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Chapter 9: Machine Dependent Features
Dn(0)
Dn
Dn(Ln,0)
Dn(Ln)
235
9.37.3.4 Instruction Formats
The Principles of Operation manuals lists 26 instruction formats where some of the formats
have multiple variants. For the ‘.insn’ pseudo directive the assembler recognizes some of
the formats. Typically, the most general variant of the instruction format is used by the
‘.insn’ directive.
The following table lists the abbreviations used in the table of instruction formats:
OpCode / OpCd
Part of the op code.
Bx
Base register number for operand x.
Dx
Displacement for operand x.
DLx
Displacement lower 12 bits for operand x.
DHx
Displacement higher 8-bits for operand x.
Rx
Register number for operand x.
Xx
Index register number for operand x.
Ix
Signed immediate for operand x.
Ux
Unsigned immediate for operand x.
An instruction is two, four, or six bytes in length and must be aligned on a 2 byte
boundary. The first two bits of the instruction specify the length of the instruction, 00
indicates a two byte instruction, 01 and 10 indicates a four byte instruction, and 11 indicates
a six byte instruction.
The following table lists the s390 instruction formats that are available with the ‘.insn’
pseudo directive:
E format
+-------------+
|
OpCode
|
+-------------+
0
15
RI format: <insn> R1,I2
+--------+----+----+------------------+
| OpCode | R1 |OpCd|
I2
|
+--------+----+----+------------------+
0
8
12
16
31