FCOM在Turbo汇编器中不起作用

Question

I have a problem with comparison in tasm. I have a code

cycle:
...some code...
fninit
fld limit
fld currentX    
fcom st(1)
jge exit
jmp cycle

But it jumps to exit mark even if conditions aren't met. I check it in turbo debugger and fpu stack values are right. How to fix it?

Answer 1

fcom only sets fpu status bits, not the cpu flags so you can't directly use a conditional jump. There is an fcomi instruction which does, but that's only available when using a P6+ cpu and an assembler that understands such instructions. According to Michael Petch, for tasm you need at least version 5.3. A workaround is to transfer the fpu status bits to cpu flags using fstsw ax and sahf .

Also note that in either case, the relevant fpu status bits C0 , C2 and C3 get transferred to CF , PF and ZF respectively, so you should use a conditional jump that checks these flags. In your case this means jae .

See also section 8.1.4 Branching and Conditional Moves on Condition Codes in the Intel® 64 and IA-32 Architectures Software Developer's Manual Volume 1: Basic Architecture and of course the instruction set reference as usual.