Re: [hatari-devel] rev 3689 : splitting cpu cycles above 20

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Hi Nicolas,

I've splitted cpu instruction not for the DSP, but for the sound (maybe it's not useful and in this case, I'll remove it) :

At 50000 Khz 16 bits stereo, the crossbar needs an internal interrupt every 60 cycles.

A movem with all the registers can take more than 150 cycles.

A demo like Amanita (DHS) uses a lot of these movem.

So, I've decided to split the big instruction (in cycle) to be sure to execute all the sound updates between the crossbar and the DSP (in SSI mode essentially).

Do you think this is a good idea ? (I think Amanita sounds better with it, but I'll have to listen many times with and without the cycle splitter to be sure of this).



Le 07/01/2012 00:13, Nicolas Pomarède a écrit :
Hello laurent

I saw your patch to split cycles when cpu cycles > 20 and call dsp on every chunk of 20 cycles.

Did you do this for a particular demo/game where this improves the emulation ? Or is this just a test ? Because I don't think doing this will change anything to cpu/dsp emulation.

The point is that once you know curr_cycles, it's too late to call the dsp in chunks of 20 cycles, because the 68030 instruction has already been emulated, the possible read/write were already done, so splitting the resulting number of cycles in chunk of 20 cycles will not help the dsp.

From the point of view of the dsp, all read/write/memory accesses made by the 68030 were made at cycles 0. If you split in chunk of 20 after, it won't change anything, the cpu has already altered its registers or the RAM or the data exchanged with the dsp.

What you really need is not call dsp_run more often *after* the cpu emulation, it's to call dsp_run every 4 or 8 cycles *while* emulating *every* cpu instructions (the same things that winuae does in the most precise 68000 cycle exact mode).

But doing that is really complicated, because you need to know the inner working of each 68030 instruction, and that's unfortunately not described in any official doc.


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