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Help with assembly instruction (IMUL)

 
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SwaggaJackin'
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PostPosted: Sun Feb 26, 2012 9:23 am    Post subject: Help with assembly instruction (IMUL) Reply with quote
Having some difficulty interpreting this instruction:

Code:

IMUL ecx


It's my understanding that if it multiplies a 32-bit value like this, it stores the product of EAX * ECX in both EAX and EDX. However, when this operation is performed its storing the 32-bit product in EAX and storing some 16-bit value inside of EDX. I don't understand how EDX is receiving this value and would like appreciate some help.

Register values before IMUL Ecx is executed:



Register values after execution:



Thanks.
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Dark Byte
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PostPosted: Sun Feb 26, 2012 9:34 am    Post subject: This post has 1 review(s) Reply with quote
ECX=CC71
EAX=51EB851F



51EB851F*CC71=416BD70A76AF

Now since eax can only contain 32-bit values the extra bits are placed in edx

Code:

EDX  : EAX
416B : D70A76AF

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SwaggaJackin'
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PostPosted: Sun Feb 26, 2012 9:35 am    Post subject: Reply with quote
I see, thanks.
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