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?
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)