Disk hardware emulation, was Re: Grandfather system RTE6/VM?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Dec 10 18:22:46 2003

>
> On Wed, 2003-12-10 at 01:07, Tony Duell wrote:
> > It was suggested that the emulator should contain enough RAM to buffer
> > the entire disk image,
>
> That does seem a little over the top to me. Ditto with suggestions for

I woulld agree...

> various ways of condensing the data stream - if the buffer just needs to
> be a couple of MB, then that seems fine - 2MB of fast RAM would not be
> unreasonable.

No, I think 512K SRAM chips are now easy to obtain and cheap. Certainly
128K ones are, and it's only 16 of those (which is not excessive). I'd
want to organise the RAM as at least 16 bits wide, maybe 32 (to
simplifiy the timing problems in the rest of the hardware), so larger RAM
chips wouldn't be a big help.

> I'm with Tony on the USB side of things :-) The maximum cable length
> between drive and controller as Tony mentioned the other day is an issue
> though, and one I'd mean to ask about a little while ago. I'd assumed
> the average length of 10" or so was more due to physical constraits than

I think it partially depends on what drivers and receivers are used, and
how well the cables are terminated. I've probably broken all the rules,
but I have sent ST506 signals down a couple of feet of cable without
problems.... Just don't have the classic one one side of the room and the
PC-with-the-emualtor on the other....

> That does imply that *something* needs to be inside the classic chassis
> due to electrical restrictions. That in turn seems to imply that the
> whole lot - in an ideal world - needs to fit in a 5.25" half-height bay;
> interface electronics and modern drive replacement. Of course most
> classics would have more space than this though, but I'd suggest that
> for a final interface things at least fit in a 5.25" bay for convenience
> of mounting, even if the replacement drive has to lurk somewhere else.

I was hoping to do that (although I was also thinking of a full-height
bay....

> Having one drive per classic doesn't help a) at all, but reliable drives
> in the order of 1GB or so can be found for free and are likely to keep

The simplest hardware would seem to take 2-3Gbytes for a maximum-size
ST506 image. Most will be a lot smaller than that. I think 20-40Gbyte IDE
drives are pretty cheap now....

> running for a good few years yet (at which point I can switch to
> whatever capacity is then the freebie of the day. So b) becomes
> irrelevant (I'd still much rather use SCSI drives as that's what I have
> spares of - I don't keep IDE drives lying around)

I don't have either 'spare'. It really comes down to which makes the
hardware interface easier. I'd like to try DMA (see the other message) --
this is not too hard on a SCSI drive, I think it's OK on most modern-ish
IDE drives.

>
> Point c) about backup can be rethought. All the interface needs is
> *some* way of getting data out of it (and back into it, presumably :)

A really kludgy way to do this is to pull the drive from the emulator,
cable it up to a PC, and copy the image over using dd or something. Not
elegant, but it'll work (remember the data on the modern drive is stored
in normal sectors on said drive, so a PC can read it, even if it can't
make sense of it).

-tony
Received on Wed Dec 10 2003 - 18:22:46 GMT