Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!ames!elroy!gryphon!cadovax!ucla-an!sm.unisys.com!csun!csuna!abcscnge
From: abcscnge@csuna.csun.edu (Scott "The Pseudo-Hacker" Neugroschl)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Need info on 16-bit DMA for AT
Keywords: 8237-5, AT Bus
Message-ID: <1751@csuna.csun.edu>
Date: 11 Mar 89 22:06:06 GMT
References: <1901@randvax.UUCP> <37068@bbn.COM>
Reply-To: abcscnge@csuna.csun.edu (Scott Neugroschl)
Organization: CSU Northridge
Lines: 34

In article <37068@bbn.COM> tchu@BBN.COM (Tom Chu) writes:
#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 !!!
#


This is true only under DOS.  When writing/designing something to be used
under a multi-tasking system (can you say UNIX?  or even OS/2? :-), one must
be careful not to hog the MOST PRECIOUS RESOURCE IN THE SYSTEM -- to wit, the
CPU.  Granted IBM really blew it with their DMA design in the AT, but
it should still be used for multitasking!  This unburdens the CPU and also
lowers the period of time that the CPU runs with interrupts locked out.

Any Xenix users out there want to talk about losing RS-232 interrupts during
disk access because of this?

-- 
Scott "The Pseudo-Hacker" Neugroschl
UUCP:  ...!sm.unisys.com!csun!csuna.csun.edu!abcscnge
-- unless explicitly stated above, this article not for use by rec.humor.funny
-- Disclaimers?  We don't need no stinking disclaimers!!!