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h3x1c
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 Posted: Mon Jul 20, 2015 2:47 pm Post subject: Quick question about fild |
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I've been studying the FPU lately, but I can't quite pin down what's going on to cause my game to crash when trying to modify a particular fild instruction.
Take this bit of code (assume the referenced memory addresses are in a currently-running instance of the game):
| Code: | fild dword ptr [007E562C] //1 is what resides in 007E562C
fadd qword ptr [007E5540]
fstp qword ptr [007E5540] |
How am I using fild incorrectly by replacing fild dword ptr [007E562C] with my own code injection consisting of the following (using the same value as what already resides in [007E562C], just to try to keep everything the same value-wise for testing purposes):
| Code: | push ebx
mov ebx,1
fild dword ptr [ebx]
pop ebx |
If I keep the original fild instruction intact but simply add a mov [007E562C],1 before it, everything still works wonderfully, like so:
| Code: | mov [007E562C],1 //I can place up to FF here and it works great
fild dword ptr [007E562C]
fadd qword ptr [007E5540]
fstp qword ptr [007E5540] |
Thanks for any help! I know it's something stupidly simple, but I'm not finding it at the moment... 
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Dark Byte
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| Posted: Mon Jul 20, 2015 3:44 pm Post subject: |
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fild dword [address] will load a 4 byte signed integer from the address, and convert it internally to a floating point value (ext-double)
| Code: |
push ebx
mov ebx,1
fild dword ptr [ebx]
pop ebx
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this code can be rewritten as:
Which is very doubtful that's what you wish to do (address 1 isn't often readable)
Your second solution is the best solution (and you should be able to go to 7fffffff without issue)
alternatively, you can use an self allocated address with the value
| Code: |
alloc(myvalue,4)
myvalue:
dd (int)1000
...
fild dword ptr [myvalue]
...
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h3x1c
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| Posted: Mon Jul 20, 2015 3:55 pm Post subject: |
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OHHHHHH, so the issue is basically that I'm trying to load 1 from a CPU register instead of 1 from an address in RAM?
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Dark Byte
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| Posted: Mon Jul 20, 2015 3:56 pm Post subject: |
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Yes. FILD only takes addresses as parameter
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h3x1c
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| Posted: Mon Jul 20, 2015 3:57 pm Post subject: |
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Excellent stuff. That's all I needed for it to click. Thanks for replying, Dark Byte! 
EDIT: Good grief, it was right under my damn nose the whole time, lmao. From the very top of here:
| Quote: | | "Integer data cannot be transferred directly between the CPU and FPU registers." |
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Zanzer
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| Posted: Mon Jul 20, 2015 4:07 pm Post subject: |
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| Just to clarify, your code was trying to read data from the address [00000001].
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h3x1c
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| Posted: Mon Jul 20, 2015 4:20 pm Post subject: |
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| Zanzer wrote: | | Just to clarify, your code was trying to read data from the address [00000001]. |
Yep, got it. Thanks for the additional feedback!
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