*= $0801 "Basic Upstart"
BasicUpstart(start)    // 10 sys$0810

*= $0810 "Program"
start:  ldx #$00
        stx $d020               // Border color register
        stx $d021               // Background color

clear:  lda #$20                // spacebar screen code
        sta $0400,x             // fill four areas of screen with 256 spacebar characters
        sta $0500,x
        sta $0600,x
        sta $06e8,x
        lda #$00                // set foreground to black in Color Ram 
        sta $d800,x
        sta $d900,x
        sta $da00,x
        sta $dae8,x
        inx
        bne clear               // bne = branch on not equal
                                // did x turn to zero already? 
                                // if yes -> stop loop
                                // if no -> continue loop
        rts