Disk hardware emulation, was Re: Grandfather system RTE6/VM? - test-drb@ccmp.vtda.org

7 Dec 2003

On Dec 7, 19:26, Tony Duell wrote:

...
  OK, many machines used Western Digital hard disk
controller chipsets and
...
  had relatively standard formats, but others
didn't. Some manufacturers
...
  used homebrew controllers -- 2910-based state
machines, 8X300-based
 microcontrollers, ASICs, etc. 
RQDXn controllers come to mind :-(

...
  > the interface is set to write and the relevant
drive is selected, and
...
  > stop when it isn't.  Similarly it can replay
the data stream when
 > requested.  There's nothing magical about a sector, it's just a
stream
...

 A minor correction : There is no 'request' for outputting data, other
 than selecting the drive, head, and cylinder. When that's done, you have
...
  to keep on squirting data to the controller. 
I meant it can stop as soon as the drive is no longer selected.   Most
systms, if they want to read a sector, assert the select, wait for the
correct header to come around, and then de-select as soon as they've
got it.  A lot don't of course; they wait until they've verified the
checksum, and if it fails, they expect to see the same sector come
around again.  Or they may keep the drive selected for a while in case
whatever software is asking for sectors, asks for another one.

...
  > So Tony's idea could handle everything
including an LLF, for any format
...
   and any
encoding scheme you like. 
 Which, IMHO, is important. 
No, it's essential if emulation is going to work for anything other
than the simplest case of a PC with an ST506 controller (which is
trivial to replace with IDE anyway). :-)

BTW, it occured to me that you can alleviate the transfer rate problem
on the real drive by splitting the data stream, like RAID systems do.

-- 
Pete						Peter Turnbull
						Network Manager
						University of York