Dirty Rotten DMA problem (?) Simultaneous tape & diskette use

[David Mason]

In article <1990Oct31.190027.8613 at informix.com> aland at infmx.informix.com
(Colonel Panic) writes:
>Situation: while doing a mass cpio to the streaming tape, I try to
>           format some 3.5" floppies or dd some disk-resident floppy
>           images to them.  I keep getting i/o errors on the diskettes.
>           If I try the same operations while the tape drive is not in
>           use, no problems arise.
>
>Since both are using DMA, I suspect that the DMA buffering in the
>kernel is going haywire.  Current pertinent kernel parms are DMAEXCL=0
>(allow simultaneous DMA) and DMAABLEBUF=256.

As a matter of interest, the Interactive 2.2 (== 3.2.2) release notes
state the following, under "Known Problems and Bugs":

        "On machines with certain types of DMA chips, concurrent
        use of two DMA-based devices (such as diskette drive
        and a tape controller) can result in erroneous data
        transfers or aborted commands."

No saying which chips, of course.  Haven't seen the problem myself. 
As far as I know, AT&T 3.2.2 is written by ISC, so it is likely that
this problem will appear in AT&T 386 Unix.
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