[stella] What Stella don't like...

Subject: [stella] What Stella don't like...
From: "Andrew Davie" <adavie@xxxxxxxxxxxxx>
Date: Sun, 4 Mar 2001 22:47:27 +1100
Some reports of screen rolling on the Stella emulator had me stuck, for a
It turns out that the following code was the problem...

        ldy #4

        lda #D1
Synch   sta WSYNC
        sta VSYNC               ; start vertical synch
        bne Synch

        sty VSYNC

This code was my 'optimised' beginning of vertical blank/screen stuff.

Here's the non-optimised (ie: working) version....

        lda #D1                ; requires D1 already in A
        sta WSYNC
        sta VSYNC               ; start vertical synch

        sta WSYNC               ; 1st
        sta WSYNC               ; 2nd   TV SYNCH (2 tested OK on PAL, 1
        sta WSYNC               ; 3rd, to satisfy ES :)

        lda #0
        sta VSYNC               ; stop vertical synch

Now, the first version (appears) to work OK under Z26, and also PCAE.  I
believe it works on the real thing, too - though I'm not exactly sure about
that.  But definitely causes screen-roll (and super-fast speed) under

Looks like the handling of writes to VSYNC is causing the problem - stella
doesn't like multiple writes.  I have no idea if my optimised version is
'legal' or not, so I'm junking it, in order to maintain stella-support - but
it's one for people to keep in mind :)

Optimisation of Qb:  -105 bytes :)

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