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)

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
BitsFieldMeaning
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 Program Mask Instruction Address 32 33 34 35 36 39 40 63 S/360 Standard PSW abbreviations Bits Field Meaning 0-7 SM System Mask 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 Program Mask Bit Meaning 36 Fixed-point overflow 37 Decimal overflow 38 Exponent underflow 39 Significance 40-63 IA Instruction Address
System MaskKeyAMWPInterruption Code
07811121314151631
ILCCCProgram MaskInstruction Address
3233343536394063
S/360 Standard PSW abbreviations Bits Field Meaning 0-7 SM System Mask 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 Program Mask Bit Meaning 36 Fixed-point overflow 37 Decimal overflow 38 Exponent underflow 39 Significance 40-63 IA Instruction Address
0-7SMSystem Mask 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
BitMeaning
0Channel 0 mask
1Channel 1 mask
2Channel 2 mask
3Channel 3 mask
4Channel 4 mask
5Channel 5 mask
6Channel 6 mask
7External Mask
8-11KeyPSW key
12AASCII
13MMachine-check mask
14WWait state
15PProblem state
16-31ICInterruption Code
32-33ILCInstruction-Length Code
34-35CCCondition Code
36-39PMProgram Mask Bit Meaning 36 Fixed-point overflow 37 Decimal overflow 38 Exponent underflow 39 Significance
BitMeaning
36Fixed-point overflow
37Decimal overflow
38Exponent underflow
39Significance
40-63IAInstruction Address
S/360 Extended PSW spare 24/32 Bit Mode Tran Ctrl I/O Mask Ext. Mask Key A M W P ILC CC Program Mask 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 S/360 Extended PSW abbreviations 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 Program Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 24-31 Spare 32-63 IA Instruction Address
spare24/32 Bit ModeTran CtrlI/O MaskExt. MaskKeyAMWPILCCCProgram Maskspare
034567811121314151617181920232431
Instruction Address
3263
S/360 Extended PSW abbreviations 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 Program Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 24-31 Spare 32-63 IA Instruction Address
BitsFieldMeaning
0-3Spare (must be 0)
424/32-bit Address mode
5Translation Control
6IOI/O Mask (Summary)
7EXExternal Mask (Summary)
8-11KeyProtection Key
12AASCII
13MMachine-check mask
14WWait state
15PProblem state
16-17ILCInstruction-Length Code
18-19CCCondition Code
20-23PMProgram Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance
BitMeaning
20Fixed-point overflow
21Decimal overflow
22Exponent underflow
23Significance
24-31Spare
32-63IAInstruction Address

S/370

IBM S/370 PSW formats
BitsFieldMeaning
S/370 Basic Control mode PSW Chan. Mask I O E X Key 0 M W P Interruption Code 0 1 2 4 5 6 7 8 11 12 13 14 15 16 31 ILC CC Program Mask Instruction Address 32 33 34 35 36 39 40 63 S/370 BC mode PSW abbreviations 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 Program Mask Bit Meaning 36 Fixed-point overflow 37 Decimal overflow 38 Exponent underflow 39 Significance 40-63 IA Instruction Address
Chan. MaskI OE XKey0MWPInterruption Code
0124567811121314151631
ILCCCProgram MaskInstruction Address
3233343536394063
S/370 BC mode PSW abbreviations 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 Program Mask Bit Meaning 36 Fixed-point overflow 37 Decimal overflow 38 Exponent underflow 39 Significance 40-63 IA Instruction Address
0-5Channel Masks for channels 0-5
6IOI/O Mask for channels > 5
7EXExternal Mask
8-11KeyPSW key
12E=0Basic Control mode
13MMachine-check mask
14WWait state
15PProblem state
16-31ICInterruption Code
32-33ILCInstruction-Length Code
34-35CCCondition Code
36-39PMProgram Mask Bit Meaning 36 Fixed-point overflow 37 Decimal overflow 38 Exponent underflow 39 Significance
BitMeaning
36Fixed-point overflow
37Decimal overflow
38Exponent underflow
39Significance
40-63IAInstruction Address
S/370 Extended Control mode PSW 0 R 0 0 0 T I O E X Key 1 M W P S 0 CC Program Mask 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 S/370 EC mode PSW abbreviations 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 Control 0=primary-space mode 1=Secondary-space mode 18-19 CC Condition Code 20-23 PM Program Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 40-63 IA Instruction Address
0R000TI OE XKey1MWPS0CCProgram Mask00000000
0124567811121314151617181920232431
00000000Instruction Address
32394063
S/370 EC mode PSW abbreviations 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 Control 0=primary-space mode 1=Secondary-space mode 18-19 CC Condition Code 20-23 PM Program Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 40-63 IA Instruction Address
BitsFieldMeaning
1RPER Mask
5TDAT mode
6IOI/O Mask; subject to channel mask in CR2
7EXExternal Mask; subject to external subclass mask in CR0
8-11KeyPSW key
12E=1Extended Control mode
13MMachine-check mask
14WWait state
15PProblem state
16SAddress-Space Control 0=primary-space mode 1=Secondary-space mode
18-19CCCondition Code
20-23PMProgram Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance
BitMeaning
20Fixed-point overflow
21Decimal overflow
22Exponent underflow
23Significance
40-63IAInstruction Address

