Re: DSK to NIB Instruction

["josephoswaldgg@hotmail.com" <josephoswald@gmail.com>]

Ed Eastman wrote:
> Linards Ticmanis wrote:
> > Actually judging by the source code, most emulators seem to "nibblize"
> > the .dsk image internally and then feed it to the emulated controller.
>
> Interesting.  The Emulator takes the sector data and converts it into a
> byte stream to feed the controller?  I guess it feeds standard sector
> headers and the like then, eh?  Probably doesn't feed any sync bytes then...

I think they include a few FF bytes just for kicks. (You need the
spacing between the address & data fields to allow time for the address
to be decoded.)

They don't feed the controller so much as feed the read data register
address. This allows them to support the RWTS code (remember also that
many DOS 3.3-compatible programs use their own stripped down version of
DOS) which reads disks; the code keeps looking at $C0E8, the simulator
changes to a new NIB data byte every 32 simulated cycles, and RWTS
finds all the markers it is looking for.

One could write a circuit-level simulation that took read pulses at
arbitrary points in time, and drive the Disk-II controller with the
low-level bus signals, and then you would have to include sync bytes
with extra zeros and the whole bit. I've done this, but I've only run
it for a few simulated bytes to make sure I understand the Disk II
design. Using it for a simulator is serious overkill---you'd have to
translate to even a lower level than NIB data, for no real gain.