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From: lphillips@lpami.wimsey.bc.ca (Larry Phillips)
Newsgroups: comp.sys.amiga
Subject: Re: What is the program Patch2090?
Message-ID: <786@lpami.wimsey.bc.ca>
Date: 5 Oct 89 12:04:20 GMT
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Return-Path: <lpami!lphillips>
To: van-bc!rnews

In <5660062@hpcvca.CV.HP.COM>, charles@hpcvca.CV.HP.COM (Charles Brown) writes:
>	Patch2090 V1.0 By: Khalid Aldoreri
>	Unable to find Devices.  Please reread Docs files.
>Naturally I don't have the docs files.  Can you (anybody) tell me what
>patch2090 does?  Where does it come from?  Is it safe?

I don't have the docs handy, but here's the scoop...

The 2090(a) suffers from problems of overrunning its FIFO  during periods of
intense DMA (hi-res, 4 bitplanes, etc.). When this happens, the disk operation
needs to be retried. This causes a slowdown, and if there are too nmany
retries, a read/write error. Unfortunately the entire operation is retried,
meaning that 5 retries of a 100K transfer could try to transfer 500K.

Patch 2090 attempts to alleviate this problem by watching for heavy DMA by
checking the screen parameters. If appropriate, it changes the driver's
MaxTransfer value to something lower (somewhere between 512 bytes and 4K I
think). This means that the driver has less data to retry, and more chance of
getting the request satisfied.

It is, in a sense, an 'unclean solution', though at this time, it is the only
solution short of putting a low MaxTransfer value into your mountlist and
suffering a slowdown all the time, whether it is needed or not. Khalid did go
'outside the rules' to implement the method. Since the MaxTransfer value that
is actually consulted by the file system is in the driver's data area, he could
not access it directly. To find it, he uses a special MaxTransfer value in the
mountlist, and then searches for that value, so that he knows where to change
it.

In spite of his bending of the rules, it is pretty slick, performing as
advertised. It may or may not break under any revision of the file system,
driver software, or OS rev.

I think the dealer in question has done your friends (and Khalid) a disservice
by supplying Patch2090 without docs. Khalid has written a substantial number of
programs, all freely distributable, for the Amiga, and when they bend the
rules, he says so. You might want to hit the dealer up for the docs. Meanwhile,
I will see if I can dig up the docs for you.

-larry

--
The Mac? Oh, that's just like a computer, only slower.
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|   //   Larry Phillips                                                 |
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