360 Assembly/360 Instructions

When the System/360 (which will be abbreviated "360") was introduced in 1964, there were four instruction types, determined by the first two bits of the operation code ("opcode"), which was the first 8 bits of an instruction. These were

• 00, Opcodes 00 to 3F/Decimal 0 to 63; RR instruction, generally two registers
• 01, Opcodes 40 to 7F/Decimal 40 to 127; RX instruction, one register and one address calculated using a base register, an index register, and a displacement of 0 to 4095 bytes
• 10, Opcodes 80 to BF/Decimal 128 to 191; RS or SI instruction
  • RS was generally one register and a storage location address
  • SI was generally one register, one storage location, and an immediate value
• 11, Opcodes C0 to FF/Decimal 192 to 255; SS instruction, two storage addresses

In order to encourage the expansion of mainframes into more and more uses, the capabilities of the processor were enhanced in later machines by adding new types of instructions to add additional functionality at the machine code level. There are several types of instructions which are now available on all models, the 360, System/370 ("370" or "S/370"), ESA/390 ("390"), and z/System ("Z") type mainframes.

• Instructions in the Available column which are marked All are available on all models.
• Otherwise the instruction is available only on the 370, 390 and/or Z machines.

In the table below, the machine type that it is listed for will indicate that it is available on

• all models
• the 370 and above,
• the 390 and above, or
• the z/System only.

Note that while an instruction type is listed available a particular model or models indicated, a specific instruction may be restricted either to

• earlier (360 only, 360 and 370) or
• later (370 only; 390 only; z/System only; 370 and 390; 390 and Z; 370, 390 and Z) models.

Where a "base-displacement" address is used, it is described as B1,D1 where one base-displacement address is used, or either B2,D2 or B2,D2,X2 where two are. If the base-displacement address is not indexed, the X2 field on the second operand is not used. The B or X values refer to a 4-bit register value from 0 to 15, and the D value is an unsigned 12-bit displacement address.

Instructions take a multiple of 2 bytes, and are 2, 4 or 6 bytes in length. The instruction types are:

All instructions must be aligned to an even address. Attempts to create an instruction at an odd address will be flagged as an error by the assembler; an attempt to branch to an odd address will result in an address exception.

There are four classes of instructions.

• 360 instructions are those which were part of the original IBM 360 mainframe instruction set, they are shown on the table below in this color: [       ]. Except for instructions marked "360 only" these instructions work on all models.
• 370 instructions were added with the creation of the IBM 370, they are shown on the table below in this color: [       ] are only available on 370 and higher series machines, and will not work ("throw an exception") if they are attempted to be executed on a 360. Instructions marked "370 Only" will also throw an exception on the 390 or Z. Instructions marked 370 and 390 Only will throw an exception on the z/System.
• ESA/390 instructions were created beginning with the 390, they are shown on the table below in this color: [       ] are only available on ESA/390 and z/System models, and will throw an exception if they are attempted to be executed on a 360 or S/370. I|nstructions marked "390 Only" will throw an exception on the z/System.
• z/System instructions were created after the 370 and 390, they are shown on the table below in this color: [       ] and will throw an exception on 360, 370 or 390 hardware, working only on z/System models.

In general, there are seven types of instructions:

• Branch Instructions, which jump from one part of a program to another
• Data Transfer Instructions, which move data from one part of memory (including registers)
• Control Flow Instructions, which make changes in the scope of execution of a program
• Arithmetic Instructions, which do computations
• Logic Instructions, which make comparisons
• Shift and Rotate Instructions, which move bits right or left,
• Privileged Instructions, used by the operating system or other programs having special permission, primarily to do input/output or to control the operations of the machine, and
• Other Instructions, instructions not otherwise classified

The table shown below can be sorted on any of its columns by clicking on the symbol at the top of that column. The numeric value of opcodes listed is shown in hexadecimal only.