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Newsgroups: comp.sys.nsc.32k
Subject: Sanity Check and 'Disconnect' Questions..
Message-ID: <9009212338.AA04249@manatee.UUCP>
Date: 22 Sep 90 03:38:29 GMT
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Hi George,

I am posting this message to the group since I assume others may find
it of interest.

I need a sanity check regarding SCSI psuedo DMA read / write operations.

As I understand it, psuedo DMA reads or writes cause the signal /nrdy
to be asserted until (a) the select SCSI device asserts its drqs 
(DMA Request) line, and (b) two wait states have elasped since the 
assertion of drgs.  Furthermore, /nrdy may be asserted for an extended 
period of time without interferring with DRAM refresh.  

    [ ] - Yes, give him a cigar.
    [ ] - No, you dummy.
    [ ] - Well, ......

The reason I ask is that off line I have been playing around with
a SCSI tape controller trying to get a feel as how best to structure
a device driver.  Well this sucker from time to time introduces 
substantial delays (at least relative to DRAM refresh periods ) between 
assertions of the SCSI bus REQ signal.  Assuming my above understanding
is correct, then psuedo DMA transfers will not cause DRAM refresh
interference.  However, excessive DRQ hold-off could lead to 
inefficient processor utilization.

By the way, the hold-off at least wrt tape reads, appears at the 
very beginning of each 'datain' phase and during streaming breaks.

Change of subject directed to the Group:  

Has any one thought about implementing SCSI disconnect / reconnect ??  
Can anyone comment on the relative performance of systems with disconnect 
versus those without ??  On the surface, it would seem the single threated 
nature of Minix's FS could make implementation of disconnect a real pain...
Assuming there is a real performace win with disconnect, how should it
be implemented ??  Like the TTY suspend / revive ??

Best regards,
johnc
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