Typewriter User Manual

5- 48 MC68340 USER’S MANUAL MOTOROLA
All unimplemented instructions are reserved for use by Motorola for enhancements and
extensions to the basic M68000 architecture. Opcode pattern $4AFC is defined to be
illegal on all M68000 family members. Those customers requiring the use of an
unimplemented opcode for synthesis of "custom instructions," operating system calls, etc.,
should use this opcode.
Exception processing for illegal and unimplemented instructions is similar to that for traps.
The instruction is fetched and decoding is attempted. When the processor determines that
execution of an illegal instruction is being attempted, exception processing begins. No
registers are altered.
Exception processing follows the regular sequence. The vector number is generated to
refer to the illegal instruction vector or in the case of an unimplemented instruction, to the
corresponding emulation vector. The illegal instruction vector number, current PC, and a
copy of the SR are saved on the supervisor stack, with the saved value of the PC being
the address of the illegal or unimplemented instruction.
5.5.2.9 PRIVILEGE VIOLATIONS. To provide system security, certain instructions can be
executed only at the supervisor access level. An attempt to execute one of these
instructions at the user level will cause an exception. The privileged exceptions are as
follows:
• AND Immediate to SR
• EOR Immediate to SR
• LPSTOP
• MOVE from SR
• MOVE to SR
• MOVE USP
• MOVEC
• MOVES
• OR Immediate to SR
• RESET
• RTE
• STOP
Exception processing for privilege violations is nearly identical to that for illegal
instructions. The instruction is fetched and decoded. If the processor determines that a
privilege violation has occurred, exception processing begins before instruction execution.
Exception processing follows the regular sequence. The vector number (8) is generated to
reference the privilege violation vector. Privilege violation vector offset, current PC, and
SR are saved on the supervisor stack. The saved PC value is the address of the first word
of the instruction causing the privilege violation.
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