Date: Sat, 16 Mar 1996 20:52:49 -0700 (MST) From: Terry Lambert <terry@lambert.org> To: jdp@polstra.com (John Polstra) Cc: nate@sneezy.sri.com, freebsd-hackers@freebsd.org Subject: Re: GAS question Message-ID: <199603170352.UAA18530@phaeton.artisoft.com> In-Reply-To: <199603152356.PAA02917@austin.polstra.com> from "John Polstra" at Mar 15, 96 03:56:56 pm
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> > Most of this is obvious, but these two lines completely baffle me.
> >
> > : "=a" (*eax), "=b" (*ebx), "=c" (*ecx), "=D" (cf)
> > : "0" (*eax), "1" (*ebx), "2" (*ecx)
>
> This is GCC's "extended asm" syntax. The documentation is in the GCC
> info pages. From the top-level node, follow the menu entry "C
> Extensions", then "Extended Asm".
>
> Here's a quick answer for this specific case. The stuff after the
> first colon specifies the output operands of the assembly code.
> The stuff after the second colon specifies the input operands.
> The quoted strings are GCC "constraints," which say more or less
> specifically where each operand will be. In parentheses is the
> corresponding C expression for each operand.
>
> For the output operands, the constraints here refer to specific hardware
> registers:
>
> "a" means %eax
> "b" means %ebx
> "c" means %ecx
> "D" means %edi
>
> The "=" on each output operand is required by gcc; it says that the
> operand will be modified.
>
> For the input operands, the "0", "1", and "2" say that the three inputs
> correspond in location to the first three output operands. In other
> words, these operands are read, modified, and written back to the same
> place.
>
> In the assembly code itself, "%3" refers to third operand. For
> this, the operands are numbered from 0, output operands first, then
> input operands. Here, "%3" refers to the local variable "cf",
> which, according to its constraint "=D", is in the register %edi.
Oh god, this sucks.
A VC++ C variable reference from an inline addembler statement:
void
foo( unsigned long locklist)
{
unsigned long delta;
...
__asm mov eax, delta
__asm mov ebx, locklist
...
}
And Assembly for a C callable function (Win95 blue screen from a VXD,
actually):
IFSMgr_SYSMODAL_Message PROC C PUBLIC USES EBX EDI ESI \
pszMessage:DWORD, \
pszCaption:DWORD, \
dwFlags:DWORD
VxDCall Get_Sys_VM_Handle ; handle in ebx
mov eax, dwFlags
mov ecx, pszMessage
mov edi, pszCaption
VxDCall SHELL_SYSMODAL_Message ; bluescreen with message
ret
IFSMgr_SYSMODAL_Message ENDP
A C function without any preamble/postamble, and block inline assembly:
__declspec(naked) void
_peneter(save_edx)
{
_asm {
mov eax, esp
pushfd
cli
push eax
mov edx, save_edx
...
popfd
ret
}
}
Variable references are plain old variable references. Why is GCC so
complicated?
Terry Lambert
terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.
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