• Source: Program status word

      • The program status word (PSW) is a register that performs the function of a status register and program counter, and sometimes more. The term is also applied to a copy of the PSW in storage. This article only discusses the PSW in the IBM System/360 and its successors, and follows the IBM convention of numbering bits starting with 0 as the leftmost (most significant) bit.
      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).


      Format




      = S/360

      =
      On all but 360/20, the PSW has the following formats. S/360 Extended PSW format only applies to the 360/67 with bit 8 of control register 6 set.


      = S/370

      =


      = S/370 Extended Architecture (S/370-XA)

      =


      = Enterprise Systems Architecture (ESA)

      =


      = z/Architecture

      =


      Notes




      References



      S360
      IBM System/360 Principles of Operation (PDF) (Eighth ed.). IBM. September 1968. A22-6821-7.
      func67
      IBM System/360 Model 67 Functional Characteristics (PDF) (Third ed.). IBM. February 1972. GA27-2719-2.
      S370
      IBM System/370 Principles of Operation (PDF) (Eleventh ed.). IBM. September 1987. A22-7000-10.
      S370-XA
      IBM System/370 Extended Architecture Principles of Operation (PDF) (Second ed.). IBM. January 1987. SA22-7085-1.
      S370-ESA
      IBM Enterprise Systems Architecture/370 Principles of Operation (PDF) (First ed.). IBM. August 1988. SA22-7200-0.
      z
      z/Architecture Principles of Operation (PDF) (Fourteenth ed.). IBM. May 2022. SA22-7832-13.

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