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cadaver
Registered: Feb 2002
Posts: 1160 |
Exomizer on-the-fly loading/decompressing
Hey,
anyone want to share, what is the lowest disk interleave you've managed to use with on-the-fly Exomizer decompression while loading?
I'm currently at 11, using 2-bit transfer and a lame drivecode (using jobcodes only) + 1 sector buffering. However I don't think the drivecode is the problem; if I try to decrease to IL 10 the C64 often doesn't have to wait for the drive at all for the next sector's data, but occasionally the depack will take too long, resulting in missed revolution.
I've already done some optimization to the depack routine, including inlining getting a single bit (literal/sequence decision, and reading the gamma).
Don't think I would switch to another packer just for speed, but nevertheless interested in any battle stories. |
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cadaver
Registered: Feb 2002
Posts: 1160 |
I want to see the crackers reaction when they want to add an intro, but there's no space free :) Ie. do they use IFFL, add a "boot side", improve compression or what?
If we're being serious, a boot side would always be an option (free around 100 blocks), actual in-game disk flipping less preferable since you travel back and forth, and I'm not using the directory track yet. But I don't even know yet if I'll run into serious trouble, if I have extra space in the end then I'll just add extra cutscene pictures, movie credits end sequence or such. |
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algorithm
Registered: May 2002
Posts: 705 |
What lft also mentioned in a previous post. Its just as important (if not more important) to transform the data to make it more compressible or generate it before the packing. Even just simple methods such as low/high nibble swaps from byte pairs (for bitmap data) or delta for samples. |
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Didi
Registered: Nov 2011
Posts: 487 |
Don't expect any cracker to be really bothered by a full disk. ;-) |
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SIDWAVE
Account closed
Registered: Apr 2002
Posts: 2238 |
in my world, exomizer is too slow to be used in constant-loading production, so i never would.
with some buffering, the problem is solved, instead complicated code to make it "real time".
no ? |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
cadaver: make sure to also use 40 tracks then :) |
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tlr
Registered: Sep 2003
Posts: 1790 |
make that 41 + kabuto's encoding scheme... ;) |
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cadaver
Registered: Feb 2002
Posts: 1160 |
Minor update to this subject: dug up ideas to speed up drive->c64 sector transfer (for example Newcomer has a nice near-optimal transfer routine, and V-Max games like Rocket Ranger transfer multiple bytes per sync). Could get below interleave 10 that way. Custom decode in drivecode still wasn't necessary, as Exomizer (and potential slowing conditions like sprites) are still the bottleneck, but when the transfer is tuned for speed, it can be hard for drive to keep up, so I used a 256-byte table for speeding up the high nybble transfer. If custom GCR decode leaves high & low nybble in separate buffers, then it's probably also going to be fast enough without a large table. |
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Krill
Registered: Apr 2002
Posts: 2980 |
I'm still not sure whether you're going up the right alley. Why is decoupling block download and decompression using out-of-order loading and in-place depacking between the arrival of new blocks not an option, again?
You'd combine good pack ratio and as-quick-as-possible loading times while being able to independently tune the serial transfer protocol. The sub-files must be split up into separate files, then, but the overhead should be negligible.
But another thing: I've considered patching Exomizer (and other compressors) to get rid of the "safety offset", i.e., the handful of clobbered bytes beyond the unpacked data after depacking in-place.
That is, ensure that the write (decompressed data) pointer always points to memory before the read (downloaded blocks, packed data) pointer. This requires picking different (possibly less efficient) packed representations of data, but this might not be required so often and may be relevant only when crossing block boundaries. Having looked at Exomizer more recently than i, do you think this is possible? |
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cadaver
Registered: Feb 2002
Posts: 1160 |
I could examine the in-place depacking, but I'm not sure if it will work due to the safety offset requirement. For example, assume that rest of the memory is used by code, level data and graphics, and thus can't be clobbered, and I need to load & depack new music into a 2KB buffer. In worst case the music module will utilize the full 2KB. |
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Krill
Registered: Apr 2002
Posts: 2980 |
But with Exomizer, this is really just a few bytes, like 3. Surely you can work around that? :) |
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