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From: george@wombat.bungi.COM (George Scolaro)
Newsgroups: comp.sys.nsc.32k
Subject: Re: Sanity Check and 'Disconnect' Questions..
Message-ID: <9009220822.AA05238@wombat.bungi.COM>
Date: 22 Sep 90 15:22:02 GMT
Sender: news@daver.bungi.com
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Approved: news@daver.bungi.com

[In the message entitled "Sanity Check and 'Disconnect' Questions.." on Sep 21, 23:38, John Connin writes:]
> 
> 
> 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 pseudo DMA read / write operations.
> 
> As I understand it, pseudo DMA reads or writes cause the signal /nrdy
> to be asserted until (a) the selected SCSI device asserts its drqs 
> (DMA Request) line, and (b) two wait states have elasped since the 
> assertion of drqs.  Furthermore, /nrdy may be asserted for an extended 
> period of time without interferring with DRAM refresh.  
> 	 
>     [X] - Yes, give him a cigar.
>     [ ] - No, you dummy.
>     [X] - Well, ......

An interrupt from the SCSI controller (which ever one is selected) will also
de-assert /NRDY for the rest of the transfer. The idea, is that the driver
will then get an interrupt and realize that the last packet was junked for
some scsi error reason.

Note also, that to use the pseudo-dma feature you must be very careful with
the transfer code. Follow the scheme that Bruce commented in the monitor
source. There is a fair bit of black magic to ensure maximum performance
with the peculiar way the 532 handles the I-cache and I/O accesses (ie it
has a major performance bug - which is documented in the monitor source).

If the comments don't make sense, then drop us a line and we'll try and
elucidate - if we can still remember all the details :-)

> interference.  However, excessive DRQ hold-off could lead to 
> inefficient processor utilization.

Yep.

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

I would have hoped that the tape would only interrupt (via scsi) when the
data was 'really' ready to be read. Is this an embedded controller type with
a reasonable local buffer or a 'dumb' external controller?

Dave & I are about to start playing with a embedded scsi tape drive also.
We should get it in the next few days. If you have some rough code worked
out we'd appreciate it if you email it to us, or any ideas you have.

> johnc


best regards,

-- 
George Scolaro
george@wombat.bungi.com                [37 20 51 N / 122 03 07 W]