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AssDruid

implementing SHR in ASM

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how can implement SHR in assembly?( without the SHR instructor, of course :-D) I thought I seperate each bit with AND and then get a new number, but this is way too long...

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;stdcall dword shiftRight (dword number, dword shiftCount)
shiftRight:
push ebp
mov ebp, esp

mov ebx, 2
mov ecx, [ebp + 12]
mov eax, [ebp + 8]

.divide:
xor edx, edx
div ebx
loop .divide

mov esp, ebp
pop ebp
ret 8






[smile]

This must be slow as hell [smile]

Oxyd

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Can’t think why anyone would want to implement a SHR in assembler without actually using the SHR instruction! That’s why it’s there isn’t it?

However, the SHR shifts all the bits in a source operand by the number in the second operand, zeroising the top bits.

Repeated DIV by 2 of the source operand will do it as has been shown already.

Another option, if the number of shifts > 1, is to do a single DIV by 2 and then a SAR by 1 less than the number of desired shifts.

The SAR shifts to the right, but retains the existing value of the top bit. If you know your source operand has a zero top bit, a SAR has the same effect as a SHR.


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