1by1textscrollertop-kick.asm

// Program: Single row scroll loop
// Author: Andrew Burch
// Site: www.0xc64.com
// Assembler: win2c64
// Notes: Use $d016 to scroll the single row
//               8 pixels and then loop
//
BasicUpstart2(plottext)
*=$c000
plottext:      ldx            #30       // fill last row with characters
               lda            #00
               sta            $0400,x
               adc            #01
               and            #15
               dex
               bpl            plottext+4
               sei                      // set up interrupt
               lda            #$7f
               sta            $dc0d     // turn off the CIA interrupts
               sta            $dd0d
               and            $d011     // clear high bit of raster line
               sta            $d011
               ldy            #$3a       // trigger on first scan line
               sty            $d012
               lda            #<noscroll// load interrupt address
               ldx            #>noscroll
               sta            $0314
               stx            $0315
               lda            #$01      // enable raster interrupts
               sta            $d01a
               cli
               rts                      // back to BASIC
noscroll:     

               lda            $d016     // default to no scroll on start of screen:
               and            #248      // mask register to maintain higher bits
               sta            $d016
               ldy            #$00     // trigger scroll on last character row
               sty            $d012
               lda            #<scroll  // load interrupt address
               ldx            #>scroll
               sta            $0314
               stx            $0315
               inc            $d019     // acknowledge interrupt
 //              lda #1
 //              sta $d021

               jmp            $ea31
scroll:         lda            $d016     // grab scroll register
               and            #248      // mask lower 3 bits
               adc            offset    // apply scroll
               sta            $d016
//               dec            smooth    // smooth scroll
//               bne            continue
            ;  dec            offset    // update scroll
             ;  bpl            resetsmooth
               lda            #07       // reset scroll offset
               sta            offset

//               lda #0
//               sta $d021

shiftrow:       ldx            #00       // shift characters to the left
               lda            $0401,x
               sta            $0400,x
               inx
               cpx            #39
               bne            shiftrow+2

resetsmooth:    ldx            #02       // set smoothing
               stx            smooth
continue:       ldy            #$3a      // trigger on first scan line
               sty            $d012
               lda            #<noscroll// load interrupt address
               ldx            #>noscroll
               sta            $0314
               stx            $0315
               inc            $d019     // acknowledge interrupt
               jmp            $ea31
offset:         .byte           01                        // start at 7 for left scroll:
smooth:         .byte           02