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5.5.2.2 BUS ERROR. A bus error exception occurs when an assertion of the BERR signal
is acknowledged. The BERR signal can be asserted by one of three sources:
1. External logic by assertion of the BERR input pin
2. Direct assertion of the internal BERR signal by an internal module
3. Direct assertion of the internal BERR signal by the on-chip hardware watchdog
after detecting a no-response condition
Bus error exception processing begins when the processor attempts to use information
from an aborted bus cycle.
When the aborted bus cycle is an instruction prefetch, the processor will not initiate
exception processing unless the prefetched information is used. For example, if a branch
instruction flushes an aborted prefetch, that word is not accessed, and no exception
occurs.
When the aborted bus cycle is a data access, the processor initiates exception processing
immediately, except in the case of released operand writes. Released write bus errors are
delayed until the next instruction boundary or until another operand access is attempted.
Exception processing for bus error exceptions follows the regular sequence, but context
preservation is more involved than for other exceptions because a bus exception can be
initiated while an instruction is executing. Several bus error stack format organizations are
utilized to provide additional information regarding the nature of the fault.
First, any register altered by a faulted-instruction EA calculation is restored to its initial
value. Then a special status word (SSW) is placed on the stack. The SSW contains
specific information about the aborted access—size, type of access (read or write), bus
cycle type, and function code. Finally, fault address, bus error exception vector number,
PC value, and a copy of the SR are saved.
If a bus error occurs during exception processing for a bus error, an address error, a reset,
or while the processor is loading stack information during RTE execution, the processor
halts. This simplifies isolation of catastrophic system failure by preventing processor
interaction with stacks and memory. Only assertion of RESET can restart a halted
processor.
5.5.2.3 ADDRESS ERROR. Address error exceptions occur when the processor attempts
to access an instruction, word operand, or long-word operand at an odd address. The
effect is much the same as an internally generated bus error. The exception processing
sequence is the same as that for bus error, except that the vector number refers to the
address error exception vector.
Address error exception processing begins when the processor attempts to use
information from the aborted bus cycle. If the aborted cycle is a data space access,
exception processing begins when the processor attempts to use the data, except in the
MOTOROLA
MC68340 USER’S MANUAL
5- 45
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