SPO600-Lab2 (Experimenting with 6502 emulator)

In this lab, I have been given a bitmap code which will fill the emulator's bitmap display with the yellow color. Later I was given some task to modify the code in such a way to get different output color for the entire display. I was also asked to show four different color in four different pages. In this display one quarter is refer to as one page. I was encouraged to do some further experiment then. So I will start from the very first task.

I assembled and run the initial code on the emulator and below is my raw code and output.

lda #$00	; set a pointer in memory location $40 to point to $0200
	sta $40		; ... low byte ($00) goes in address $40
	lda #$02	
	sta $41		; ... high byte ($02) goes into address $41

	lda #$07	; colour number

	ldy #$00	; set index to 0

loop:	sta ($40),y	; set pixel colour at the address (pointer)+Y

	iny		; increment index
	bne loop	; continue until done the page (256 pixels)

	inc $41		; increment the page
	ldx $41		; get the current page number
	cpx #$06	; compare with 6
	bne loop	; continue until done all pages

This was the initial code that was given which is supposed to produce yellow color.

                                                Here you can see the yellow bitmap display

Calculating execution time:

Modifying the code:

1. Here at first I was asked to change the given code to fill the display with light blue instead of yellow.

For this task I have given a reference chart showing below

• $0: Black
• $1: White
• $2: Red
• $3: Cyan
• $4: Purple
• $5: Green
• $6: Blue
• $7: Yellow
• $8: Orange
• $9: Brown
• $a: Light red
• $b: Dark grey
• $c: Grey
• $d: Light green
• $e: Light blue
• $f: Light grey

From here we can see that to change the display to light blue we need to modify the value of color number lda #$07 to #$e.

After modifying that code below is my output display.

                                     

2. In this task I was asked to modify the code to display four different color in four page. Each quarter of the display is a page. For this I stored the color in memory location $10. Next is when the page incremented I use increment value of $10 so that the accumulator loads with the incremented color code value of $10.

lda #$00 ; set a pointer in memory location $40 to point to $0200

sta $40 ; ... low byte ($00) goes in address $40

lda #$02

sta $41 ; ... high byte ($02) goes into address $41

lda #$5 ; colour number

sta $10    ; store colour number to memory location $10

ldy #$00 ; set index to 0

loop: sta ($40),y ; set pixel colour at the address (pointer)+Y

iny ; increment index

bne loop ;  continue until done the page (256 pixels)

inc $41 ; increment the page

  inc $10  ; increment the color number

        lda $10 ; colour number

ldx $41 ; get the current page number

cpx #$06 ; compare with 6

bne loop ; continue until done all pages 

This is my modified code to obtain four color in four page

                                              Here is the output of having four color in four pages

Here you can see that I start with value 05 which is green, then it incremented to 06 which is color blue,

then 07 which is yellow and lastly 08 which is orange. We can also set the initial color to a random choice by assigning it $fe so that everytime we run the code it will produce different output colors.