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Strange looking assembly code

Atm97fin · 2003-11-30T19:40:35

Should this code work? .data FixedScale dd 65536.0 pFixednumber dd ? .code ;... ;stuff ;... ;strange code: st(0) fmul [FixedScale] ; some number fistp [pFixednumber] Now the idea of this code is to translate some number to fixedpoint. The strange thing is that fmul direction, which(for what I know) should multiply the st(0) with the number adressed by Fixedscale. But here the fixedscale is not an address, but a dword variable.

General and Gameplay Programming Programming

Started by Atm97fin November 29, 2003 05:45 AM

14 comments, last by Atm97fin 20 years, 4 months ago

Jolle

178

November 30, 2003 10:53 AM

AP is right if we are talking about a 6809 processor and the like, but I don''t think that''s the case... I pretty sure I''m right if we are talking about normal x86, using for example NASM.

Atm97fin

136

Author

November 30, 2003 02:45 PM

Then what''s the difference between fmul and fistp instructions?

EasyGL - easy to use graphics library.

Jolle

178

November 30, 2003 03:23 PM

fmul multiplies st0 with what you specify (here [FixedScale], i.e. 65536.0)
fistp stores st0 as an integer at the place specified (here at the place pFixedNumber), then pops floating point buffer... or whatever it should be called.

Atm97fin

136

Author

November 30, 2003 04:41 PM

So, what''s the difference between these commands:

.dataNumberOne    dd 65536.0FixedNumber  dd   ?.code;1.     fmul [NumberOne]     fistp [FixedNumber];2.     fmul NumberOne     fistp FixedNumber

EasyGL - easy to use graphics library.

Puzzler183

540

November 30, 2003 05:34 PM

The second one doesn''t work? You really need to not do floating point now, and build a basic understand of assembly first.

Jan Wassenberg

1,000

November 30, 2003 07:40 PM

They are equivalent in MASM (see above). This stupid quirk has led to much confusion *sigh*

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Strange looking assembly code

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