December Adventure 2024

01

Today starts December Adventure ✨ a month of cozy coding. See @eli_oat@tenforward.social‘s page for details. I want to get something small done with Minicube64 this time, to learn how games for the 6502 were made. I look at some assembly last year, mainly Nick Morgan’s 6502 and compudanzas’ uxn tutorials. But most is lost by now and I had already had some problems following Minicube64’s code.

For now I just tried to paint the screen a bit through a loop, and I learned about the difference between the main loop and the VBLANK interrupt, although I have to look that up the later. So updates to the screen need to be done in the IRQ routine if I want to have something frame based. Minicube64 offer some macros to work easier with such common things.

ldx #$01
_setw IRQ, VBLANK_IRQ
 
IRQ:
	inx
	rti

The other thing I struggled with were adressing modes. Need to get them into my head again.

02

Ehrm. Frustration. I tried to implement grabbing the arrow keys and moving that pixel on screen around but I run into so many problems that I need to take two steps back and than maybe another three or four. I tried to cross-reference my problems with Skilldrick’s tutorial and Masswerk’s 6502 instruction set, but to no avail. There is a lot I didn’t understand and maybe I should just go through Skilldrick’s tutorial again. The Minicube64 documentation is not very beginner’s friendly, so that might be a thing that could grow out of this :)

---

I got myself together, researched a bit more and was at least able to properly paint the screen, one pixel per interrupt.

The magic part was hidden in the indirect indexed adressing mode. First I set up the low and high byte of the video adress.

    PIXELS_PER_ROW  = 64
    VIDEO_MEM       = $00
 
    lda #$00                        ; Low byte
    sta VIDEO_MEM                   ; Store low byte of VIDEO_MEM
    lda #$F0                        ; High byte
    sta VIDEO_MEM+1                 ; Store high byte of VIDEO_MEM
 
    ldy #$00

Then I use that when drawing my pixel.

IRQ:	    						; Set 'IRQ' label
    jsr Draw
	rti	    						; Return from interrupt
 
Draw:
 
    lda #$20                                ; Load A with a value (e.g., pixel color)
    sta (VIDEO_MEM),y                       ; Store A at current pixel position
 
    iny                                     ; Increment X (move to next column)
    cpy #PIXELS_PER_ROW * #PIXELS_PER_ROW   ; Check if end of row is reached
    bne Continue                            ; If not, loop back for next column
 
    clc
    lda VIDEO_MEM+1
    adc #$01
    sta VIDEO_MEM+1
 
    ldy #$00                                ; reset X
    rts
 
    Continue:
    rts

I’m pretty sure there are better ways to do that, but that’s good enough for today.