Re: [Qemu-devel] [PATCH] PC speaker emulation

[malc]

On Thu, 19 Jan 2006, Joachim Henke wrote:

> Ok, here is it - my first attempt for emulating the PC speaker using the 
> audio API. This needs some testing, tough it seems to work well with full 
> system emulation on my iMac G5. Would be nice if someone could test this in a 
> different environment (on a PC, in user mode, ...).
>
> With my patch, QEMU plays a sine wave, that is calculated on base of the 
> count value in PIT channel 2. So it only supports tone generation (typical 
> for system beeps), but no sample playing using two PITs (I mean that noisy 
> sound from old DOS programs :) - It would probably be not that easy to 
> emulate this in realtime.
>
> The attached zip file also includes a DOS program, that came with an old 
> computer book. It plays a long tone scale, each tone half a second. You can 
> use this for testing, but maybe you have some more interesting applications.
>
> The patch is probably a bit too hackish to be included yet. As I am a bit 
> busy these days, I just sent it as a request for comments. I would prefer to 
> use a sample rate of 32000 instead of 44100 Hz, but when playing with sample 
> rates of 22050 or 32000 Hz, QEMU becomes so slow and unusable, that I need to 
> kill it. I don't know if this is something Mac specific. malc, could you 
> please have a look, if I use your audio API correctly?

Nope.

a. Writing more than `free' bytes of data is pointless
b. Once AUD_write return zero one should stop writing untill some space
   becomes available (for all intents and purposes after callback
   will be called next time)

Item b is probably why it hangs: you try to write N bytes, AUD_write
returns zero, yet you are tring again immediately - ad nauseam. We
have only one thread - hence audio can not push the data out of
internal buffers into the host, and what you get is an infinite loop.

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