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asmdemon



Joined: 18 Jan 2004
Posts: 97
Location: Virginia Beach, VA
asmdemon
i have this code that works:
Code:
fp_real dt ? ; 80-bit floating point number
    

but when i want to assign it with a value:
Code:
fp_real dt 0 ; 80-bit floating point number
    

i get an error. i can compile tword pointer instructions but if i want to define data i have to use dq and dw like this:
Code:
fp_real dw 0
fp_real_2 dq 0
    

Is this a but or builtin?

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Post 01 Mar 2005, 18:28
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7724
Location: Kraków, Poland
Tomasz Grysztar
It accepts floating point values only, so it should be:
Code:
fp_real dt 0.0    

or
Code:
fp_real dt 0f    
Post 01 Mar 2005, 18:41
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asmdemon



Joined: 18 Jan 2004
Posts: 97
Location: Virginia Beach, VA
asmdemon
ah, sorry bout that and thanx for the fast reply
Post 01 Mar 2005, 23:30
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r22



Joined: 27 Dec 2004
Posts: 805
r22
Speaking of floating point (new-bie question). How do you assign and output an 80bit value like that.

mov [fp_val], st0
but how would you pass it to say a wsprintf function or the console?
Post 02 Mar 2005, 00:23
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MCD



Joined: 21 Aug 2004
Posts: 604
Location: Germany
MCD
you can't use usual integer "mov" instruction to copy a whole 80bit FPU register. If you want to store a FPU register into memory, you must use FPU instructions:
Code:
fst [fp_val]    
or
Code:
 fstp [fp_val]    
depending on whether you want to keep the FPU stack top itself or discard it.

An optimization tip: if you want to copy FPU values from memory to memory, you should better avoid FPU instruction unless you need to do some floating point calculations. For only copying them, use integer (or MMX/SSE...) instructions.
Code:
;This examples copies an 80bit value from memory in [esi] to [edi]
mov eax,[esi]
mov edx,[esi+4]
mov cx,[esi+8]
mov [edi],eax
mov [edi+4],edx
mov [edi+8],cx
    

You can naturally loop this instruction cluster for copying arrays. But I recommend you to align/use your 80bit fp values as 12byte (or even 16byte) values, thus you may replace the "mov cx,[esi+8]" by "mov ecx,[esi+8]" which speed/size optimizes this a bit. When you need it super fast, try considering MMX/SSE with dqword alignment, but anyway, supper fast fp_copying should try to avoid 80bit values, since they are a bit difficult to handle outside the FPU stack.
Post 02 Mar 2005, 09:05
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