1×1 PETSCII scroller

Ok, here’s where it starts to get interesting, the source for a simple one character scroller that scrolls the first line of the screen.

Due to popular request, this source has more comments for easier readability.

*=$0801
; BASIC runnable stub, handy snippet!
.word ss,2005
.null $9e,^init;will be sys 4096
ss	.word 0 
init  	LDX #$00
; Clear the screen
-   LDA #$20 
 	STA $0400,x
    STA $0500,x
    STA $0600,x
    STA $0700,x
 ;  Clear colour memory
    LDA #$01
    STA $D900,x
    STA $DA00,x
    STA $DB00,x
    STA $DBE0,x
    INX
    BNE -
SEI 
LDA $DC0D
LDA $DD0D
    JSR $1000 ; init music
  	LDA #$1F
    STA $DC0D    ;CIA1: CIA Interrupt Control Register
   	STA $DD0D    ;CIA2: CIA Interrupt Control Register
    LDA #$1B
    STA $D011    ;VIC Control Register 1
LDA #$01
    STA $D01A    ;VIC Interrupt Mask Register (IMR)
LDA #$06
STA $D020    ;Border Color
LDA #$00
    STA $D021    ;Background Color 0
    LDA #$00
    STA $D012    ;Raster Position
    LDA #<start
    STA $FFFE    ;IRQ
    LDA #>start
    STA $FFFF    ;IRQ
    LDX #<nmi
    LDY #>nmi
    STX $FFFA    ;NMI
    STY $FFFB    ;NMI
    LDA #$35
    LDX #$FF
    STA $01
TXS 
    INC $D019    ;VIC Interrupt Request Register (IRR)
   	LDA $DC0D    ;CIA1: CIA Interrupt Control Register
    LDA $DD0D    ;CIA2: CIA Interrupt Control Register
    CLI 

• JMP -

  start STA nexta+1 STX nextx+1 STY nexty+1 LDA #$00 STA $D020 STA $D021 JSR $1003 JSR scroller LDA #$1B STA $D011 ;VIC Control Register 1 LDX #<next LDY #>next LDA #$32 STX $FFFE ;IRQ STY $FFFF ;IRQ STA $D012 ;Raster Position INC $D019 ;VIC Interrupt Request Register (IRR) nexta LDA #$00 nextx LDX #$00 nexty LDY #$00 nmi RTI

  next STA nexta1+1 STX nextx1+1 STY nexty1+1 LDA xpos ; store the current xpos in $D016 STA $D016 LDA #$00 STA $D021 STA $D020 LDX #<start LDY #>start LDA #$00 STX $FFFE STY $FFFF STA $D012 INC $D019 nexta1 LDA #$00 nextx1 LDX #$00 nexty1 LDY #$00 RTI

  scroller LDA xpos CLC SBC speed ; Subtract speed from xpos BCC + STA xpos ; if carry set (value is minus) then reset to zero RTS

  • AND #$07 ; Make sure the value is between 0 and 7 STA xpos LDX #$00 ; Scroll the first line of the screen
  • LDA $0401,x STA $0400,x INX CPX #$27 BNE - LDA gettext+1 CLC ADC #$01 ; add one to the gettext location STA gettext+1 BCC gettext INC gettext+2 gettext LDA scrolltext BNE + ; If a zero is detected in the scrolltext, loop LDX #<scrolltext LDY #>scrolltext STX gettext+1 STY gettext+2 JMP gettext
  • STA $0427 RTS xpos .byte 0 speed .byte 2 scrolltext .enc screen .text " welcome to this simple 1x1 scroller, your next challenge is to create a 1x2 scroller!!! stay tuned.... " .byte 0 *=$1000 .binary "music.prg",2

Your challenge for this episode, try to create a 1×2 scroller. Good luck!