SixtyPical/eg/demo.60p

 assign byte table screen $0400
assign byte table screen2 1274
assign byte table screen3 1524
assign byte table screen4 1774

assign byte table colormap 55296
assign byte table colormap2 55546
assign byte table colormap3 55796
assign byte table colormap4 56046

assign byte vic_border 53280
assign byte table vic_bg 53281

assign vector cinv 788
reserve vector save_cinv

assign word position $fb

reserve byte value

reserve word m
reserve word n

routine reset_position {
    lda #$00
    sta <position
    lda #$04
    sta >position
}

routine increment_pos {
    clc
    lda <position
    adc #1
    sta <position
    lda >position
    adc #0
    sta >position
}

routine compare_16_bit {
    lda >m
    cmp >n
    if beq {
        lda <m
        cmp <n
    } else {
    }
}

routine compare_pos {
    lda <position
    sta <m
    lda >position
    sta >m
    lda #$07
    sta >n
    lda #$e8
    sta <n
    jsr compare_16_bit
}

routine clear_screen {
    ldy #0
    repeat bne {
        lda #1
        sta colormap, y
        sta colormap2, y
        sta colormap3, y
        sta colormap4, y
        
        lda #32
        sta screen, y
        sta screen2, y
        sta screen3, y
        sta screen4, y

        iny
        cpy #250
    }
}

routine our_cinv {
    inc value
    lda value
    ldy #0
    sta (position), y
    jsr increment_pos
    jsr compare_pos
    if beq {
        jsr reset_position
    } else {
    }
    jmp (save_cinv)
}

routine main {
    lda #5
    sta vic_border
    lda #0
    sta vic_bg
    jsr reset_position
    jsr clear_screen
    sei {
        copy cinv save_cinv
        copy routine our_cinv to cinv
    }
    clc
    repeat bcc { }
}