IBM

Hursley Museum

THE IBM System/360 INSTRUCTION SET PRESENTED AS A PERIODIC SYSTEM


The IBM System/360 instruction set belongs to the simplest and clearest processor instruction sets in past and present computers. This is because of three main reasons: (A) there are only four addressing modes; (B) there is no hardware stacking mechanism; (C) there is a strong orthogonal aspect as regards the actual operation (arithmetical, logical) on the one hand and the kinds of operands and addressing modes on the other hand.
A knowledge of the System/360 architecture as defined in the IBM System/360 Principles of Operation is assumed in what follows.

(A) the addressing modes are register-to-register (RR), register-to-storage (RX and RS), instruction-to-storage (SI) and storage-to-storage (SS).
(B) Stacking operations, necessary in the correct treatment of subroutines and interrupt routines within subroutines and interrupt routines, have to be implemented entirely in software.
(C) Machine operation codes take one byte in storage. In many instructions the front half-byte represents the addressing mode, while the rear half-byte represents the specific operation to be performed, for instance, the four Compare Logical operations, with opcodes X’15’,  X’55’,  X’95’ and X’D5′, are in the same column; all full-word RX operations, with X’5′ as their leftmost half-bytes, are in the same row; on the intersection of column and row we find the opcode X’55’, mnemonic CL, Compare logical register and full word in main storage.

It is foremost this orthogonal design, the separation of addressing modes from functions performed, that gives the instruction set the looks of a periodic system, very much like the periodic system of chemical elements .

In the first table below the opcodes are arranged in ascending order row-wise; so the rows are numbered by the front half-byte of the opcode, running from 0− to F− , and the columns by the rear half-byte of the opcode, running from −0  to −F.  The grey cells represent invalid operation codes.
The second table is the first table transposed, so the opcodes are ordered column-wise, in line with the IBM handbooks and reference cards.

The tables are based on the pocket reference card on the System/360 and the supplementary card on the Model 67, affectionately known as the Green Card and Blue Card , and for the Model 44 information on the Model 44 Functional Characteristics handbook .

NOTES on the Model 44: (a) storage protection (controlled by the SSK and ISK instructions) is not standard but is an optional feature; (b) none of the decimal and variable-field instructions (rows D− and F− in the first table below) are available.

tooltips: roll the mouse cursor over the mnemonic to read the name of the instruction in full

COLOUR CODES: Standard: on all System/360 models except Model 44
 Not on Model 44; standard on all other models
 Model 67 only Floating-point feature Extended-floating-point feature Privileged instructions: on all System/360 models Additional privileged instructions for the Model 67 Decimal Arithmetic feature; not available on Model 44

ORDERED ROW-BY-ROW, LIKE THE PERIODIC SYSTEM OF CHEMICAL ELEMENTS

−0 −1 −2 −3 −4 −5 −6 −7 −8 −9 −A −B −C −D −E −F
0− 0−
1− 1−
2− 2−
3− 3−
4− 4−
5− 5−
6− 6−
7− 7−
8− 8−
9− 9−
A− A−
B− B−
C− C−
D− D−
E− E−
F− F−
−0 −1 −2 −3 −4 −5 −6 −7 −8 −9 −A −B −C −D −E −F

ORDERED COLUMN-BY-COLUMN, LIKE ON THE IBM SYSTEM/360 REFERENCE CARD

0− 1− 2− 3− 4− 5− 6− 7− 8− 9− A− B− C− D− E− F−
−0 −0
−1 −1
−2 −2
−3 −3
−4 −4
−5 −5
−6 −6
−7 −7
−8 −8
−9 −9
−A −A
−B −B
−C −C
−D −D
−E −E
−F −F
0− 1− 2− 3− 4− 5− 6− 7− 8− 9− A− B− C− D− E− F−

COLOUR CODES: Standard: on all System/360 models except Model 44
 Not on Model 44; standard on all other models
 Model 67 only Floating-point feature Extended-
floating-point feature		 Privileged instructions: on all System/360 models Additional privileged instructions for the Model 67 Decimal Arithmetic feature; not available on Model 44