Hallo Albert

 > I wondered if ADD must be used for physical device and load under
 > BASEDEV= ?

A physical device driver need not be a BASEDEV, however for a device driver to 
be recognized by a FLT/DMD/..., it has to be. The device helpers which interface 
to the kernel data structures linking ADDs and DMDs together are unavailable in 
standard device drivers (checked in 2.0GA). Else I would have implemented my DMD 
tape driver as plain DEVICE driver.

 > If writing it as an ADD is possible for a non-physical device

Why not? But it doesn't have to be an ADD, such a driver looks a lot more like a 
DMD. Anyway, the extension only determines the order in which these drivers are 
loaded, it does not detemine the way they react on calls. Whether a BASEDEV acts 
like an ADD, a FLT or a DMD is solely detemined by the init phase code of the 
driver itself.

A DMD like driver does not have to present more than the normal OS/2 1.x based 
DEVICE driver interface and it obeys the same rules, it can even use blocking 
i/o. Just init phase is significantly different as it runs at ring 0 instead of 
ring 3 and doesn't support DosXxx AIPs. I don't think it is a big job to convert 
some VDISK.SYS source code into a VDISK.DMD.

There is one restriction however: HPFS386 requires a device driver to work 
without blocking. So HPFS386 forbids use of virtual memory within a block device 
driver.

		Gruss
		Andreas

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