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. + IBM S/360 PSW formats+ S/360 Standard PSW System Mask Key A M W P Interruption Code 0 7 8 11 12 13 14 15 16 31 ILC CC ProgramMask Instruction Address 32 33 34 35 36 39 40 63+ Bits Field Meaning 0-7 SM Bit Meaning 0 Channel 0 mask 1 Channel 1 mask 2 Channel 2 mask 3 Channel 3 mask 4 Channel 4 mask 5 Channel 5 mask 6 Channel 6 mask 7 External Mask 8-11 Key PSW key 12 A ASCII 13 M Machine-check mask 14 W Wait state 15 P Problem state 16-31 IC Interruption Code 32-33 ILC Instruction-Length Code 34-35 CC Condition Code 36-39 PM Bit Meaning 36 Fixed-point overflow 37 Decimal overflow 38 Exponent underflow 39 Significance 40-63 IA Instruction Address+ S/360 Extended PSW spare 24/32BitMode TranCtrl I/OMask Ext.Mask Key A M W P ILC CC ProgramMask spare 0 3 4 5 6 7 8 11 12 13 14 15 16 17 18 19 20 23 24 31 Instruction Address 32 63+ Bits Field Meaning 0-3 Spare (must be 0) 4 24/32-bit Address mode 5 Translation Control 6 IO I/O Mask (Summary) 7 EX External Mask (Summary) 8-11 Key Protection Key 12 A ASCII 13 M Machine-check mask 14 W Wait state 15 P Problem state 16-17 ILC Instruction-Length Code 18-19 CC Condition Code 20-23 PM Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 24-31 Spare 32-63 IA Instruction Address

S/370 + IBM S/370 PSW formats+ S/370 Basic Control mode PSW Chan.Mask IO EX Key 0 M W P Interruption Code 0 1 2 4 5 6 7 8 11 12 13 14 15 16 31 ILC CC ProgramMask Instruction Address 32 33 34 35 36 39 40 63+ Bits Field Meaning 0-5 Channel Masks for channels 0-5 6 IO I/O Mask for channels > 5 7 EX External Mask 8-11 Key PSW key 12 E=0 Basic Control mode 13 M Machine-check mask 14 W Wait state 15 P Problem state 16-31 IC Interruption Code 32-33 ILC Instruction-Length Code 34-35 CC Condition Code 36-39 PM Bit Meaning 36 Fixed-point overflow 37 Decimal overflow 38 Exponent underflow 39 Significance 40-63 IA Instruction Address+ S/370 Extended Control mode PSW 0 R 0 0 0 T IO EX Key 1 M W P S 0 CC ProgramMask 0 0 0 0 0 0 0 0 0 1 2 4 5 6 7 8 11 12 13 14 15 16 17 18 19 20 23 24 31 0 0 0 0 0 0 0 0 Instruction Address 32 39 40 63+ Bits Field Meaning 1 R PER Mask 5 T DAT mode 6 IO I/O Mask; subject to channel mask in CR2 7 EX External Mask; subject to external subclass mask in CR0 8-11 Key PSW key 12 E=1 Extended Control mode 13 M Machine-check mask 14 W Wait state 15 P Problem state 16 S Address-Space Control0=primary-space mode1=Secondary-space mode 18-19 CC Condition Code 20-23 PM Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 40-63 IA Instruction Address

S/370 Extended Architecture (S/370-XA) + IBM Extended Architecture (XA) PSW format+ Extended Architecture Extended Control mode PSW 0 R 0 0 0 T IO EX Key 1 M W P S 0 CC ProgramMask 0 0 0 0 0 0 0 0 0 1 2 4 5 6 7 8 11 12 13 14 15 16 17 18 19 20 23 24 31 A Instruction Address 32 33 63+ Bits Field Meaning 1 R PER Mask 5 T DAT mode 6 IO I/O Mask; subject to channel mask in CR2 7 EX External Mask; subject to external subclass mask in CR0 8-11 Key PSW key 12 E=1 Extended Control mode 13 M Machine-check mask 14 W Wait state 15 P Problem state 16 S Address-Space Control0=primary-space mode1=Secondary-space mode 18-19 CC Condition Code 20-23 PM Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 32 A Addressing mode0=24 bit; 1=31 bit 33-63 IA Instruction Address

Enterprise Systems Architecture (ESA) + IBM Enterprise Systems Architecture (ESA) PSW format+ Enterprise Systems Architecture Extended Control mode PSW 0 R 0 0 0 T IO EX Key 1 M W P AS CC ProgramMask 0 0 0 0 0 0 0 0 0 1 2 4 5 6 7 8 11 12 13 14 15 16 17 18 19 20 23 24 31 A Instruction Address 32 33 63+ Bits Field Meaning 1 R PER Mask 5 T DAT mode 6 IO I/O Mask; subject to channel mask in CR2 7 EX External Mask; subject to external subclass mask in CR0 8-11 Key PSW key 12 E=1 Extended Control mode 13 M Machine-check mask 14 W Wait state 15 P Problem state 16-17 AS Address-Space Control00=primary-space mode01=Access-register mode10=Secondary-space mode11=Home-space mode 18-19 CC Condition Code 20-23 PM Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 32 A Addressing mode0=24 bit; 1=31 bit 33-63 IA Instruction Address

z/Architecture + IBM z/Architecture PSW formats+ z/Architecture long PSW 0 R 0 0 0 T IO EX Key 0 M W P AS CC ProgramMask RI 0 0 0 0 0 0 EA 0 1 2 4 5 6 7 8 11 12 13 14 15 16 17 18 19 20 23 24 30 31 BA 0 32 33 63 Instruction Address 64 95 Instruction Address (Continued) 96 127+ Bits Field Meaning 1 R PER Mask 5 T DAT mode 6 IO I/O mask 7 EX External Mask 8-11 Key PSW key 12 E=0 Must be zero for LPSWE 13 M Machine-check mask 14 W Wait state 15 P Problem state 16-17 AS Address-Space Control00=primary-space mode01=Access-register mode10=Secondary-space mode11=Home-space mode 18-19 CC Condition Code 20-23 PM Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 HFP Exponent underflow 23 HFP Significance 24 RI Reserved for IBM 31 EA Extended Addressing mode0=defined by BA below; 1=64-bit, BA must be zero 32 BA Basic Addressing mode0=24 or 64; 1=31 64-127 IA Instruction Address+ z/Architecture short PSW 0 R 0 0 0 T IO EX Key 1 M W P AS CC ProgramMask RI 0 0 0 0 0 0 EA 0 1 2 4 5 6 7 8 11 12 13 14 15 16 17 18 19 20 23 24 25 30 31 BA Instruction Address 32 33 63+ Bits Field Meaning 1 R PER Mask 5 T DAT mode 6 IO I/O mask 7 EX External Mask 8-11 Key PSW key 12 E=1 Must be one for LPSW 13 M Machine-check mask 14 W Wait state 15 P Problem state 16-17 AS Address-Space Control00=primary-space mode01=Access-register mode10=Secondary-space mode11=Home-space mode 18-19 CC Condition Code 20-23 PM Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 HFP Exponent underflow 23 HFP Significance 24 RI Reserved for IBM 31 EA Extended Addressing mode0=defined by BA below; 1=64-bit, BA must be zero 32 BA Basic Addressing mode0=24 or 64; 1=31 33-63 IA Instruction Address

Notes

References

S360

func67

S370

S370-XA

S370-ESA

z