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assemblyx86divisionsign-extension

Assembly Language: cbw

I am unsure of what the cbw command actually does. I have a snippet of code:

mov  ax,0FF0h
cbw
idiv ah

How does the value of ax change after cbw?

Solution

  • The cbw instruction sign-extends a byte into a word. In this case, it'll take the sign bit of AL (which happens to be 1) and copy it into every bit of AH.

    This means that the two's-complement value of AX will be the same, but the binary representation will be different.

    The value of AX after the cbw instruction will be FFF0h (a 16-bit -16 value, just like AL was originally an 8-bit -16)