SixtyPical/eg/game.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 byte joy2 $dc00

assign vector cinv 788

; ---------

reserve vector save_cinv
assign word position $fb
reserve word delta
reserve byte value
reserve word compare_target

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

routine our_cinv {
    lda value
    inc value
    ldy #0
    sta (position), y
    jsr read_stick
    jsr advance_pos

    lda #$07                 ; just past bottom of screen
    sta >compare_target
    lda #$e8
    sta <compare_target
    jsr compare_pos

    if bcs {
        jsr reset_position
    } else {

        lda #$04
        sta >compare_target
        lda #$00
        sta <compare_target
        jsr compare_pos

        if bcc {
            jsr reset_position
        } else { }
    }
    jmp (save_cinv)
}

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

routine advance_pos {
    clc
    lda <position
    adc <delta
    sta <position
    lda >position
    adc >delta
    sta >position
}

routine compare_pos {
    lda >position
    cmp >compare_target
    if beq {
        lda <position
        cmp <compare_target
    } else {
    }
}

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 read_stick {
    lda #0
    sta <delta
    sta >delta
    ldx joy2
    txa
    and #1                   ; up
    if beq {
        lda #216             ; -40
        sta <delta
        lda #255
        sta >delta
    } else {
        txa
        and #2               ; down
        if beq {
            lda #40
            sta <delta
        } else {
            txa
            and #4           ; left
            if beq {
                lda #255     ; -1
                sta <delta
                lda #255
                sta >delta
            } else {
                txa
                and #8       ; right
                if beq {
                    lda #1
                    sta <delta
                } else { }
            }
        }
    }
}