Motorola M68000 User's Guide download pdf (Page 169)

C/MPTER 12

EXCEPTION PROCESSING,

SYSTEM CONTROL

OPERATIONS, AND I/O

In this chapter we will cover a number of topics and instructions

that don’t fit into the category of general programming instructions. I

mentioned in Chapter 2 that the 68000 doesn’t have any input/output

instructions. This is not quite true. A number of instructions are used

to process interrupts, a certain type of exception which is a vital part

of I/O activity. In addition, a special version of the MOVE instruction is

provided to make certain kinds of I/O easier and faster. I also mentioned

in Chapter 2 that the 68000 has a user and supervisor mode of operation.

The system control operations are used to coordinate these two modes.

The Status Register and System Control

When we are in the supervisor mode, the condition code register,

CCR, is part of a 16-bit register known as the status register, SR. Let’s

take a closer look at the high-order bits of the register found on the

following page. If the trace bit is set to a 1, the CPU is placed into trace

mode. This special mode is used for debugging purposes. When in trace

mode, an exception is generated for each instruction executed. We will

discuss trace mode when we talk about exceptions.

If the supervisor state bit is set, we are in supervisor mode. However,

this bit cannot be arbitarily set at any time. If we are in user mode

(the supervisor bit clear), we can’t modify the high-order bits of the

status register. How then do we get into supervisor mode? There are

two ways. The first is when we initially turn on our system, or push the

reset button (if so equipped). This causes a system reset to supervisor

mode. The second method is related to the handling of exceptions. When

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Motorola M68000 User's Guide Page 169