Should the RIOT timer decrement immediately after initialization?

[Smidge]

I've been writing a 2600 emulator from scratch and using Stella's debugger mode to help track down some timing issues, and I noticed something odd with how Stella handles the RIOT timer after it is initialized. In the game Dolphin, TIM64T is initialized to $30, and after the next instruction (LDY SWCHA, 4 cycles), the timer immediately decrements to $2f instead of holding the value $30 for 64 cycles. After that the timer is decremented every 64 cycles as expected. This I identified as the reason my emulator started drawing the first non-garbage frame of Dolphin one scan line later than Stella. I tend to trust that Stella is doing everything correctly, but is this actually the right behavior?

 

To add more background, here is the relevant segment of code from Dolphin:

f647 LDA #$30   ;2 cycles
f649 STA WSYNC  ;3 cycles
f64b STA TIM64T ;4 cycles
f64e LDY SWCHA  ;4 cycles

Immediately after STA TIM64T (scan cycle = 4), Stella sets TIM64T and INTIM to $30 and resets TIMINT and Total Clks to 0. INTIM Clks is unchanged from its previous value of 1.

Immediately after LDY SWCHA (scan cycle = 8), Stella decrements INTIM to $2f. TIMINT remains at 0. Total Clks is now 4, and INTIM Clks in $3d.

 

I checked another game (Adventure) and observed the same behavior. So who's right, me or Stella? (Okay obviously Stella's right, but why?)

 

[Thomas Jentzsch]

AFAIK the timer is described that way in the tech documents.

[Smidge]

Interesting, that means the example given in the Stella guide that setting TIM64T to 100 would cause the timer to take 6,400 cycles to reach 0 is inaccurate (it would need to be set to 101).

 

I wrote a quick program to test this behavior on real hardware (source attached). It sets TIM64T to 1 and then immediately checks INTIM. If it's 0, the background is set to blue. Otherwise, it's set to red. I confirmed that in Stella and on real hardware the result is a blue screen. As expected my emulator incorrectly displays red, but it won't for long.

timer-init-test.asm