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

IBM Extended Architecture (XA) PSW format
BitsFieldMeaning
Extended Architecture Extended Control mode PSW 0 R 0 0 0 T I O E X Key 1 M W P S 0 CC Program Mask 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 S/370-XA EC mode PSW abbreviations 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 Control 0=primary-space mode 1=Secondary-space mode 18-19 CC Condition Code 20-23 PM Program Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 32 A Addressing mode 0=24 bit; 1=31 bit 33-63 IA Instruction Address
0R000TI OE XKey1MWPS0CCProgram Mask00000000
0124567811121314151617181920232431
AInstruction Address
323363
S/370-XA EC mode PSW abbreviations 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 Control 0=primary-space mode 1=Secondary-space mode 18-19 CC Condition Code 20-23 PM Program Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 32 A Addressing mode 0=24 bit; 1=31 bit 33-63 IA Instruction Address
1RPER Mask
5TDAT mode
6IOI/O Mask; subject to channel mask in CR2
7EXExternal Mask; subject to external subclass mask in CR0
8-11KeyPSW key
12E=1Extended Control mode
13MMachine-check mask
14WWait state
15PProblem state
16SAddress-Space Control 0=primary-space mode 1=Secondary-space mode
18-19CCCondition Code
20-23PMProgram Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance
BitMeaning
20Fixed-point overflow
21Decimal overflow
22Exponent underflow
23Significance
32AAddressing mode 0=24 bit; 1=31 bit
33-63IAInstruction Address

Enterprise Systems Architecture (ESA)

IBM Enterprise Systems Architecture (ESA) PSW format
BitsFieldMeaning
Enterprise Systems Architecture Extended Control mode PSW 0 R 0 0 0 T I O E X Key 1 M W P AS CC Program Mask 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 ESA EC mode PSW abbreviations 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 Control 00=primary-space mode 01=Access-register mode 10=Secondary-space mode 11=Home-space mode 18-19 CC Condition Code 20-23 PM Program Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 32 A Addressing mode 0=24 bit; 1=31 bit 33-63 IA Instruction Address
0R000TI OE XKey1MWPASCCProgram Mask00000000
0124567811121314151617181920232431
AInstruction Address
323363
ESA EC mode PSW abbreviations 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 Control 00=primary-space mode 01=Access-register mode 10=Secondary-space mode 11=Home-space mode 18-19 CC Condition Code 20-23 PM Program Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 32 A Addressing mode 0=24 bit; 1=31 bit 33-63 IA Instruction Address
1RPER Mask
5TDAT mode
6IOI/O Mask; subject to channel mask in CR2
7EXExternal Mask; subject to external subclass mask in CR0
8-11KeyPSW key
12E=1Extended Control mode
13MMachine-check mask
14WWait state
15PProblem state
16-17ASAddress-Space Control 00=primary-space mode 01=Access-register mode 10=Secondary-space mode 11=Home-space mode
18-19CCCondition Code
20-23PMProgram Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance
BitMeaning
20Fixed-point overflow
21Decimal overflow
22Exponent underflow
23Significance
32AAddressing mode 0=24 bit; 1=31 bit
33-63IAInstruction Address

