Assembly language is a type of low-level programming language that is intended to communicate directly with a computer's hardware. In this lab, there are four options for us to choose:
- Bouncing Graphic
- Numeric Display
This time, I will choose Kaleidoscope because I already know this game when I was young, so I partially have the idea of how it is going to work.
The code I am creating permits user to draw pixels on one-quarter of the screen in any of several different colors. Moreover, the code also allows user to mirror the pixel to the other three quadrants. For instance, a pixel drawn at (0,0) should be mirrored to (31,31), (0,31), and (31, 0). Here is the source code I have create for this game:
; 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 $04 ; purple 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
Even though the code is working, but it possibly has some errors. Before writing the code, it is better if we think about which components should be fixed or checked in order to make it work successfully. Also, giving it multiple tests to see where the errors come from.