Chapter 2. Programming Model
2-37
Instruction Set Summary
2.3.1 Classes of Instructions
The MPC7400 instructions belong to one of the following three classes:
¥
¥
¥
DeTned
Illegal
Reserved
Note that while the deTnitions of these terms are consistent among the PowerPC
processors, the assignment of these classiTcations is not. For example, PowerPC
instructions deTned for 64-bit implementations are treated as illegal by 32-bit
implementations such as the MPC7400.
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 deTned
instruction or of a reserved instruction, the instruction is illegal.
Instruction encodings that are now illegal can become assigned to instructions in the
architecture or can be reserved by being assigned to processor-speciTc instructions.
2.3.1.1 DeTnition of Boundedly UndeTned
If instructions are encoded with incorrectly set bits in reserved Telds, the results on
execution can be said to be boundedly undeTned. If a user-level program executes the
incorrectly coded instruction, the resulting undeTned 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-undeTned results for a given instruction can vary between implementations,
and between execution attempts in the same implementation.
2.3.1.2 DeTned Instruction Class
DeTned instructions are guaranteed to be supported in all PowerPC implementations,
except as stated in the instruction descriptions in Chapter 8, òInstruction Set,ó in
The
Programming Environments Manual
. The MPC7400 provides hardware support for all
instructions deTned for 32-bit implementations. It does not support the optional
fsqrt
,
fsqrts
, and
tlbia
instructions.
A PowerPC processor invokes the illegal instruction error handler (part of the program
exception) when it encounters a PowerPC instructions that has not been implemented. The
instruction can be emulated in software, as required. Note that the architecture speciTcation
refers to exceptions as interrupts.
A deTned instruction can have invalid forms. The MPC7400 provides limited support for
instructions represented in an invalid form.