secondary FDC on PC with MS-DOS?

Chuck Guzis cclist at
Tue Feb 19 13:07:43 CST 2008

>From Andrew Lynch:

> I searched around a bit and found SDRIVE.SYS which refuses to
> acknowledge any of the other FDCs I place in the computer at the
> secondary address (IO address $170).

I/O port 0170H refers to the secondary *hard drive* address (the 
primary being 01f0H).  The secondary FDC address is 037xH (the 
primary is 03FxH).

Let me look around--I've got several old MS-DOS secondary port 
drivers kicking around here.

A word of warning, however.  For a secondary controller using the 
same DMA and IRQ (2 and 6) as the primary, the primary needs to have 
implemented the "enable" bit at I/O port 03F2H so it can float its 
own DMA and IRQ lines when the secondary controller is active.  Not 
all controllers will do this, even though it's documented as part of 
the standard PC architecture.

If you want to check out the operation of your secondary controller, 
hook a drive to it and find copy of FORMATQM or COPYQM (or 22DISK...) 
and set up a DISKETTE.CFG file for it to use.

In the meantime, I'll look for a good driver.


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