
3-6
G2 PowerPC Core Reference Manual
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MOTOROLA
Instruction Set Summary
memory and a set of 32 GPRs. It also provides for word and double-word operand loads
and stores between memory and a set of 32 FPRs.
Arithmetic and logical instructions do not read or modify memory. To use the contents of a
memory location in a computation and then modify the same or another memory location,
the memory contents must be loaded into a register, modified, and then written to the target
location using load and store instructions.
The description of each instruction includes the mnemonic and a formatted list of operands.
To simplify assembly language programming, a set of simplified mnemonics (extended
mnemonics in the architecture specification) and symbols is provided for some of the
frequently-used instructions; see Appendix F, “Simplified Mnemonics,” in the
Programming Environments Manual
, for a complete list of simplified mnemonic examples.
3.2.1
Classes of Instructions
The G2 core instructions belong to one of the following three classes:
Defined
Illegal
Reserved
Note that although the definitions of these terms are consistent among the processors of this
family, the assignment of these classifications is not. For example, an instruction that is
specific to 64-bit implementations is considered defined for 64-bit implementations but
illegal for 32-bit implementations such as the G2 core.
The class is determined by examining the primary opcode and the extended opcode, if any.
If either is not that of a defined instruction or of a reserved instruction, the instruction is
illegal.
In future versions of the PowerPC architecture, instruction codings that are now illegal may
become assigned to instructions in the architecture or may be reserved by being assigned to
processor-specific instructions.
3.2.1.1
Definition of Boundedly Undefined
If instructions are encoded with incorrectly set bits in reserved fields, the results on
execution can be said to be boundedly undefined. If a user-level program executes the
incorrectly coded instruction, the resulting undefined results are bounded in that a spurious
change from user to supervisor state is not allowed, and the level of privilege exercised by
the program in relation to memory access and other system resources cannot be exceeded.
Boundedly undefined results for a given instruction may vary between implementations,
and between execution attempts in the same implementation.
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