as: s390 Operands
9.40.3.3 Instruction Operands
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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 content of a single general R
* The sum of the content of a general register called the base
register B plus the content of a displacement field D
* The sum of the contents of two general registers called the index
register X and the base register B plus the content of a
displacement field
* The sum of the current instruction address and a 32-bit signed
immediate field multiplied by two.
The length of a storage operand can be:
* Implied by the instruction
* Specified by a bitmask
* Specified by a four-bit or eight-bit length field L
* Specified by the content of a general register
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
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Dn(0,Bn) Dn(Bn)
Dn(0,0) Dn
Dn(0) Dn
Dn(Ln,0) Dn(Ln)
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