XEX player does not initialize POKEY correctly
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#40 XEX player does not initialize POKEY correctly
When exporting SAP to XEX with ASAP, the XEX player contains the following initialization sequence.
The sequence does not properly initialize SKCTL and both POKEYs. Before setting it or $03, it must be set to $00 to trigger the initialization sequence. Also the last STA AUDF should be and STA SKCTL.
Play the attached .XEX/ATR on a STEREO POKEY machine in Altirra. The XEX will work (as there is no SIO changing POKEY settings), the ATR will sound wrong,
013A: A2 08 LDX #$08
013C: 9D 10 D2 L013C STA $D210,X
013F: 9D 00 D2 STA AUDF1,X
0142: CA DEX
0143: 10 F7 BPL $013C
0145: A9 03 LDA #$03
0147: 8D 1F D2 STA $D21F
014A: 8D 00 D2 STA AUDF1
Fix merged.
The missing SKCTL setting was a huge issue if one plays with a stereo upgrade. Thanks for catching that!
Triggering POKEY initialization doesn't seem necessary. From my experience, most of Atari games and demos don't do that. It only initializes the polys and /28 and /114 divisors. They are already in a valid state because the OS "reset" routine initialized POKEY. You could argue that you get reproducible POKEY state (including RANDOM), but I don't think that's the case without full time sync with ANTIC. Zeroing IRQEN is a good idea so that a CLI in SAP followed by a keypress doesn't crash. Only the xexd routine did that. Writing STIMER doesn't really help as it's done before zeroing AUDF's.
I chose the route to clear all registers, because I didn't want to cause any changes in the file offsets. From the docs, my understanding was that setting D1/D0=$00 in SKCTL is required to perform the missing initialization. Now reading the docs again I understand that this only refers to the internal timer counters and the problem we have here is that D4=$1 in SKCTRL after SIO operations, creating unwanted behavior of "use timer 4 as input clock and reset timers 3+4 when waiting for start bit or a zero is received".
Nevertheless initializing all registers makes the code less error prone in all cases.
I wanted to sync my fork to you latest commits, but I cannot find a way to do that in SF. Do you have tip? And will you create a new build?
Last edit: Peter Dell 2025-02-04