Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!dg!rec
From: rec@dg.dg.com (Robert Cousins)
Newsgroups: comp.arch
Subject: Re: DMA on RISC-based systems
Message-ID: <181@dg.dg.com>
Date: 30 May 89 12:49:47 GMT
References: <46500067@uxe.cso.uiuc.edu>
Reply-To: rec@dg.UUCP (Robert Cousins)
Organization: Data General, Westboro, MA.
Lines: 41

In article <46500067@uxe.cso.uiuc.edu> sandrock@uxe.cso.uiuc.edu writes:
>
>I am interested in the pros and cons of DMA transfers in RISC systems.
>In particular I am interested in the notion that the DECsystem 3100 has
>no DMA to its main memory, but instead relies upon the CPU to copy i/o
>buffers to/from an auxilliary memory. First, is this statement accurate?

Yes, there is no DMA in the traditional form on the PMAX.

>And second, if true, is this a reasonable tradeoff to make on a RISC system?

IMHO, no.  When designing the AViiON, 88K based workstations, we found
that it is possible to provide the increased performance from DMA
at a low cost (lower than the 3100).

>Mark Sandrock
>=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=
>BITNET:   sandrock@uiucscs	    University of Illinois at Urbana-Champaign
>Internet: sandrock@b.scs.uiuc.edu   School of Chemical Sciences Computing Serv.
>Voice:    217-244-0561		    505 S. Mathews Ave., Urbana, IL  61801  USA
>Home of the Fighting Illini of 'Battle to Seattle' fame. NCAA Final Four, 1989.
>=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=

There are some basic requirements, IMHO, which must be met to be considered
state-of-the art:  I/O which does not involve the CPU for moving every
byte, graphics which does not require total CPU dedication for normal
operations such as line drawing or bit blitting, dedicated LAN controllers
to handle the low levels of the LAN protocol and a number of similar 
minimums which most new machines have.  It is interesting, however, to
notice the number of machines which do not meet up with even the most
basic criteria.  

I make this point to begin discussion.  What are some of the minimum
standards which should be applied to these classes of machines and which
machines fail to meet them?

Robert Cousins
Dept. Mgr, Workstation Dev't.
Data General Corp.

Speaking for myself alone.