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FLD - Flexible Line Distance

FLD means the ability to move blocks of graphics vertically on the screen. More precisely FLD operates on char-lines (8 pixels high) as its smallest element. With FLD you can make gaps between char-lines, thus the graphics below the FLD will be pushed down on the screen.

Here is a short piece of code that shows a very basic FLD-effect. FLD does not require exact timing and doesn't even have to be updated each rasterline (only every ~6:th rasterline is ok). FLD can even be done with periodic interrupts instead of looped code that just wastes all the precious rastertime. That exercise however, i leave to you..



        bit $d011 ; Wait for new frame
        bpl *-3
        bit $d011
        bmi *-3

        lda #$1b ; Set y-scroll to normal position (because we do FLD later on..)
        sta $d011

        jsr CalcNumLines ; Call sinus substitute routine

        lda #$40 ; Wait for position where we want FLD to start
        cmp $d012
        bne *-3

        ldx NumFLDLines
        beq loop1 ; Skip if we want 0 lines FLD
        lda $d012 ; Wait for beginning of next line
        cmp $d012
        beq *-3

        clc ; Do one line of FLD
        lda $d011
        adc #1
        and #7
        ora #$18
        sta $d011

        dex ; Decrease counter
        bne loop2 ; Branch if counter not 0

        jmp loop1 ; Next frame

        lda #0
        bpl *+4
        eor #$ff
        sta NumFLDLines
        inc CalcNumLines+1

        .byte 0
base/fld.txt · Last modified: 2015-06-27 16:31 by