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        riddles >> microsoft >> ASM question
        
(Message started by: vishal_changrani on Feb 15^th, 2006, 12:07pm)

Title: ASM question
Post by vishal_changrani on Feb 15^th, 2006, 12:07pm

Hi,
I am back with one more question.....supposedly asked in an MS interview...

Q. The following asm block performs a common math function, what is it?
CWD XOR AX,DX
SUB AX,DX

My guess earlier was it is exchangin AX and DX but doesnt seemt to be that.
Also is CWD carried out first or XOR? in the first line

any and all replies would be appreciated.
Thank u
Vishal

Title: Re: ASM question
Post by vishal_changrani on Feb 15^th, 2006, 12:35pm

Sorry but i think i found the answer...its just doing a MOD operation :)
:-[ sorry for posting it in the first place
-vishal

Title: Re: ASM question
Post by Grimbal on Feb 16^th, 2006, 4:22am

No, it does an ABS(AX)

In fact, it splits AX into sign and absolute value:

DX = sign(AX)
AX = abs(AX)

Title: Re: ASM question
Post by ruhela_nitk on Jul 28^th, 2006, 7:48am

Hi
If anyone can explain the answer , please let me?

Title: Re: ASM question
Post by Grimbal on Jul 29^th, 2006, 11:48am

CWD converts a word to double. It does so by extending the sign of AX into DX. Practically, it means it does DX=0 if AX is >=0 or DX=-1 (all bits set) if AX<0.

The XOR and SUB do nothing if DX=0.
If DX=-1, the XOR reverses all bits of AX and the SUB adds 1 to AX. Together, it does
AX = (not AX) + 1
which is exactly how you compute AX=-1 in the 2's complement binary representation of integers.

So, the effect is to set DX=0 or -1 depending on AX's sign, and to set AX = abs(AX).