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

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Dec 10 17:54:54 2003

> Is it not true that the precomp circuits are driven by head position
> info - thus the 'solid' version (emulation) of any drive would just be

Generally, yse...

> permanently 'stuck' in one mode or the other would the controller care, or
> even know, what track and cylinder the actual 'data' were coming from?

Ih, but it does matter...

The point of precompensation is tha when you write magnetic transitions
to the real disk, they don't end up quite where you expect (this is a
sort-of cimplification, but it will do for the moment). So you change the
timing of some of the pulses in the bitstream, so that when you write the
_modified_ bitstream and read it back you get what you had in the first
place (before precompensation). Thus, on reading, the controller sees an
_unmangled_ bitstream coming from the drive. By unmangled, I don't mean
what was fed into the drive when it was written. I mean what was fed into
the precompensation circuit, which might well be part of the controller.

Now, the emulator doesn't ever mangle the bitstream. So if you write a
precompensated bitstream to the emulator, and then read it back, what you
see is the precompensated bitstream, with the timing of some pulses
changed. This is not what the controller expects. Maybe it'll work, maybe
not.

-tony
Received on Wed Dec 10 2003 - 17:54:54 GMT

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