Modifying the 6502 Assembly Program

Hello, Blog!

As promised, in this post, I’ll share the small tweaks I made to modify the functionality of the screen colouring 6502 assembly program.

• Check out the previous post for complete breakdown of the base program's logic: 6502 Assembly: Calculating Code Performance
• Find the 6502 emulator I used here: 6502 Emulator

If you have just stumbled upon my SPO600 series of blog posts, it has been created to document and share my learnings as I progress through my Software Portability and Optimization college course.

Taking a look at the given program

The following code fills the emulator's display with the yellow colour:

 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 

Filling the display with a different colour

To fill the display with the colour of choice I just needed to load the value associated with the colour into the accumulator.

• You can find information about the colour values by clicking the "Notes" button in the emulator and scrolling down to the "Bitmapped Display" section.

We can see that light blue = $e

lda #$00 sta $40 lda #$02 sta $41 lda #$0e ; load $0e (light blue) instead of $07 (yellow) into the accumulator ldy #$00 loop: sta ($40),y iny bne loop inc $41 ldx $41 cpx #$06 bne loop 

Filling the display with different colour on each page

To colour each of the four pages with different colours, I decided to increment the colour value stored in the accumulator each time a page gets filled.

 lda #$00 sta $40 lda #$02 sta $41 lda #$07 ldy #$00 loop: sta ($40),y iny bne loop inc $41 adc #$01 ; increment the value in the accumulator, so the next page is coloured with different colour ldx $41 cpx #$06 bne loop 

Making each pixel a random colour

Achieving this was a bit trickier, as getting random colour values involved using pseudo-random number generator located at address $FE in the emulator's Zero Page.

 lda #$00 sta $40 lda #$02 sta $41 ldy #$00 loop: lda $fe ; load colour value from the pseudo-random number generator on each loop iteration sta ($40),y iny bne loop inc $41 ldx $41 cpx #$06 bne loop 

P. S.

Playing around with the Assembly code provided by the professor was fun. I found that the key to effectively tweaking the code was a solid understanding of the logic behind the original program.

Thank you for reading my post!