Program Status Word (PWS)

• A special CPU register holding status information and control bits.

• Updated automatically after most instructions.
• Used by CPU to determine conditional operations, branching, and interrupts.

Contents of PSW

1. Status Flags (Condition Codes) – Reflect result of last ALU operation:
   • CF (Carry Flag) → 1 if carry out (unsigned overflow) or borrow occurs.
   • ZF (Zero Flag) → 1 if result is 0.
   • SF (Sign Flag) → Copies MSB of result (1 = negative in signed representation).
   • OF (Overflow Flag) → 1 if signed overflow occurs.
   • PF (Parity Flag) → 1 if result has even parity (even no. of 1s).
   • AC (Auxiliary Carry) → Carry from bit 3 to bit 4 (used in BCD).
2. Control Bits – Control processor operation:
   • Interrupt Enable/Disable → Allows or blocks interrupts.
   • Mode bits → User mode / Supervisor mode.
   • Trap Flag → Enables single-step execution for debugging.

Example: After ADD AL, BL in x86:

• ZF = 1 if AL = 0.
• CF = 1 if unsigned carry occurs.
• OF = 1 if signed overflow occurs.

Uses of PSW

• Conditional branching (JZ, JC, JNE, etc.).
• Exception handling & interrupts.
• Debugging & single-step execution.