Program status word

The Program status word^[1][2] (PSW) is an IBM System/360 architecture and successors control register which performs the function of a Status register and Program counter in other architectures, and more.

Although certain fields within the PSW may be tested or set by using non-privileged instructions, testing or setting the remaining fields may only be accomplished by using privileged instructions.

Contained within the PSW are the two bit condition code, representing zero, positive, negative, overflow, and similar flags of other architectures' status registers. Conditional branch instructions test this encoded as a four bit value, with each bit representing a test of one of the four condition code values, 2³ + 2² + 2¹ + 2⁰. (Since IBM uses big-endian bit numbering, mask value 8 selects code 0, mask value 4 selects code 1, mask value 2 selects code 2, and mask value 1 selects code 3.)

The 64-bit PSW describes (among other things)

• Interrupt masks
• Privilege states
• Condition code
• Instruction address

In the early instances of the architecture (System/360 and early System/370), the instruction address was 24 bits; in later instances (XA/370), the instruction address was 31 bits plus a mode bit (24 bit addressing mode if zero; 31 bit addressing mode if one) for a total of 32 bits.

In the present instances of the architecture (z/Architecture), the instruction address is 64 bits and the PSW itself is 128 bits.

The PSW may be loaded by the LOAD PSW instruction (LPSW or LPSWE). Its contents may be examined with the Extract PSW instruction (EPSW).

References

1. ↑ IBM System/360 Principles of Operation (PDF). EIGHTH EDITION. IBM. September 1968. A22-6821-7. 
2. ↑ z/Architecture Principles of Operation (PDF). Tenth Edition. IBM. September 2012. SA22-7832-09.