Re: GCR <--CP/M --> MS-DOS and vice versa?

["Michael Evenson" <mevenson@nospam.adelphia.net>]

"Matthew Montchalin" <mmontcha@OregonVOS.net> wrote in message
Pine.LNX.4.44.0303060306150.30400-100000@lab.oregonvos.net">news:Pine.LNX.4.44.0303060306150.30400-100000@lab.oregonvos.net...
> On Thu, 6 Mar 2003, Tony Cianfaglione wrote:
> |Could a common ground be found, using these formats, to read/write Apple
> |II disks on a PC.  Since both computers can use CP/M, could software be
> |written to convert MFM material to CP/M which is then written to a disk.
> |The disk is then put in a II and read via software into something the II
> |can understand from CP/M.  Material could be written to CP/M on teh II
and
> |put into a PC to translate form CP/M.
> |
> |Let me guess...there is GCR CP/M and MFM CP/M and neither the twain shall
> |meet? :-(
>
> Well, irrc, the Z80 cartridge for the old C-64 allowed you to write
> "1541" style GCR.
>

I think the point that is being missed here is this: The Apple II disk
controller was really nothing more than a card to allow the software in the
Apple II to turn the motor on/off, move the heads in/out and cause the
read/write coils in the head to generate a magnetic field or detect a change
in magnetic field. All of this work was done by turning bits on/off in the
controllers address space and accessing routines in the cards's firmware.
The software in the Apple II was in total control. This is VERY much unlike
the process in a PC. A PC uses an NEC 765 (in the beginning - the actual
chip has changed since then, but the idea is the same). This chip does all
of the work that the software in the Apple II did. It is responsible for
turning things on and off through commands that are sent to it. These
commands did not allow the software in the PC to see the individual flux
transistions or cause individual flux transitions to occur. To move the
head, you send a command to the 765 and tell it how may tracks and in what
direction. The stepping is then done by the controller and not the software
in the PC. The PC is unaware that any stepping has taken place until the
controller tells it via either an error or a complete status. The Apple II
was capable of half stepping. Something the PC diskette controller cannot
do. This is totally aside from the GCR/MFM differences. They are two totally
separate recording algorithms. It would be akin to trying to read SDLC
streams of data with a UART. It just cannot be done. To read an Apple II
diskette in a PC would require that someone come up with an IWM chip on a PC
card. and have a separate drive in the PC to do it. Woz not only came up
with the controller card for the Apple II, he also designed the 'Analog'
board that was resident on the disk drive. This board was totally different
than the stock analog board. Plugging a PC floppy controller into an Apple
drive would be like plugging a PCI card into a newbus slot. Not only does it
not fit, but the signals are all wrong.

Back 'in the day', I used to build my own Apple II hardware from blank
boards I acquired in Japan. I had access to PROM programmers, and all of the
components required to assemble the machines, even the Analog boards for the
Shugart disk drives. I can remember even having to wrap my own chokes for
the analog card. Ah - those were the days.


Well that's my 2 cents worth.

Mike