Radio Shack EC-293 Technical Information download pdf (Page 21)

TECHNICAL INFORMATION

25 mS Heartbeat Interrupt

A clock circuit in the Expansion Interface provides an

Interrupt

the Z-80,

times each second. When such an

Interrupt

is received,

the Z-80 gets instructions from ROM telling it to

a specific

RAM

address for further instructions. Normally, these

"further

instructions" simply tell the Z-80

return from the interrupt

sequence and continue where it left off.

you

have enough experience with the Z-80 Instruction Set (and

with assembly-language programming in general),

you

can link up an

Interrupt Service Routine which accomplishes some simple fore-

ground task before it returns control

the interrupted program.

For

example,

you

can set up a real-time clock which keeps time

regardless

what

the

current program is - BASIC command mode,

BASIC program execution, machine-language routine, etc. Here's

how it's done:

Write an

Interrupt

Service Routine

a. Disable Interrupts

b. Read FDC status register 37ECH

clear it

c. Read

Interrupt

Status in

37E0H

d. Increment

the

software clock

counter

e. Read from 37E0H again to reset

Interrupt

Latch

Enable Interrupts

Return

interrupted program

2. Disable Interrupts

3. Replace the

Interrupt

Default link at

4012H

- 4014H with a

jump

your

Service Routine

4. Enable Interrupts

turn on the clock

Here's a BASIC program which POKES the necessary code

into

addresses

32680

- 32767. Note: Before using this program, reset

your

TRS-80 and answer MEMORY SIZE with

the

number

32679.

Note: This program

for

LEVEL II only.

Don't

try

to use

with

DISK BASIC (use the TRSDOS clock instead!).

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Radio Shack EC-293 Technical Information Page 21

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