Illegal Instruction Class - IBM PowerPC 604 User Manual

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A defined instruction can have invalid forms. The 604 provides limited support for
instructions that are represented in an invalid form. Appendix B, "Invalid Instruction
Forms," lists all invalid instruction forms and specifies the operation of the 604 upon
detecting each.
2.3.1.3 Illegal Instruction Class
Illegal instructions can be grouped into the following categories:
• Instructions not defined in the PowerPC architecture.The following primary
opcodes are defined as illegal but may be used in future extensions to the
architecture:
l,4,5,6,9,22,56,57,60,61
Future versions of the Power PC architecture may define any of these instructions to
perform new functions.
• Instructions defined in the PowerPC architecture but not implemented in a specific
PowerPC implementation. For example, instructions that can be executed on 64-bit
PowerPC processors are considered illegal by 32-bit processors such as the 604.
The following primary opcodes are defined for 64-bit implementations only and are
illegal on the 604:
2,30,58,62
• All unused extended opcodes are illegal. The unused extended opcodes can be
determined from information in Section A.2, "Instructions Sorted by Opcode," and
Section 2.3.1.4, "Reserved Instruction Class." Notice that extended opcodes for
instructions defined only for 64-bit implementations are illegal in 32.:.bit
implementations, and vice versa. The following primary opcodes have unused
extended opcodes.
17, 19, 31, 59, 63 (Primary opcodes 30 and 62 are illegal for all 32-bit
implementations, but as 64-bit opcodes they have some unused extended opcodes.)
• An instruction consisting of only zeros is guaranteed to be an illegal instruction. This
increases the probability that an attempt to execute data or uninitialized memory
invokes the system illegal instruction error handler (a program exception). Note that
if only the primary opcode consists of all zeros. The instruction is considered a
reserved instruction, as described in Section 2.3.1.4, "Reserved Instruction Class."
The 604 invokes the system illegal instruction error handler (a program exception) when it
detects any instruction from this class or any instructions defined only for 64-bit
implementations.
See Section 4.5. 7, "Program Exception (Ox00700)," for additional information about
illegal and invalid instruction exceptions. With the exception of the instruction consisting
entirely of binary zeros, the illegal instructions are available for further additions to the
PowerPC architecture.
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PowerPC 604 RISC Microprocessor User's Manual