DISK device drivers

[THOMPSON,JOHN C]

I am working on project for a scsi 2 device which fully supports multiple
requests to this device simultaneously. Scsi 2 supports tagged commands for
disk devices so that you may send multiple commands to a device while others
are still pending completion. My question is does XENIX or any version of UNIX
support queing commands to disk devices? If multiple processes make independent
requests to the same device will each request be sent to the physical device
driver one at a time as the previous command completes or will all requests be
sent to the pdd for queing? If multiple requests are indeed forwarded to the
pdd are there any existing pdds for scsi host adaptors which will forward the
requests to the device rather than process them single file? Sort of multithreading to one device.
Also does sco have a physical device driver (pdd) development kit? Thanks.
-- 
THOMPSON,JOHN C
Georgia Institute of Technology, Atlanta Georgia, 30332
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