Motorola M68000 User's Guide download pdf (Page 177)

Exception Processing, System Control Operations, and I/O 163

multi-user, it might not allow a user to do anything in supervisor mode.

This is not a disadvantage, but one of the intended features of the 68000

family. Unlike many microprocessors, including the 8086, the 68000 does

allow the writing and implementation of a secure multi-user operating

system. Here is how to set up an exception handler for the TRAP #0

instruction:

TRAP0V: EQU 32*4 TRAP 0 VECTOR ADDRESS

*GET INTO SUPERVISOR MODE IP NOT ALREADY IN IT

LEA T0HAND/A0 GET ADDRESS OF HANDLER

MOVE.L A0,TRAP0V STICK IN VECTOR

♦TRAP 0 HANDLER

T0HAND: MOVEM.L . ...,-(SP) SAVE REGISTERS WE USE

MOVEM (SP)+,.... RESTORE REGISTERS WE USED

RTE END WITH AN RTE

Notice that the exception handler saves and restores any registers it uses.

This is important because they are not automatically saved and restored

by the exception mechanism—only the program counter and the status

major consequences.

Serial I/O

In Chapter 2 the concept of memory mapped I/O was introduced.

All input/output on the 68000 is via I/O devices with control and data

registers that are present as addresses in the 68000’s address space. This

has the advantage that rather than being restricted to a small number of

input/output instructions, the 68000 programmer can use any memory

reference instruction with input/output devices as well as actual physical

memory locations.

To cover the wide variety of I/O devices that can be connected

to the 68000 would take up a book in itself. Additionally, so many

system-dependent aspects of these devices would make the topic almost

limitless in scope. Instead, in order to give you an idea of how to program

an I/O device, we will take a look at one very popular I/O chip, the

MC6850 Asynchronous Communications Interface Adapter (ACIA).

Many microcomputers are equipped with serial communications in

terfaces. Your CRT terminal or printer can be connected via a serial

interface. Another popular interface is the parallel interface. The terms

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Motorola M68000 User's Guide Page 177