Re: Commodore PLC TIB DD-001 / Drive 2001

From: Mia Magnusson <mia_at_plea.se>
Date: Fri, 25 May 2018 17:03:56 +0200
Message-ID: <20180525170356.00001953@plea.se>
Den Thu, 24 May 2018 13:17:50 +0100 skrev smf <smf@null.net>:
> On 24/05/2018 00:59, Mia Magnusson wrote:
> > But not worth the effort with a mechanical disk, as even only using
> > half the capacity will be enough for any disk you'll find in the
> > junk box.
> 
> No good for data transfer though & supporting 16 bit transfers is
> pretty trivial hardware wise.

Yeah, unless you use some proprietary file system which anyway needs
conversion to be read/written on a modern computer. But you wouldn't
use mechanical disks to transfer data (in general - for the one-off
transfer they are fine) as you would soon end up with a dead disk
because you did move it while it were spinning down.

Anyone who actually wants to move data will probably use CF cards and
they should be able to handle 8-bit transfers.

> > You will not gain anything by using the various "DMA" modes on an
> > IDE/CF disk with a 1MHz 6502 system as the disk is faster than the
> > computer even in the disks slowest mode.
> 
> You would gain quite a bit. A 6502 copy loop will take something like
> 14 cycles per byte to copy, while using DMA you should be able to
> average 1 cycle per byte.

DMA here refers to various bus modes for data transfer to/from the
disk, not actual DMA in the host system.

It's only the stupid PC computers where PIO and DMA mode of an IDE disk
corresponds to actual host bus activity, as the IDE connector maps 1:1
to the ISA/x86 bus. (An IDE controller on a PC is actually only an
adress decoder and maybe bus buffers).

Re speed, you will have to dig deep down the box of old mechanical
disks to find a disk that can't cope with 1MHz 6502 bus speed. I can't
remember how slow you are supposed to run with the older PIO mode
0/1/2/3, but the fastest PIO mode 4 surely is faster than even a full
DMA on a 1MHz 6502 bus. And I see no reason to use mechanical disks
that are smaller than say 0.5GB or somewhere around where PIO mode 4
became standard on all disks.

Those older slower disks would fit better in an old PC, for example a
5,25 Connor 60MB disk were one option for the original Compaq Deskpro
386/20.

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Received on 2018-05-25 18:00:08

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