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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
Programming Model
Page 88 of 377
gx_02.fm.(1.2)
March 27, 2006
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 ille-
gal, but might be defined to perform new functions in future extensions to the architecture:
1, 4, 5, 6, 9, 22, 56, 60, 61
• Instructions defined in the PowerPC Architecture but not implemented in a specific PowerPC implemen-
tation. For example, instructions that can be executed on 64-bit PowerPC processors are considered ille-
gal by 32-bit processors such as the 750GX.
The following primary opcodes are defined for 64-bit implementations only and are illegal on the 750GX:
2, 30, 58, 62
• All unused extended opcodes are illegal. The unused extended opcodes can be determined from infor-
mation in Section 2.3.1.4. Notice that extended opcodes for instructions defined only for 64-bit implemen-
tations 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 proba-
bility 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.
The 750GX 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 on page 170 for additional information about illegal and invalid instruction exceptions.
Except for an instruction consisting of binary zeros, illegal instructions are available for additions to the
PowerPC Architecture.