Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!uwvax!provolone.cs.wisc.edu!shekita
From: shekita@provolone.cs.wisc.edu (Eugene Shekita)
Newsgroups: comp.arch
Subject: desperately seeking explanation of typical SCSI configuration
Summary: was *big iron* I/O
Message-ID: <8774@spool.cs.wisc.edu>
Date: 8 Oct 89 22:41:34 GMT
Sender: news@spool.cs.wisc.edu
Lines: 47

I've found this discussion on *big iron* I/O very interesting, but I don't 
have a clear picture in my mind of what a typical workstation SCSI
configuration looks like. Surely, there is someone out there who could fill 
in the gaps, and I'd *really* appreciate it if they did. I think a lot of
other people would too. (Unfortunately, this is the kind of stuff that 
generally gets ignored in Architecture 101 so please forgive my ignorance.)

From what I've read over the net, the way I view a typical SCSI configuration
is as follows:

    -------------      ----------
    | Processor |      | Memory |
    -------------      ----------
          |                |
     ==================================   Main Bus
          |
     -------------------    -------
     | SCSI Controller |--- | Disk|
     -------------------    -------

Is this right, or what else is there? Other basic questions I have are:

   1) How does DMA logic get added to the picture -- is it typically
      a separate unit with its own buffer, thereby allowing full-word
      transfers over the main bus instead of 8-bit SCSI words (ala Amiga)?
      Or do DMA units typically have no buffer?

   2) Dual-ported memory systems let DMA transfers go straight into memory 
      without tieing up the main bus, right? Roughly, how much cost does
      a dual-ported memory system add, and is that why they're usually only
      found on *big iron*? And while we're at it, what level of multi-porting
      do some of these *big iron* machines have (a Cray, for example)?
   
   3) Why does a VME bus help here? Do they allow bock-mode 
      transfers and that's why?

   4) Is it becoming common for SCSI controllers to have a track-size 
      buffer of their own and always do track-size reads? (I am told 
      that Conner contollers have this feature.)

   5) What does the corresponding picture look like for a *big iron*
      machine?

   6) Finally, any good reading references I could take a look at to
      find out more?

thanks in advance -- Gene