The X 86 Microprocessors: Architecture, Programming and Interfacing (8086 to Pentium) 2nd Edition by Pearson

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THE

x86 MICROPROCESSORS

The point to remember here is that each group of 4 bits represents a ‘decimal number’, the

base of which is ten. Thus, when we try to subtract a bigger number from a smaller number,

we have to consider the ‘four bits together’ as a decimal number. Let us review the steps in

the above subtraction.

First step

Thus, when we have to subtract 6 from 0 in the rightmost group of four bits, we need to

borrow. Borrow from the group on the left a decimal 10, and add it to the ‘0000’ on the

right. That makes it ‘1010’ (because of borrowing, the 0011 on the left is now ‘0010’).

Then, subtract 0110 from this. The result is 0100 ...