Lab - 3 6502 Math and Strings Lab

Assembly language is a low-level programming language used to interface directly with the hardware of a computer. 

For this I have to create a program that fits the following criteria:

1. Your application must run in the 6502 Emulator.
2. You must output to both the character screen and the graphics (bitmapped) screen.
3. You must take user input from the keyboard in some way.
4. You must employ some arithmetic/math instructions (to add, subtract, do bitwise operations, or rotate/shift).

I'll go with Kaleidoscope because I remember playing it when I was younger and have a rough concept of how it works. The code I'm writing allows the user to draw pixels in any of several distinct colours on one-quarter of the screen. In addition, the code allows the user to mirror the pixel to the other three quadrants. A pixel drawn at (0,0), for example, should be mirrored to (31,31), (0,31), and (0,31). (31, 0).

Here is the source code for this game that I wrote: 

; zero-page variable locations
 define ROW     $20 ; current row
 define COL     $21 ; current column
 define POINTER     $10 ; ptr: start of row
 define POINTER_H   $11

 ; constants
 define DOT     $01 ; dot colour location
 define CURSOR      $05 ; green colour


 setup: lda #$0f    ; set initial ROW,COL
    sta ROW
    sta COL
    LDA #$01
    STA DOT

 draw:  jsr draw_cursor

 getkey:    lda $ff     ; get a keystroke

    ldx #$00    ; clear out the key buffer
    stx $ff

    cmp #$30
    bmi getkey
    cmp #$40
    bpl continue

    SEC
    sbc #$30
    tay
    lda color_pallete, y
    sta DOT
    jmp done

continue:   cmp #$43    ; handle C or c
    beq clear
    cmp #$63
    beq clear

    cmp #$80    ; if not a cursor key, ignore
    bmi getkey
    cmp #$84
    bpl getkey

    pha     ; save A

    lda DOT ; set current position to DOT
    sta (POINTER),y
    jsr draw_on_quads

    pla     ; restore A

    cmp #$80    ; check key == up
    bne check1

    dec ROW     ; ... if yes, decrement ROW
    jmp done

 check1:    cmp #$81    ; check key == right
    bne check2

    inc COL     ; ... if yes, increment COL
    jmp done

 check2:    cmp #$82    ; check if key == down
    bne check3

    inc ROW     ; ... if yes, increment ROW
    jmp done

 check3:    cmp #$83    ; check if key == left
    bne done

    dec COL     ; ... if yes, decrement COL
    clc
    bcc done

 clear: lda table_low   ; clear the screen
    sta POINTER
    lda table_high
    sta POINTER_H

    ldy #$00
    tya

 c_loop:    sta (POINTER),y
    iny
    bne c_loop

    inc POINTER_H
    ldx POINTER_H
    cpx #$06
    bne c_loop

 done:  clc     ; repeat
    bcc draw


 draw_cursor:
    lda ROW     ; ensure ROW is in range 0:31
    and #$0f
    sta ROW

    lda COL     ; ensure COL is in range 0:31
    and #$0f
    sta COL

    ldy ROW     ; load POINTER with start-of-row
    lda table_low,y
    sta POINTER
    lda table_high,y
    sta POINTER_H

    ldy COL     ; store CURSOR at POINTER plus COL
    lda #CURSOR
    sta (POINTER),y

    rts

 draw_on_quads:     
    LDA POINTER ;; save the pointer to the
    PHA         ;; original location in top_left_quad
    LDA POINTER_H
    PHA

    LDA #$10
    CLC
    SBC COL
    CLC
    ADC #$10
    TAY
    LDA DOT
    STA (POINTER),y

    TYA
    PHA ; save the y offset

    lda #$10    ; load POINTER with start-of-row
    CLC
    SBC ROW
    CLC
    ADC #$10
    TAY
    lda table_low,y
    sta POINTER
    lda table_high,y
    sta POINTER_H

    ldy COL     ; store CURSOR at POINTER plus COL
    lda DOT
    sta (POINTER),y

    PLA
    TAY
    lda DOT
    sta (POINTER),y

    PLA
    STA POINTER_H
    PLA
    STA POINTER

    RTS

 ; these two tables contain the high and low bytes
 ; of the addresses of the start of each row

 table_high:
 dcb $02,$02,$02,$02,$02,$02,$02,$02
 dcb $03,$03,$03,$03,$03,$03,$03,$03
 dcb $04,$04,$04,$04,$04,$04,$04,$04
 dcb $05,$05,$05,$05,$05,$05,$05,$05,

 table_low:
 dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
 dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
 dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
 dcb $00,$20,$40,$60,$80,$a0,$c0,$e0

color_pallete:
dcb $01,$02,$03,$04,$05,$06,$07,$08,$09,$0a

Result of the code:

What have I learnt from this lab?

Even when the code is functional, it may have mistakes. It is preferable to think about which components should be fixed or tested before developing the code. Also, run numerous tests to identify where the issues are coming from.