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From: sph@logitek.co.uk (Stephen Hope)
Newsgroups: comp.periphs.scsi
Subject: Re: DOS/SCSI DMA?
Message-ID: <sph.669040581@krypton>
Date: 15 Mar 91 12:36:21 GMT
References: <27080001@hplred.HP.COM> <2154@atlas.tegra.COM>
Organization: Logitek Plc.
Lines: 47

vail@tegra.COM (Johnathan Vail) writes:


>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.

>   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.


>Now if you were running unix....

>jv

This is not strictly true. DMA is not as fast as an I/O loop, but the
latency between a request for transfer and response for DMA is on the
order of 1uS or less. I/O loop transfers are only "better" for peripherals
with a buffer (so that latency is less important), and where transfers
are block oriented, ie each transfer moves a large amount of data. For
example, older LAN cards with no buffering (eg 3Com 3C501) only work well
if DMA is used - Yes  I know this is an opinion, so no flames. 

I can also think of examples where DMA is better, eg where disk to tape
backups are taking place. If DMA is used for disk to memory and tape to
memory transfers, then the processor can just supervise the whole
process. I believe something like this is used by DOS backup packages.

Stephen
>"Everything that gives us pleasure gives us pain to measure it by."
>    -- The Residents, GOD IN THREE PERSONS
> _____
>|     | Johnathan Vail | n1dxg@tegra.com
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