Subtract Registers and Borrow from Accumulator

The  contents of the  register  and borrow  flag  are subtracted from   the contents of the accumulator and the  result is  stored  in the accumulator. The carry  flag behaves like the borrow flag in subtract operation. The instruction format is

SUB R                   (A - A- (R + Borrow)

Actually  the previous  borrow  is added to the subtrahend fist. Then  2  complement  of the results  is obtained and finally it is added to the accumulator to obtain  the final result of  the operation  similar  to SUB  instructions.