MC68020 查看數據表（PDF） - Freescale Semiconductor

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MC68020

MICROPROCESSORS USER’S MANUAL

Freescale Semiconductor 

Freescale Semiconductor, Inc.

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.

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 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.

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

M68020 USER’S MANUAL

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