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Index > Compiler Internals > FASM accept negative immediates in ENTER and INT

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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7756
Location: Kraków, Poland
Tomasz Grysztar
bitRAKE wrote:
If the processor operates with an unsigned number then negative numbers are incorrect. Since FASM correctly errors in some cases and has the precision to detect a number out of bounds, I assume the implementation is just incomplete rather than having this behavior as a feature. Hiding coding errors is never desirable.

That's right - there isn't much to discuss here.
Fix will be coming soon.
Post 26 Jan 2010, 10:34
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17476
Location: In your JS exploiting you and your system
revolution
Code:
bt ax,-1
bt eax,-1    
Should fasm allow negative values for register operands?
Post 26 Feb 2010, 10:08
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
It is documented the same way for btc, btr, bts, shr, shl, shld and shrd, it takes the modulo bits_of_dest of the immediate.

As for legitimate uses of this could be "counting from the left" immediate, so if you always want to take the bit just before the sign bit, you use -2 and then you don't care of the dest register your macro will receive because the immediate will always pick the correct.

For the others you can think of them as "keep the last/first n bits" again with independence of the destination size.

I've just woke up so I hope I didn't say complete nonsense Laughing
Post 26 Feb 2010, 17:19
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17476
Location: In your JS exploiting you and your system
revolution
Code:
 pshufd  xmm0,xmm1,-1    
Question
Post 26 Feb 2010, 22:45
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
Since imm8 there is working as some sort of bit set, I think it is right to accept negative values.

The only problem I see however, is that since fasm can't distinguish between 64-bit negatives and real negatives, there is some kind of odd thing in that fasm accepts all ones or all zeros but not any other combination in the upper 48-bits. I think that the imm8 in pshufd xmm0, xmm1, $FFFF'FFFF'FFFF'FFxx may have to be considered out of range because the Fs are not acting as selectors of any muxer. On the other hand, in $0000'0000'0000'00xx the 0s are not handling any muxer neither but it is a correct representation of the $xx number and for that reason the all Fs version may be a correct representation of a negative imm8 as well...
Post 26 Feb 2010, 23:48
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17476
Location: In your JS exploiting you and your system
revolution
LocoDelAssembly wrote:
... upper 48-bits.
I presume you mean 56-bits?

For the BT case I think that negatives are okay. But for the PSHUFD (and similar) there is no notion of the imm8 having a sign. Indeed is it not even an integer value, it is a bitmap selection. To use negative numbers is a strange way to define a bitmap constant.
Post 26 Feb 2010, 23:59
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