carry look ahaed adder

[rajakash]

hi friends,

am using a carry look ahead adder for adding two 8bit numbers....... using tat adder i want to do subtraction ..... how to do tat..... am doing alu as my project............

 

[hunter694]

to do (a - b):

1/ calculate 2's complement of b (call it ~b)
2/ add a and ~b using your adder
3/ if the carry out bit is 1, the output is the answer (ignore the carry out)
__if the carry out bit is 0, take 2's complement of the output to get the answer