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base:repeating_char-lines

Repeating char-lines

By HCL/Booze Design

Here is a small program that doubles one char-line. If you run it from the basic screen you should see the “64k ram system…” text-line being doubled. The trick can of course be used several times, and by changing char-bank or video-bank at each repetition, large areas of graphics can be moved by just storing one char-line.

        *= $0900
	sei
loop
        lda #$51 ; Wait for a line
        cmp $d012
        bne *-3

        ldx #10 ; Wait one more line..
        dex
        bne *-1
        nop

	cmp $d012 ; ..and make a bit more stabel raster
	bne *+5
	bit $ea
	nop

	ldx #5 ; Wait for the perfect moment..
	dex
	bne *-1
	nop
	nop
	nop

        inc $d021 ; Display on screen
        dec $d021

        lda #$1a ; Do repeat char-line
        sta $d011

	lda #$54 ; Wait a bit more..
        cmp $d012
        bne *-3

        lda #$1b ; ..before setting $d011 back to original value.
        sta $d011
        nop

        ldx #0 ; Make raster unstable again :)
        inc *-1
        inx
	bne *-1

        jmp loop
base/repeating_char-lines.txt · Last modified: 2016-09-09 18:21 by ftc