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are left pending. An instruction breakpoint is not made pending until the instruction
corresponding to the request is executed.
A pending breakpoint can be acknowledged between instructions or at the end of
exception processing. To acknowledge a breakpoint, the CPU performs a read from CPU
space $0 at location $1E (see Section 3 Bus Operation).
If the bus cycle terminates normally, instruction execution continues with the next
instruction, as if no breakpoint request occurred. If the bus cycle is terminated by BERR,
the CPU begins exception processing. Data returned during this bus cycle is ignored.
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Exception processing follows the regular sequence. Vector number 12 (offset $30) is
internally generated. The PC of the currently executing instruction, the PC of the next
instruction to execute, and a copy of the SR are saved on the supervisor stack.
5.6.2.7 FORMAT ERROR. The processor checks certain data values for control
operations. The validity of the stack format code and, in the case of a bus cycle fault
format, the version number of the processor that generated the frame are checked during
execution of the RTE instruction. This check ensures that the program does not make
erroneous assumptions about information in the stack frame.
If the format of the control data is improper, the processor generates a format error
exception. This exception saves a four-word format exception frame and then vectors
through vector table entry number 14. The stacked PC is the address of the RTE
instruction that discovered the format error.
5.6.2.8 ILLEGAL OR UNIMPLEMENTED INSTRUCTIONS. An instruction is illegal if it
contains a word bit pattern that does not correspond to the bit pattern of the first word of a
legal CPU32 instruction, if it is a MOVEC instruction that contains an undefined register
specification field in the first extension word, or if it contains an indexed addressing mode
extension word with bits 5–4 = 00 or bits 3–0 ≠ 0000.
If an illegal instruction is fetched during instruction execution, an illegal instruction
exception occurs. This facility allows the operating system to detect program errors or to
emulate instructions in software.
Word patterns with bits 15–12 = 1010 (referred to as A-line opcodes) are unimplemented
instructions. A separate exception vector (vector 10, offset $28) is given to unimplemented
instructions to permit efficient emulation.
Word patterns with bits 15–12 = 1111 (referred to as F-line opcodes) are used for M68000
Family instruction set extensions. They can generate an unimplemented instruction
exception caused by the first extension word of the instruction or by the addressing mode
extension word. A separate F-line emulation vector (vector 11, offset $2C) is used for the
exception vector.
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.
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MC68330 USER'S MANUAL
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