Simultaneous DMA

[Thomas Hoberg]

In article <2247 at pdxgate.UUCP>, brians at eecs.cs.pdx.edu (Brian Smith) writes:
|> The manual for Interactive Unix states that it is possible for some DMA chips
|> to malfunction when more than one allocated DMA channel is used simultaneously.
|> Is there any way to tell whether a motherboard has this defect?  One of the
|> boards in my system which uses DMA is the SCSI disk controller, so I'd rather
|> not test this under Unix and risk losing my filesystem.  Even a DOS program
|> to test for this problem would be fine.
|> 
This should really only concern usage of the motherboards DMA chips (Intel 8237).
Since these are really left overs from 8-bit days and run at 4MHz by default,
they are quite combersome to use and slow. Some boards like those with a C&T
chipset will allow them to clock at 8MHz, though. In any case *NO* SCSI host
adapter that I am aware of will use these on board DMA a chip, but revert to bus
mastering DMA, if they use DMA at all. Bus mastering means, they pull a DREQ line
on the I/O bus to force the CPU off the bus and then generate memory cycles
themselves. The Adaptec 154x works that way for example. The only devices
actually using the 8237s that I know of, are the floppy disk and some QIC-02
cartrige tape controllers like those from Archive (maybe some network cards,
too?). I don't know what happens if you try to use to floppys at the same time,
since I guess they have to share DMA channel 2 (probably just doesn't work,
after all the PC was a DOS machine). Whether you can use a QIC-02 tape streamer
and a floppy at the same time depends on a tunable parameter called DMAEXCL or
something like that, I *think*. In any case your file system shouldn't be
trashed by your playing around with two DMA channels at the same time.

|> My motherboard is the AMI Voyager 486-25.
|> 
|> Thanks,
|> Brian
|> 
|> Inet: brians at cs.pdx.edu
|> UUCP: tektronix!pdxgate!brians
|> 
-- tom
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