Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!unmvax!gatech!bbn!bbn.com!tchu
From: tchu@bbn.com (Tom Chu)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Need info on 16-bit DMA for AT
Keywords: 8237-5, AT Bus
Message-ID: <37068@bbn.COM>
Date: 10 Mar 89 13:16:53 GMT
References: <1901@randvax.UUCP>
Sender: news@bbn.COM
Reply-To: tchu@BBN.COM (Tom Chu)
Organization: Bolt Beranek and Newman Inc., Cambridge MA
Lines: 38

In article <1901@randvax.UUCP> edhall@randvax.UUCP (Ed Hall) writes:
>I'm developing (for my own education) an AT interface card using
>DMA for data transfers.  I know how to do this for PC's, and by
>extention, how to do 8-bit DMA interfaces for the AT.  But what
>is different about the 16-bit channels on the AT?
>
>For instance, I know that the second DMA controller (channels 5 through
>7) control address lines A1-A16, thus covering a 128K window of two-byte
>words.  Other than this, what is different about 16-bit DMA?  Is the MEM
>CS16 signal asserted by the AT during DMA reads?  Writes?  How about I/O
>CS16?  What is the layout of the channel 5-7 page registers?
>
>I've got piles of documentation but nothing answers these particular
>questions.  Help me graduate from 8 to 16 bits!  Answers or pointers
>to documentation are both appreciated.
>
>		-Ed Hall
>		edhall@rand.org
>		uunet!sdcrdcf!randvax!edhall


Unless you want to do things very slowly do not use DMA !! The DMA on the PC-AT
is very slow. In fact the only device that I know of that uses DMA is the
floppy controller. The DMA circuitry of the PC-AT is running at 5 MHZ (I think,
but it has been awhile since I looked at a PC-AT). Hard disks on the PC-AT use
MOVE strings, software to move data to and from the hard disk.

I/O_CS_16, MEM_CS_16 are signals controlled by peripheral boards to indicate
that the current I/O more Memory access is a 16 bit access. The 80286 has to
types of address space, a memory and an I/O space. In real mode the 80286 has
an address capability of 1 megabyte and an I/O address space of 64K. Accessing
memory basically uses MOVE instructions and I/O accesses require INPUT and
OUTPUT instructions.

So in conclusion, don't use the DMA !! DMA Performance on the AT really bites
the big one !!!

					T. Chu