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1990-07-05
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ID:VS Virtual DMA Service (VDS) Interface
Quarterdeck Technical Note
by Don Carroll
Q: Why do I need a VDS driver ?
A: All software running in Virtual 8086 mode (i.e. QEMM) on
a system with a bus-mastering device such as a SCSI Hard
disk controller , Network card etc. will experience the
inability to determine the correct address of the
DIRECT MEMORY ACCESS (DMA) region for that software.
The Problem of QEMM running with a Bus-mastering device,
(such as a SCSI hard disk controller) is that the device
will not put the data in the right memory address. This is
because QEMM moves the physical address. One of the
features of QEMM is its VDS Interface. This is the part of
QEMM that the VDS driver makes a request to so the correct
address can be determined.
Examples using Adaptec 1542 SCSI VDS driver & QEMM.
There are 3 potential ways to make this work
1) ASPI4DOS is only 5K in size. That is not much to worry
about on most systems. If you have the room then leave it
in conventional memory.
(Note: ASPI4DOS must be loaded before QEMM!)
2) The next method is to set the QEMM parameter DISKBUF=1
and have QEMM load ASPI4DOS into HIGHRAM. This only uses
1K of Conventional memory. This lets ASPI4DOS load and
start running. ASPI4DOS then takes over control of QEMM's
DISKBUF Function.
3) If you are running on a system that is tight on memory
and you need all the conventional memory you can get, then
the following is a way that you can get the ASPI4DOS driver
loaded into HIGHRAM and not use any conventional memory.
You have to have a 386 with C&T Shadow Ram Hardware and
also buy QRAM . You load QRAM so that it uses 16k of
shadow ram , I used B000-B3FF Because I am running a VGA
card . You then Loadhi ASPI4DOS.SYS and any other SYS
drivers that will fit , for example I loaded VDISK.SYS .
This used 6k out of 16k. Next comes QEMM which you have
EXCLUDE the 16k area used by QRAM or if you know how much
memory the drivers are using then only EXCLUDE that amount
. In my case it was 8k (B000-B1FF). You can now load
everything normally
* * * E N D O F F I L E * * *