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From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.arch
Subject: Re: tiny COBOL, Article on new mainframe use
Date: Mon, 02 Sep 2024 10:38:07 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
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Terje Mathisen <terje.mathisen@tmsw.no> writes:
>Thomas Koenig wrote:
[Commodore 1541]
>> And were extremely slow - around 300 bytes per second, comparable
>> to a card reader.  But fast loaders could improve on that up to 10 kB/s.
>
>They did that by reading every 1/2 sector, then reassembling after 2 
>rotations, instead of having to wait a full rotation between every 
>sector read?

No.  The disk-access part of the disk drive was actually pretty cool,
supporting variable data rate with a 6502.  The variable data rate and
group-code recording allowed getting 170KB on a single-sided
single-density disk, while FM disk controllers typically got 85KB, and
Wozniak got 140KB.  I don't remember what was done about sector
interleaving, but when you replaced the slow interface (see below) to
the computer with a fast one (I have Prologic DOS), the drive was up
to 28 times faster.

The problem is in the data transfer between the drive (which had its
own CPU) and the computer.  They replaced the parallel IEEE-488
interface of the PET with a serial interface for cost reasons, and
then they botched the serial transfer.  There were some contributing
factors, like the 6522 bug (the C64 had no 6522, though), and the
desired compatibility with the VIC-20 (which then did not happen
anyway: the Commodore 1540 (for the VIC-20) does not work unmodified
with the C64).

They probably could still have fixed the problem by giving some more
love to the firmware for the computer-to-disk-drive interface;
according to <https://www.c64-wiki.de/wiki/Schnelllader>, the fastest
serial fast-loaders were 15 times as fast as the serial routines in
the firmware.  I guess that, when the C64 was designed, disk drives
were rare for computers with the price of the C64, so it did not seem
that important to make that interface fast.

- anton
-- 
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>