Processing States; Exceptions - Motorola MC68030 User Manual

SECTION 4

PROCESSING STATES

This section describes the processing states of the MC68030. It describes the functions of
the bits in the supervisor portion of the status register and the actions taken by the processor
in response to exception conditions.
Unless the processor has halted, it is always in either the normal or the exception processing
state. Whenever the processor is executing instructions or fetching instructions or operands,
it is in the normal processing state. The processor is also in the normal processing state
while it is storing instruction results or communicating with a coprocessor.
Exception processing refers specifically to the transition from
normal processing of a program to normal processing of system
routines, interrupt routines, and other exception handlers. Ex-
ception processing includes all stacking operations, the fetch of
the exception vector, and filling of the instruction pipe caused by
an exception. It has completed when execution of the first in-
struction of the exception handler routine begins.
The processor enters the exception processing state when an interrupt is acknowledged,
when an instruction is traced or results in a trap, or when some other exceptional condition
arises. Execution of certain instructions or unusual conditions occurring during the execution
of any instructions can cause exceptions. External conditions, such as interrupts, bus errors,
and some coprocessor responses, also cause exceptions. Exception processing provides
an efficient transfer of control to handlers and routines that process the exceptions.
A catastrophic system failure occurs whenever the processor receives a bus error or
generates an address error while in the exception processing state. This type of failure halts
the processor. For example, if during the exception processing of one bus error another bus
error occurs, the MC68030 has not completed the transition to normal processing and has
not completed saving the internal state of the machine, so the processor assumes that the
system is not operational and halts. Only an external reset can restart a halted processor.
(When the processor executes a STOP instruction, it is in a special type of normal
processing state, one without bus cycles. It is stopped, not halted.)
MOTOROLA
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MC68030 USER'S MANUAL 4-1