zerox86 version 0.06 beta released!

July 28th, 2014, 20:28 Posted By: wraggster

This version has a couple of major architectural changes, a few configuration improvements, and several game-specific changes. I wrote about some of the changes in my previous blog post, so I will only mention those briefly in this post. I did some more work to zerox86 since my blog post from last weekend, and will write longer chapters about these new changes. I like to order my change logs so that the new features are first, then the fixes that affect several games, and lastly the game-specific fixes. This is why the changes are not usually listed in chronological order.

Enabled Virtual Memory (paging) support in zerox86
This is the major architectural change in this version. I wrote a rather detailed blog post about this change last weekend, so see below for more information about this. Please note that this does not mean that every game that uses virtual memory will now work, it just means that support for those games can now be added.

Implemented a slowdown feature using key MHZ in zerox86.ini
This is a new configuration feature that you can use to slow down zerox86 for those very old games. The value of the new MHZ key should be an integer corresponding to the clock speed of a 80386 processor you wish to emulate. If you give any value outside the range of 1 .. 79, there will be no slowdown applied, so zerox86 runs at the approximate speed of a 80MHz 80386 (or 40MHz 80486, as the 80486 CPU is about twice as fast per clock cycle as the 80386).

When running those old games, you should use rather low values, anything between 1 and 4 should be worth a try. In this version the MHz value needs to be an integer, but if you feel this is too coarse, let me know and I'll look into implementing finer control in the next version. Since the value is only an approximation, the actual slowdown will vary depending on how the game is programmed.

Implemented further optimizations to IRQ emulation
This is again something that I already mentioned in my previous blog post. This change should allow me to add support for the PC Speaker digitized music that some games use, but I did not have time to implement it into this version yet.

Increased max number of game configs in zerox86.ini to 512
I had only allowed for 128 game-specific configurations in the zerox86.ini. This turned out to be pretty low (some of you seem to really like your DOS games!), so I increased this limit to 512 in this version. The limit can be increased still further, but this is probably enough for now, I believe.

Fixed Jazz Jackrabbit audio crackling
The compatibility wiki mentioned Jazz Jackrabbit having an audio crackling problem in addition to the pause menu selector problem that I wrote about in the previous blog post. Since I got the menu fixed, I thought it would be a good idea to make this game fully supported and fix the audio quality as well. I had not listened to the game all that closely, and when I did, it was pretty obvious that the audio quality was not as good as it could be.

First I needed to find out what actually goes wrong in the audio rendering. I added a memory buffer where I wrote the last 16 audio blocks that my SBEmulation() routine creates. I did not add any file writing feature to my code, as it occurred to me that I could simply use GDB for that. This is what the new SDL audio_callback routine looked like, after I added my memory logging mechanism (with the JAZZDBG define) for the created audio samples:

#define JAZZDBG 1
#if JAZZDBG
#define JAZZBUFSIZE (256*16)
short jazzbuf[JAZZBUFSIZE];
int jazzbufpos = 0;
#endif

void audio_callback(void *userdata, Uint8 *stream, int len)
{
// Write the Sound Blaster samples to the stream (or clear the stream if nothing to play).
int Need_SB_IRQ = SBEmulation(stream);
#if JAZZDBG
memcpy(jazzbuf + jazzbufpos, stream, 256*2);
jazzbufpos = (jazzbufpos+256)&(JAZZBUFSIZE-1);
#endif
// Write the AdLib emulation samples to the stream. 2 times 128 samples.
AdlibEmulation(stream);
AdlibEmulation(stream+256);
if (Need_SB_IRQ)
SendSBIRQ();
}

I then ran Jazz Jackrabbit, and attached GDB to the running zerox86.exe, looked up the address of my jazzbuf, and dumped that to a file using the GDB command dump binary memory jazzdump.raw 0x1ac8620 0x1ac8620+256*16*2. Next I started up Audacity and imported the raw sample data.

http://zerox86.patrickaalto.com/zblog.html

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