Motorola M68000 Uživatelská příručka stáhnout pdf (Strana 183)

Exception Processing, System Control Operations, and I/O 169

31 24

23 1G 15 0

7 0

hi-order

low-order

Contents of Memory

B 7

hi-order

low-order

find that an instruction is to be substituted by one that requires one less

word in memory, a NOP can be used to fill the excess word.

The RESET instruction can only be used in supervisor mode. When

it is executed it asserts the hardware reset line. This normally causes all

external devices to be reset. The exact results of using this instruction

will depend on what sort of devices are connected to the hardware reset

line. Normally this instruction is executed by the operating system to

ensure an initialized state for all external devices. Unless you are writing

an operating system, you should never have need for this instruction. If

you execute it by mistake in the user mode, a trap will be generated.

The STOP instruction has the general form:

STOP #xxx

The STOP instruction must be executed when in the supervisor mode

or else a trap will be generated. The immediate operand of the STOP

instruction is moved into the entire status register, the program counter is

advanced to the next instruction, and the CPU stops executing. However,

if an interrupt occurs with a priority higher than the current interrupt

priority level in the SR, then an interrupt exception will be generated;

otherwise nothing happens. If the exception handler executes an RTE,

the next instruction following the STOP will be executed. If an external

reset is generated, the processor will execute the standard reset sequence.

The test and set instruction, TAS, is primarily used in a multi-processor

environment. The general form of the instruction is:

TAS <ea>

The operand size used with the TAS instruction must be byte. It first tests

the byte at the effective address and sets the N and Z bits of the CCR

accordingly. It then sets bit 7 of the operand to a 1. This instruction is

special in that a read-modify-write bus cycle is used so that the test and

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Motorola M68000 Uživatelská příručka Strana 183