MC68020 Motorola, MC68020 Datasheet - Page 30

• Manufacturer Part Number: MC68020
• Description: (MC68020 / MC68EC020) MICROPROCESSORS USERS MANUAL
• Manufacturer: Motorola

SECTION 2
PROCESSING STATES
This section describes the processing states of the MC68020/EC020. It describes the
functions of the bits in the supervisor portion of the SR 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.
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 exception condition
arises. Execution of certain instructions or unusual conditions occurring during the
execution of any instruction 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 MC68020/EC020 has not completed the transition to normal
processing and has not completed saving the internal state of the machine; therefore, 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
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 processing includes all stacking
operations, the fetch of the exception vector, and the filling of
the instruction pipe caused by an exception. Exception
processing has completed when execution of the first
instruction of the exception handler routine begins.
M68020 USER’S MANUAL
NOTE
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