z/Architecture

IBM z/Architecture PSW formats
BitsFieldMeaning
z/Architecture long PSW 0 R 0 0 0 T I O E X Key 0 M W P AS CC Program Mask R I 0 0 0 0 0 0 E A 0 1 2 4 5 6 7 8 11 12 13 14 15 16 17 18 19 20 23 24 30 31 B A 0 32 33 63 Instruction Address 64 95 Instruction Address (Continued) 96 127 Long PSW abbreviations 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 Control 00=primary-space mode 01=Access-register mode 10=Secondary-space mode 11=Home-space mode 18-19 CC Condition Code 20-23 PM Program Mask 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 mode 0=defined by BA below; 1=64-bit, BA must be zero 32 BA Basic Addressing mode 0=24 or 64; 1=31 64-127 IA Instruction Address
0R000TI OE XKey0MWPASCCProgram MaskR I000000E A
012456781112131415161718192023243031
B A0
323363
Instruction Address
6495
Instruction Address (Continued)
96127
Long PSW abbreviations 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 Control 00=primary-space mode 01=Access-register mode 10=Secondary-space mode 11=Home-space mode 18-19 CC Condition Code 20-23 PM Program Mask 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 mode 0=defined by BA below; 1=64-bit, BA must be zero 32 BA Basic Addressing mode 0=24 or 64; 1=31 64-127 IA Instruction Address
1RPER Mask
5TDAT mode
6IOI/O mask
7EXExternal Mask
8-11KeyPSW key
12E=0Must be zero for LPSWE
13MMachine-check mask
14WWait state
15PProblem state
16-17ASAddress-Space Control 00=primary-space mode 01=Access-register mode 10=Secondary-space mode 11=Home-space mode
18-19CCCondition Code
20-23PMProgram Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 HFP Exponent underflow 23 HFP Significance
BitMeaning
20Fixed-point overflow
21Decimal overflow
22HFP Exponent underflow
23HFP Significance
24RIReserved for IBM
31EAExtended Addressing mode 0=defined by BA below; 1=64-bit, BA must be zero
32BABasic Addressing mode 0=24 or 64; 1=31
64-127IAInstruction Address
z/Architecture short PSW 0 R 0 0 0 T I O E X Key 1 M W P AS CC Program Mask R I 0 0 0 0 0 0 E A 0 1 2 4 5 6 7 8 11 12 13 14 15 16 17 18 19 20 23 24 25 30 31 B A Instruction Address 32 33 63 Short PSW abbreviations 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 Control 00=primary-space mode 01=Access-register mode 10=Secondary-space mode 11=Home-space mode 18-19 CC Condition Code 20-23 PM Program Mask 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 mode 0=defined by BA below; 1=64-bit, BA must be zero 32 BA Basic Addressing mode 0=24 or 64; 1=31 33-63 IA Instruction Address
0R000TI OE XKey1MWPASCCProgram MaskR I000000E A
01245678111213141516171819202324253031
B AInstruction Address
323363
Short PSW abbreviations 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 Control 00=primary-space mode 01=Access-register mode 10=Secondary-space mode 11=Home-space mode 18-19 CC Condition Code 20-23 PM Program Mask 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 mode 0=defined by BA below; 1=64-bit, BA must be zero 32 BA Basic Addressing mode 0=24 or 64; 1=31 33-63 IA Instruction Address
BitsFieldMeaning
1RPER Mask
5TDAT mode
6IOI/O mask
7EXExternal Mask
8-11KeyPSW key
12E=1Must be one for LPSW
13MMachine-check mask
14WWait state
15PProblem state
16-17ASAddress-Space Control 00=primary-space mode 01=Access-register mode 10=Secondary-space mode 11=Home-space mode
18-19CCCondition Code
20-23PMProgram Mask Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 HFP Exponent underflow 23 HFP Significance
BitMeaning
20Fixed-point overflow
21Decimal overflow
22HFP Exponent underflow
23HFP Significance
24RIReserved for IBM
31EAExtended Addressing mode 0=defined by BA below; 1=64-bit, BA must be zero
32BABasic Addressing mode 0=24 or 64; 1=31
33-63IAInstruction Address

Notes

S360

(PDF) (Eighth ed.). IBM. September 1968. A22-6821-7.

func67

(PDF) (Third ed.). IBM. February 1972. GA27-2719-2.

S370

(PDF) (Eleventh ed.). IBM. September 1987. A22-7000-10.

S370-XA

(PDF) (Second ed.). IBM. January 1987. SA22-7085-1.

S370-ESA

(PDF) (First ed.). IBM. August 1988. SA22-7200-0.

z

(PDF) (Fourteenth ed.). IBM. May 2022. SA22-7832-13.