MC68030 Datasheet, PDF (281/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Exception Processing
8.2 BUS FAULT RECOVERY
An address error exception or a bus error exception indicates a bus fault. The saving of the
processor state for a bus error or address error is described in 8.1.2 Bus Error Exception,
and the restoring of the processor state by an RTE instruction is described in 8.1.13 Return
from Exception
Processor accesses of either data items or the instruction stream can result in bus errors.
When a bus error exception occurs while accessing a data item, the exception is taken
immediately after the bus cycle terminates. Bus errors reported by the on-chip MMU are also
processed immediately. A bus error occurring during an instruction stream access is not
processed until the processor attempts to use the information (if ever) that the access should
have provided. For instruction faults, when the short format frame applies, the address of
the pipe stage B word is the value in the program counter plus four, and the address of the
stage C word is the value in the program counter plus two. For the long format, the long word
at SP+$24 contains the address of the stage B word; the address of the stage C word is the
address of the stage B word minus two. Address error faults occur only for instruction stream
accesses, and the exceptions are taken before the bus cycles are attempted.
MOTOROLA
MC68030 USER’S MANUAL
8-27