Is there some rules to use registers?

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LiuJunfeng

Joined: 28 Nov 2003
Posts: 48
Location: China

LiuJunfeng 09 Apr 2004, 13:22

Which registers can be used freely and which must be saved before use?

09 Apr 2004, 13:22

comrade

Joined: 16 Jun 2003
Posts: 1140
Location: Russian Federation

comrade 09 Apr 2004, 14:05

ebx esi edi ebp esp must be saved, rest u can use

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09 Apr 2004, 14:05

f0dder

Joined: 19 Feb 2004
Posts: 3175
Location: Denmark

f0dder 09 Apr 2004, 21:39

this also goes for calls to external routines (like the win32 api) - you should always assume eax,ecx,edx are trashed, even if they aren't on your windows version.

Don't modify segment registers.

If you std, always cld before returning from a callback.

Always keep stack aligned to a 4-byte boundary, the same goes for various structures and such (not only is this better speedwise, you will get problems on NT with some APIs and some structures if they aren't 4byte aligned).

Can't think of any more things right now, but I'm sure there's a few more that will pop up.

09 Apr 2004, 21:39

LiuJunfeng

Joined: 28 Nov 2003
Posts: 48
Location: China

LiuJunfeng 10 Apr 2004, 07:50

Thank you a lot! That's a good start for me.

10 Apr 2004, 07:50

MazeGen

Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia

MazeGen 11 Apr 2004, 07:07

A few things more from Agner Fog's (c) optimizations manual (pdf):

Quote:

4.5 Register usage in 32 bit Windows
[...] The interrupt flag cannot be cleared. The floating point
register stack is empty at the entry of a procedure and must be empty at return, except
for ST(0) if it is used for return value. MMX registers may be changed by the procedure
and if so cleared by EMMS before returning and before calling any other procedure that may
use floating-point registers. All XMM registers may be modified by procedures. Rules for
passing parameters and return values in XMM registers are described in Intel's application
note AP 589. A procedure can rely on EBX, ESI, EDI, EBP and all segment registers being
unchanged across a call to another procedure.

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11 Apr 2004, 07:07

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