains about
"Can't find a register in class DREGS"
For DJGPP gcc 3.4.1,
1) -O, -O2, etc. causes the output assembly to switch from %ebx to %eax
2) every combination of options I tried reloaded %eax (or %ebx if
no -O, -O2, etc.)
Since you didn't post some C code, I used the following C snippet (it is
_incorrect_, ldt should be a an array of structures eight 8-bit bytes in
length with a layout matching either the segment or gate descriptors,
code_base and data_base aren't inited etc..etc..and I don't know how you set
them up...) Your GCC options produced the assembly listing following the C
snippet. In all cases, it appeared that the register is reloaded... One
suggestion: If you know C and are using it in this project, it would be much
easier to just use C without the assembly.
---C snippet---
#define set_base(addr,base) \
__asm__("movw %%dx,%0\n\t" \
"rorl $16,%%edx\n\t" \
"movb %%dl,%1\n\t" \
"movb %%dh,%2" \
::"m" (*((addr)+2)), \
"m" (*((addr)+4)), \
"m" (*((addr)+7)), \
"d" (base) \
)
int main(void)
{
unsigned long *ldt[4];
void *code_base, *data_base;
set_base(ldt[1],code_base);
set_base(ldt[2],data_base);
}
---Asm snippet---
.file "lidt.c"
.section .text
..globl _main
_main:
pushl %ebp
movl %esp, %ebp
subl $24, %esp
andl $-16, %esp
subl $16, %esp
movl -20(%ebp), %eax
/APP
movw %dx,8(%eax)
rorl $16,%edx
movb %dl,16(%eax)
movb %dh,28(%eax)
/NO_APP
movl -16(%ebp), %eax
/APP
movw %dx,8(%eax)
rorl $16,%edx
movb %dl,16(%eax)
movb %dh,28(%eax)
/NO_APP
leave
ret
.ident "GCC: (GNU) 3.4.1"
Sorry,
Rod Pemberton
.
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