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.