MC68030 Datasheet, PDF (261/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Exception Processing
As described in 7.5.4 Double Bus Fault, if bus error or address error occur during the
exception processing sequence for a reset, a double bus fault occurs. The processor halts,
and the STATUS signal is asserted continuously to indicate the halted condition.
Execution of the reset instruction does not cause a reset exception, nor does it affect any
internal registers, but it does cause the MC68030 to assert the RESET signal, resetting all
external devices.
8.1.2 Bus Error Exception
A bus error exception occurs when external logic aborts a bus cycle by asserting the BERR
input signal. If the aborted bus cycle is a data access, the processor immediately begins
exception processing. If the aborted bus cycle is an instruction prefetch, the processor may
delay taking the exception until it attempts to use the prefetched information. The assertion
of the BERR signal during the second, third, or fourth access of a burst operation does not
cause a bus error exception, but the burst is aborted. Refer to 6.1.3.2 Burst Mode Filling
and 7.5.1 Bus Errors for details on the effects of bus errors during burst operation.
A bus error exception also occurs when the MMU detects that a successful address
translation is not possible. Furthermore, when an ATC miss occurs and an external bus
cycle is required, the MMU must abort the bus cycle, search the translation tables in memory
for the mapping, and then retry the bus cycle. If a valid translation for the logical address is
not available due to a problem encountered during the table search (the attempt to access
the appropriate page descriptor in the translation tables for that page), a bus error exception
occurs when the aborted bus cycle is retried.
The problem encountered could be a limit violation, an invalid descriptor, or the assertion of
the BERR signal during a bus cycle used to access the translation tables. A miss in the ATC
causes the processor to automatically initiate a table search but does not cause a bus error
exception unless one of the specific conditions mentioned above is encountered.
MOTOROLA
MC68030 USER’S MANUAL
8-7