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Instruction operands can be grouped into three classes, operands located in registers, immediate operands, and operands in storage.
A register operand can be located in general, floating-point, access, or control register. The register is identified by a four-bit field. The field containing the register operand is called the R field.
Immediate operands are contained within the instruction and can have 8, 16 or 32 bits. The field containing the immediate operand is called the I field. Dependent on the instruction the I field is either signed or unsigned.
A storage operand consists of an address and a length. The address of a storage operands can be specified in any of these ways:
The length of a storage operand can be:
The notation for storage operand addresses formed from multiple fields is as follows:
Dn(Bn)the address for operand number n is formed from the content of general register Bn called the base register and the displacement field Dn.
Dn(Xn,Bn)the address for operand number n is formed from the content of general register Xn called the index register, general register Bn called the base register and the displacement field Dn.
Dn(Ln,Bn)the address for operand number n is formed from the content of general register Bn called the base register and the displacement field Dn. The length of the operand n is specified by the field Ln.
The base registers Bn and the index registers Xn of a storage operand can be skipped. If Bn and Xn are skipped, a zero will be stored to the operand field. The notation changes as follows:
full notation |
short notation |
|---|---|
Dn(0,Bn) |
Dn(Bn) |
Dn(0,0) |
Dn |
Dn(0) |
Dn |
Dn(Ln,0) |
Dn(Ln) |
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