Processing States - 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.
NOTE
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. Exception pro-
cessing 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 instruction of the excep-
tion 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 pro-
MOTOROLA MC68030 USER'S MANUAL
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