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

From: Eric Smith <eric_at_brouhaha.com>
Date: Tue Dec 9 19:00:42 2003

I wrote:
> If the RAM is organized 8 bits wide, the cycle time doesn't need to be
> faster than 200 ns (assuming 25 ns sampling). That's not particularly
> fast.

I should clarify that I'm assuming that the host uses buffered seek
and properly uses the -SEEK COMPLETE signal from the drive.

If the drive does not use buffered seek, it is necessary to transfer
a new cylinder's data into the buffer RAM in less than the seek time.
As far as I know, the highest capacity 5.25-inch Winchester drives that
did not use buffered seek had no more than four heads, 615 cylinders,
and no less than 20 ms seek time, so the maximum transfer rate needed
to transfer data to and from the backing disk is about 33 MB/s. This
would require RAM with a 30 ns cycle time. More practically, the
buffer RAM could have a x16 or x32 organization, stretching the
cycle time requirement to 60 ns or 120 ns.

I think a disk emulator would still be a useful product if it ONLY
supported emulation of drives with buffered seek capability. Perhaps
a different model could be used for drives without buffered seek; such
a model might simply buffer the entire drive contents (about 205 MB).

Eric
Received on Tue Dec 09 2003 - 19:00:42 GMT