Your own interrupt on the ZX81

Any discussions related to the creation of new hardware or software for the ZX80 or ZX81
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dr beep
Posts: 1638
Joined: Thu Jun 16, 2011 8:35 am
Location: Boxmeer

Your own interrupt on the ZX81

Post by dr beep »

On this forum several questions about the use of interrupts is asked.
Problem is that your routine must be finished before the next interrupt starts so screenupdate can be done correctly.

What if your routine would need more time than available? You can solve it with this routine.

The program has an exampleroutine which can be your own, as well as the frequency your routine will be activated.

Code: Select all

; interrupt on a ZX81 

; activate interruptroutine
	ld ix,introut
	ret

intfreq equ 4				; can be set by user

introut	ld a,r				; timed display
	ld a,#f5
	ld bc,#1901			; 25 newlines 
	call #2b5			; lowres display
	call #292
	call #220			; all routines needed for a full display
	ld ix,introut			; reactivate your intrupt routine 

; here the routine starts, but we need to test if intrupt is allowed
	ld hl,counter			; we test interrupt within intrupt
	dec (hl)			; or interrupt not yet allowed
	jr nz,exit			; if so exit routine
	ld (hl),intfreq			; your own routine each intfreg 

; here your routine starts
	ld hl,frames			; this routine uses
	ld a,(hl)			; multiple screenupdates
	sub 2
longint	cp (hl)
	jr nz,longint

exit	jp #2a4				; back to mainroutine

counter	db intfreq
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