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--- base:fld [2015-06-16 00:10] – theryk
+++ base:fld [2015-06-27 16:31] – added binary and screenshot joriszwart.nl
@@ Line 1: / Line 1: @@
+====== 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.
+{{:base:fld.png?nolink|}}
+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..
+/HCL
+Binary: {{:base:fld.zip|}}
+<code>
+        sei
+loop1
+        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
+loop2
+        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 $d011
+        dex ; Decrease counter
+        bne loop2 ; Branch if counter not 0
+        jmp loop1 ; Next frame
+CalcNumLines
+        lda #0
+        bpl *+4
+        eor #$ff
+        lsr
+        lsr
+        lsr
+        sta NumFLDLines
+        inc CalcNumLines+1
+        rts
+NumFLDLines
+        .byte 0
+</code>