Hi,

Mark Willis wrote about using a PIC to make a flash card appear as a
floppy drive to the FDC.

I think that it is *very* difficult to implement something that appears as
a floppy drive to a FDC. Remember that the FDC expects serial data from
the heads, using MFM or whatever, at 4MHz or so, sector gaps, issues
stepper (track change) impulses, generates index marks, etc. The MFM part
is a bit difficult for a PIC imho. Although I know of someone who tried
and partially succeeded to make a *network* using two specially programmed
FDCs and some hardware glue. The network was peer to peer MFM (which has
the property of passing through RF transmitters and other media). This was
a hack and did not take off either. imho 4MBps MFM for the price of a
little hardware glue using a plain PC architecture is not a bad deal. Some
ham might yet use it some day on 1296 MHz or 10 GHz ;-). Come to think of
it, a PIC could translate the track select impulses to do PLL synthesizer
programming ;-).

The usual way to implement what Mark was thinking about, is to add a BIOS
extension that will modify the BIOS boot vector and/or the floppy
interface vector. When the boot process starts, it will run code in your
ROM extension instead of doing anything with the floppy. Now, here one can
use a PIC to implement a port-mapped flash memory. For example, using a
16C64 with a PSP port connected to the PC address space, and a big flash
memory on the other side. The BIOS extension implementation requires only
a HCT688 decoder and the EPROM proper. A buffer may be required if there
are more than 2 cards on the ISA. I have done a similar thing (not a flash
device, but a solid state disk nevertheless) once. It worked but did not
take off. It was a special purpose 'drive' with setup data for another
project.  Elektor once had a project on a solid state disk using similar
methods and buckets of SRAMs for storage. My device only had 64kBytes of
SRAM storage and a 8k EPROM for BIOS extension, but it used only 6 chips
and a DIP switch (no buffer). With today's devices it is easy to make a 2M
disk like this.

Peter

PS: Sorry for the long post.