Definition
Program Status Word
The Program Status Word (PSW) is a hardware register encapsulating the processor’s current state. It contains status information defining the execution context and CPU operating mode.
Essential part of the process’s context, saved in the PCB during a process switch to ensure correct resumption.
Components
Condition Codes (Flags)
Bits set by arithmetic/logic instructions: Zero, Negative, oVerflow, Carry. Used for conditional branching.
Interrupt Mask
Controls whether certain interrupts are enabled or disabled (masked).
Execution Mode
Bit(s) indicating User Mode or Kernel Mode. See Execution Mode.
Stack Pointer
Sometimes part of PSW. Points to current execution stack. See Stack Pointer.
OS Control
Hardware-Assisted Transition
When a Supervisor Call, Trap, or Interrupt occurs, hardware automatically saves the current PSW and loads a new one (pointing to kernel entry). This safely transitions from user space to protected kernel environment.