Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!sdd.hp.com!hplabs!hplred!piety
From: piety@hplred.HP.COM (Bob Piety)
Newsgroups: comp.periphs.scsi
Subject: Re: DOS/SCSI DMA?
Message-ID: <27080002@hplred.HP.COM>
Date: 14 Mar 91 20:21:27 GMT
References: <27080001@hplred.HP.COM>
Organization: Hewlett Packard Labs, Palo Alto CA
Lines: 48


 / vail@tegra.COM (Johnathan Vail) /  9:11 am  Mar 12, 1991 /

> In article <27080001@hplred.HP.COM> piety@hplred.HP.COM (Bob Piety) writes:

>	Does anyone know of the availability of a driver that hooks into DOS
>	and lets DOS and, hence, application programs communicate to a SCSI
>	hard disk via DMA?

> The DMA on an AT is much slower than the equivelent loop in software.
> The DMA is there mostly for the refresh, I believe.  If DMA is used
> then the bios for the disk would have the "driver" and DOS would still
> wait for the IO to complete.

True.  But the machine in question is an HP Vectra 486, with an EISA
bus that is capable of 33 MHZ DMA rates, and the controller is an
Adaptec 1740 HBA capable if the same data rates.

>	It sure is sad to to see a high-performance 386 or 486 spinning its
>	wheels while a disk head seeks to a given track.....

> What *else* would a PC running DOS be doing?  DOS is not multitasking
> and if a program is waiting for data from the disk then it waits for
> that data.

Agreed-- but in this case, I want to *write* a block of data to the
disk, and not wait around while it does so, freeing the 486 for
much-needed, real-time data monitoring.

I see the point you make, but it seems that it would be a relatively
simple matter to create a driver that would permit DMA writes and only
hold off the CPU if attempts were made to access the same buffer, no?  
Or is that too much to do under DOS?

> Now if you were running unix....

I'm seriously considering it!  But the additional cost and learning
curve make it less attractive at the moment.  That's why I'm hoping
for an easier solution for this specific problem, short of having to
write directly to the HBA, and maintaining my own (DOS-incompatible)
file structure, or duplicating DOS structure.

I *do* appreciate your comments, but I'm surprised that there doesn't
seem to be an off-the-shelf solution for DOS.

Thanks,

Bob          piety@hplred.hpl.hp.com      (415)857-4759