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Instruction Format

An instruction is one, two, or three halfwords in length and must be located in main storage on an integral halfword boundary. Each instruction is in one of five basic formats: RR, RX, RS, SI, and S.

Some instructions contain fields that vary slightly from the basic format, and in some instructions the operation performed does not follow the general rules stated in this section. All such exceptions are explicitly identified in the individual instruction descriptions.

The format names express, in general terms, the classes of operands which participate in the operation: RR denotes a register-to-register operation; RX. a register-and-indexed-storage operation; RS, a register-and-storage operation; SI, a storage-and- immediate operation. The S format denotes an operation using all implied operand and storage.

The first byte and, in the S format, the first two bytes of an instruction contain the operation code (op code). For some instructions in the S format, all or a portion of the second byte is ignored. The first two bits of the operation code specify the length and format of an instruction, as follows:

	Bit Positions  Instruction  Instruction 
	0-1		 Length		Format 
	00	 One halfword		RR 
	01	 Two halfwords		RX 
	10	 Two halfwords	 RS/SI/S/RX

In the format illustration for each individual instruction description. the opcode field shows the opcode in hexadecimal representation. The hexadecimal representation uses one graphic for a four-bit code, and therefore two graphics for an eight-bit code. The graphics 0-9 are used for the codes 0000 - 1001 ; the graphics A-F are used for codes 1010 - 1111.

The remaining fields in the format illustration for each instruction are designated by code names, consisting of a letter and possibly a subscript number. The subscript number denotes the operand to which the field applies.

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