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

From: Eric Smith <eric_at_brouhaha.com>
Date: Tue Dec 9 20:30:05 2003

I wrote:
>> 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.

"James Dickens" <jdickens_at_ameritech.net> wrote:
> okay not an expert, but most systems using these drives used interleaving
> of
> sectors(because there was no way the system was fast enough to handle the
> data), the IBM PC used a factor of 6.

Interleaving by the host is completely irrelevant to the disk emulator,
which doesn't even have a concept of "sectors". To the disk emulator,
a track is just a big collection of samples of the data line, which
don't even have a 1:1 correspondence with host data bits (or even host
channel code bits).

> The machine the device would not
> know
> what to do with 30MB/s of data if you produce it.

The 33 MB/s I quoted was the necessary transfer rate between the buffer
memory and the drive *inside* the disk emulator. It has nothing to
do with the host computer.
Received on Tue Dec 09 2003 - 20:30:05 GMT