Accidentally doing interlace mode?

[AGDDeveloper]

Hello,

 

I'm trying to get started writing some 2600 homebrew code, and I've written a very simple program to just set up a framework and get a good handle on the TIA timing.  I'm using the current version of Stella to test my code.  Right now, I just have it doing a simple loop where it counts down the 192 lines in the display region and stores the current value of the loop counter in PF1.  When I run it in Stella, most of the time I get the predicted results, but sometimes I get the image scrunched in half.  It looks like I'm inadvertently going into interlace mode, and that the resolution is cut in half, but I'm not certain that's the case or if so, why it's happening.  Any insight you might offer would be greatly appreciated!

Here's the code.  I'm also attaching Stella screenshots of the expected and unexpected results.  The unexpected results happen randomly, maybe 1 in 5 times.

Thanks in advance!

 

John
 

init:
    lda     #$00     ; Initialize some key registers.
    sta     ENABL
    sta     ENAM0
    sta     ENAM1
    sta     GRP0
    sta     GRP1
    sta     COLUBK
    sta     PF0
    sta     PF1
    sta     PF2
    sta     COLUPF
    sta     CTRLPF
    jmp     main

main:
    lda     #$0
    sta     CTRLPF   ; Zero out the PF control bits.
    lda     #$2
    sta     VBLANK   ; Turn vertical blanking on.
    sta     WSYNC
    sta     VSYNC    ; Start VSYNC.
    
; The start of our main loop.
.vsync:
    sta     WSYNC
    sta     WSYNC
    sta     WSYNC    ; There's our 3 scanlines of VSYNC.
    lda     #$0
    sta     VSYNC    ; Turn off VSYNC.
    lda     #37      ; 37 scanlines in VBLANK for the upper border.
    clc
.vblLoop:
    sta     WSYNC
    sbc     #1
    bne     .vblLoop
.frame:
    lda     #$0
    sta     VBLANK   ; Stop vertical blanking.
    tya
    sta     COLUPF   ; Set a cycling Playfield color.
    dey
    lda     #192     ; 192 scanlines in the visible region.
    clc
.loop1:
    sta     PF1      ; Store the current value of the loop counter in the PF1, to give us a pattern.
    sbc     #1
    sta     WSYNC
    bne     .loop1
    lda     #$2
    sta     VBLANK   ; Start vertical blanking.
    lda     #29      ; 29 lines of overscan (we'll do 30th line below, right before VSYNC.)
    clc
.loop2:
    sta     WSYNC
    sbc     #1
    bne     .loop2
    lda     #2
    sta     WSYNC    ; This is the 30th line of overscan.
    sta     VSYNC    ; Turn vsync on.
    jmp     .vsync

 

[+splendidnut]

Looks like PAL50 mode is being triggered.  You can use Alt-L to turn on display stats in Stella to confirm.

[AGDDeveloper]

Thank you - I figured out the problem.  I wasn't clearing the decimal flag in the 6502, so my counters were all decrementing in BCD.  It looks like Stella randomizes the decimal flag on reset.  Anyway, I fixed the bug - thank you!

[Thomas Jentzsch]

On 3/16/2021 at 1:17 AM, AGDDeveloper said:

Thank you - I figured out the problem.  I wasn't clearing the decimal flag in the 6502, so my counters were all decrementing in BCD.  It looks like Stella randomizes the decimal flag on reset.  Anyway, I fixed the bug - thank you!

Check Stella's developer settings and enable them. This will help you to notice the most common development problems (like clearing the BCD flag) early on.

[Just Jeff]

Nice job! 

 

Are you interested in refining this program?  You have some more learning opportunities here if so.    I see 8 or 9 things ranging from inefficient to incorrect here.

 

Here are two that are actually affecting this program negatively:

As spendidnut mentioned earlier, hit alt-L to see if you are displaying the proper number of lines (262).  This program is running only 259 which brings us to issue #2- you're using the carry flag incorrectly.  The carry flag should be set for subtraction, not cleared.  If you step through the code in Stella (hit the ~ key), you will see that you are actually subtracting 2 on your first time through your loops, even though you have sbc #1.  If you clear that up, you'll be running at 262 lines.

 

There are other things too..  Let me know if you are interested in reworking them..