Classes Of Instructions; Definition Of Boundedly Undefined; Defined Instruction Class - IBM PowerPC 604 User Manual

mnemonics. Note that the architecture specification refers to simplified mnemonics as
extended mnemonics. Programs written to be portable across the various assemblers for the
PowerPC architecture should not assume the existence of mnemonics not described in that
document.
2.3.1 Classes of Instructions
The 604 instructions belong to one of the following three classes:
• Defined
• Illegal
• Reserved
Note that while the definitions of these terms are consistent among the PowerPC
processors, the assignment of these classifications is not. For example, a PowerPC
instruction defined for 64-bit implementations are treated as illegal by 32-bit
implementations such as the 604.
The class is determined by examining the primary opcode and the extended opcode, if any.
If the opcode, or combination of opcode and extended opcode, is not that of a defined
instruction or of a reserved instruction, the instruction is illegal.
Instruction encodings that are now illegal may become assigned to instructions in the
architecture or may be reserved by being assigned to processor-specific instructions.
2.3.1.1 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.
2.3.1.2 Defined Instruction Class
Defined instructions are guaranteed to be supported in all PowerPC implementations,
except as stated in the instruction descriptions in Chapter 8, "Instruction Set," in The
Programming Environments Manua.l.
The 604 provides hardware support for all
instructions defined for 32-bit implementations.
A PowerPC processor invokes the illegal instruction error handler (part of the program
exception) when the unimplemented Power PC instructions are encountered so they may be
emulated in software, as required. Note that the architecture specification refers to
exceptions as interrupts.
The 604 provides hardware support for all instructions defined for 32-bit implementations.
The 604 does not support the optional fsqrt, fsqrts, and tibia instructions.
Chapter
2.
PowerPC 604 Proceeeor Programming Model
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