Motorola Axiom CMD11E1 Uživatelská příručka stáhnout pdf (Strana 79)

7.3. INTERRUPTS 79

(b) However, if the mask is set, then request for service is generated.

i. The I bit in the CCR is a master disable switch. If this is set

(by using the command SEI), then the request for service does not

interrupt the computer and is hence ignored.

ii. However if I bit is cleared (by using the command CLI), then the

CPU is interrupted.

Note: It is extremely important that you have an SEI before any code

that can turn on an interrupt and and CLI after all relevant and required

initialization is performed.

3. When a CPU is interrupted, it stops its current task and starts the service.

4. When performing the service, you get a completely new set of registers.

So you can not assume that the registers will have any speciﬁc value. Also,

when the service terminates, the new set of registers is destroyed. So you can

not assume that the rest of the code can see what you stored in the register

as part of the service. In fact, the interrupted task would be oblivious to

the fact that a service was provided. The only way it can ﬁnd out is if the

service modiﬁes some memory location.

5. The location of the service that is associated with a particular interrupt is

deﬁned by the hardware manufacturer, and is called the jump vector. This

would be in read only memory and can not be chanted. The operating

system, BUFFALO, sets the start of the service to a known location and

sets aside three bytes at that location. Your service will start with JMP

instruction to the actual code for the service. Your service must end with

the RTI instruction.

The address of services to three important interrupts is given in the following

table:

Interrupt Service location

TOF $00D0

RTIF $00EB

OC2F $00DC

6. Your service code should, as part of the service, turn oﬀ the ﬂag that gen-

erated the interrupt. If not, the request for service will still be active and

will generate a new service request as soon as the current service end!

Here is a short checklist for what you should do:

1. In your main routine, enable an interrupt by turning on the associated mask.

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Motorola Axiom CMD11E1 Uživatelská příručka Strana 79