NetNews Usenet Archive 1992 #18/NN_1992_18.iso/spool/comp/os/coherent/3643

Text File | 1992-08-13 | 3.2 KB | 58 lines

Newsgroups: comp.os.coherent
Path: sparky!uunet!think.com!cass.ma02.bull.com!mips2!bubba!sje
From: [e-mail] (Steven J. Edwards)
Subject: Coherent 4.0.1 and the 16 Mbyte limit
Reply-To: [e-mail]
Organization: Bull HN, Worldwide Information Systems, Billerica, Mass., USA
Distribution: comp
Date: 13 Aug 92 14:48:12
Message-ID: <[e-mail]>
In-Reply-To: [e-mail]'s message of 13 Aug 92 12:05:24 GMT
References: <[e-mail]>
    <[e-mail]>
Sender: [e-mail] (Usenet News Manager)
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In article <[e-mail]> [e-mail] (David L. Parker) writes:
> Are you sure the 24-bit memory addressing limitation you refer to is not
> a function of the AT-bus 24-bit memory addressing limitation, rather than
> a limitation of the OS??

The ISA bus is limited to 24 bit addressing.  It was designed to
handle 286 class processors which are limited to 16 Mbytes of physical
memory.  All modern 386+ machines have memory on a local memory bus
that runs at a much faster speed (33 MHz in mine) than the ISA bus
(8.3 MHz, quarter speed).  Therefore, these boxes do not have 24 bit
memory limitations per se.  This is shown by the fact that MS-DOS 5.0
has no problem using memory well past the old ISA limit.  Of course,
the mememory can't sit on the ISA bus.  A problem results if the OS
has to communicate with ISA bus peripheral cards via DMA (Direct
Memory Access); the transfer buffer address must reside in the bottom
16 Mbytes for the DMA controller circuitry to work as it itself is
only aware of 24 bit addressing.  There are other restrictions such as
the 64 Kbyte straddle problem that must also be handled, but these
were treated long ago.  The solution to the DMA problem is to ensure
that the memory transfer buffer be located in ISA bus addressing
space.  This is the responsibility of the OS as user processes have no
say as to where this buffer goes.  MS-DOS 5.0 does this.  Coherent
4.0.1 can't.  Not yet, at least.

The EISA bus does pass all 32 bits of addressing to peripheral cards.
This doesn't help all that much because the EISA bus has the same
speed limitation as the ISA bus.  Also, EISA systems are outnumbered
by about fifty to one by ISA boxes.  I think that EISA is a good thing
for installations with multiple big drives running under heavy loads
as industrial strength file servers.  While MWC may have EISA
solutions in the works, I doubt that there will be many customers in
the short term.  Certainly none of the ISA owners will benefit.  I can
jam 64 Mbyte of ever-less-expensive DRAM SIMMs in my system; why
should I or any of you have to be limited by an old stle drive
controller limitation?  EISA is just a red herring for the vast
majority of the enthusiast and small business communities.

 [The above opinions expressed are my own; not necessarily held by others.]
      == Steven J. Edwards           Bull HN Information Systems Inc. ==
      == [phone]              300 Concord Road         MS 820A ==
      == [e-mail]     Billerica, MA 01821          USA ==
"That Government which Governs the Least, Governs Best." -- Thomas Jefferson