Program Status Word

The program status word (PSW) register is an 8-bit register. It is also referred to as the flag register. Although the PSW register is 8 bits wide, only 6 bits of it are used by the 8051. The two unused bits are user-definable flags. Four of the flags are called conditional flags, meaning that they indicate some conditions that result after an instruction is executed. These four are CY (carry), AC (auxiliary carry), P (parity), and OV (overflow).

CY, (carry flag): This flag is set whenever there is a carry out from the D7 bit. This flag bit is affected after an 8-bit addition or subtraction. It can also be set to 1 or 0 directly

P, (parity flag): The parity flag reflects the number of 1 s in the A (accumulator) register only. If the A register contains an odd number of Is, then P = 1. Therefore, P = 0 if A has an even number of Is.

OV, (overflow flag): This flag is set whenever the result of a signed number operation is too large, causing the high-order bit to overflow into the sign bit. The overflow flag is only used to detect errors in signed arithmetic operations.