IBM System/370-XA

IBM System/370-XA is an instruction set architecture introduced by IBM in 1983 with the IBM 308X processors. It extends the IBM System/370 architecture to support 31-bit virtual and physical addresses, and includes a redesigned I/O architecture.

History of IBM mainframes, 1952–present
Market name
Architecture
System/370-XA
DesignerIBM
Bits32-bit
Introduced1983 (1983)
DesignCISC
TypeRegister–Register
Register–Memory
Memory–Memory
EncodingVariable (2, 4 or 6 bytes long)
BranchingCondition code, indexing, counting
EndiannessBig
PredecessorSystem/370
SuccessorESA/370
Registers
General-purpose16
Floating point4 64-bit
IBM S/370-XA registers
General Registers 0-15

Two's complement value
0 31
Control Registers 0-15

See Principles of Operation: 4-6–4-8 
0 31
Floating Point Registers 0-6

S Biased exponent Mantissa
0 1 7 8 31

Mantissa (continued)
32 63
Extended Architecture Extended Control mode PSW: 4-5–4-6 

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
  • IBM documentation numbers the bits from high order to low order; the most significant (leftmost) bit is designated as bit number 0.
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