as: s390 Operands

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1 9.40.3.3 Instruction Operands
1 .............................
1 
1 Instruction operands can be grouped into three classes, operands located
1 in registers, immediate operands, and operands in storage.
1 
1    A register operand can be located in general, floating-point, access,
1 or control register.  The register is identified by a four-bit field.
1 The field containing the register operand is called the R field.
1 
1    Immediate operands are contained within the instruction and can have
1 8, 16 or 32 bits.  The field containing the immediate operand is called
1 the I field.  Dependent on the instruction the I field is either signed
1 or unsigned.
1 
1    A storage operand consists of an address and a length.  The address
1 of a storage operands can be specified in any of these ways:
1 
1    * The content of a single general R
1    * The sum of the content of a general register called the base
1      register B plus the content of a displacement field D
1    * The sum of the contents of two general registers called the index
1      register X and the base register B plus the content of a
1      displacement field
1    * The sum of the current instruction address and a 32-bit signed
1      immediate field multiplied by two.
1 
1    The length of a storage operand can be:
1 
1    * Implied by the instruction
1    * Specified by a bitmask
1    * Specified by a four-bit or eight-bit length field L
1    * Specified by the content of a general register
1 
1    The notation for storage operand addresses formed from multiple
1 fields is as follows:
1 
1 'Dn(Bn)'
1      the address for operand number n is formed from the content of
1      general register Bn called the base register and the displacement
1      field Dn.
1 'Dn(Xn,Bn)'
1      the address for operand number n is formed from the content of
1      general register Xn called the index register, general register Bn
1      called the base register and the displacement field Dn.
1 'Dn(Ln,Bn)'
1      the address for operand number n is formed from the content of
1      general register Bn called the base register and the displacement
1      field Dn.  The length of the operand n is specified by the field
1      Ln.
1 
1    The base registers Bn and the index registers Xn of a storage operand
1 can be skipped.  If Bn and Xn are skipped, a zero will be stored to the
1 operand field.  The notation changes as follows:
1 
1      full notation          short notation
1      ----------------------------------------------
1      Dn(0,Bn)               Dn(Bn)
1      Dn(0,0)                Dn
1      Dn(0)                  Dn
1      Dn(Ln,0)               Dn(Ln)
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