OK back to business. Here is a short addition to the previous “character stars” program that adds some random colors to it. The new file can be downloaded here. As before, the file suffix has been changed to fool WordPress. The file is plain ascii asm code.
So what has changed to the previous routine you ask anxiously. Not much actually. First we add some new hard-coded memory locations to the routine just like we did with the screen memory. This is the memory area that defines the color of a specific location on the screen. $D800 (55296) is the address where the color memory starts by default.
SCRMEM1 .byte $00, $04, $FF, $04, $FE, $05, $FF, $06 ; screen memory
COLMEM1 .byte $00, $D8, $FF, $D8, $FE, $D9, $FF, $DA ; color memory
Then we modify the TRANSFER routine so that we take up (to zero page addresses $BD & $BE) the screen color address for the same screen location we randomly chose to draw our character to:
TRANSFER1
CLC
ROL ; get correct screen memory location
TAY
LDA SCRMEM1,Y ;first half of screen address
STA $BB
LDA COLMEM1,Y ; first half of color address
STA $BD
INY
LDA SCRMEM1,Y ; second half of screen address
STA $BC
LDA COLMEM1,Y ; second half of color address
STA $BE
JSR TAILPART
And finally we add some new lines to the DRAW routine that will eventually draw the character on the screen:
DRAW
STA ($BB),Y ; print character
JSR RNDBYT
AND #$0F ; scale to 0-15
STA ($BD),Y ; store value to color memory
RTS
What next?
I have been looking at cc65 a C-crosscompiler that can generate code for Commodore64. I am most likely to go on creating a simple game using sprites and character graphics either with C or assembly. I am also looking at how to generate high resolution (300×200 pixels) graphics in assembler, but that may take a while. I may also expand this blog to cover some of my beginner electronics projects such as this.
Filed under: Commodore64
