Newsgroups: comp.sys.cbm
Path: utzoo!utgpu!jarvis.csri.toronto.edu!godzilla.eecg.toronto.edu!leblanc
From: leblanc@eecg.toronto.edu (Marcel LeBlanc)
Subject: Re: SEQ file access speedup
Message-ID: <89Feb18.010253est.2384@godzilla.eecg.toronto.edu>
Summary: no read-ahead for PRG, SEQ files
Keywords: Interleave, fast SEQ access
Organization: EECG, University of Toronto
References: <89Feb10.182100est.2732@godzilla.eecg.toronto.edu> <89Feb14.171816est.2394@godzilla.eecg.toronto.edu> <335@wn2.sci.kun.nl>
Date: Sat, 18 Feb 89 01:02:49 EST

In article <335@wn2.sci.kun.nl> janhen@wn2.sci.kun.nl (Jan Hendrikx) writes:
>In article <89Feb14.171816est.2394@godzilla.eecg.toronto.edu>, leblanc@eecg.toronto.edu (Marcel LeBlanc) writes:
>>                     That's because the dos in those drives only buffers a
>> sector at a time, which forces it to use an interleave scheme.
>
>That is not true. 1541 DOS does do read-ahead when there are enough
>free buffers. When a new buffer is needed, and all are occupied,
>one of the read-ahead buffers is discarded.
>
>Source: Inside Commodore DOS, and a ROM disassembly.

I don't remember under what situations the DOS will do read-ahead, but the
point was that since the DOS follows the interleave chain, it won't send the
file any faster than the interleave allows, no matter what sort of interface
you are using.  In the case of a 1541, following the interleave chain will
only get you about a 5-6x speedup with the standard 10 sector interleave.
If this problem isn't addressed, using faster hardware buys you nothing.

Since you brought it up, under what situations will the DOS do read-ahead?

Marcel A. LeBlanc	  | University of Toronto -- Toronto, Canada
leblanc@eecg.toronto.edu  | also: LMS Technologies Ltd, Fredericton, NB, Canada
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