Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!jarthur!nntp-server.caltech.edu!toddpw
From: toddpw@nntp-server.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.sys.apple2
Subject: Re: DMA?
Message-ID: <1991Apr16.042905.18910@nntp-server.caltech.edu>
Date: 16 Apr 91 04:29:05 GMT
References: <10493@mentor.cc.purdue.edu>
Organization: California Institute of Technology, Pasadena
Lines: 25

grochoci@mentor.cc.purdue.edu (Paul Grochocinski) writes:

>	I have seen DMA referred to several times. Could somebocy please
>	explain just what it stands for and what it is used for.

DMA stands for Direct Memory Access. It is a faster way to get data in and
out of the computer, usually to disks or other storage devices but it can and
has been used for a variety of purposes.

DMA is basically any method of transferring to data to/from the computer's
memory that bypasses the main CPU (hence the term 'Direct'). It is nearly
always faster than a programmed memory move (executed by the main CPU to
a peripheral device, say) because there is no overhead for reading the
instructions that perform the move. Also, DMA can be made to occur in the
background as far as the main CPU is concerned, so it can run normally
while the transfer is taking place (with a slight speed hit because it
may have to wait for the memory).

In general, DMA is a performance enhancing technique used by nearly every
general-purpose computer in existence. I don't know how to explain much
more without going into more detail than you probably want, so I'll stop
here -- I think this answered your question though.

Todd Whitesel
toddpw @ tybalt.caltech.edu