Date: Sat, 31 Oct 1998 19:55:32 -0800 From: David Greenman <dg@root.com> To: Greg Lehey <grog@lemis.com> Cc: John Polstra <jdp@polstra.com>, hardware@FreeBSD.ORG Subject: Re: Mixing 8- and 16-bit shared memory ISA cards Message-ID: <199811010355.TAA01865@implode.root.com> In-Reply-To: Your message of "Sun, 01 Nov 1998 10:44:00 %2B1030." <19981101104400.A28493@freebie.lemis.com>
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>> I'm trying to turn an old 486 box into an ethernet <-> frame relay >> router. It has a 16-bit SMC 8013 card with shared memory, and also >> an 8-bit ET Inc. card with shared memory. I have this vague >> recollection that you can run into problems with a mix like that in >> an ISA machine. In particular, I seem to recall something along the >> lines that each 128K chunk of the address space (a0000-bffff, >> c0000-dffff, e0000-fffff) has to do either all 8-bit accesses or all >> 16-bit accesses, but not a mix of the two. > >Yes, there was something like that, and I think you've remembered it >pretty well. > >> Is that correct? Is it true for FreeBSD systems, or was it just an >> MS-DOS thing? Is such a mix guaranteed not to work, or does it just >> sometimes not work? > >IIRC (and that's not sure), it was a hardware limitation with 286 (and >maybe 386) machines. It's quite conceivable that it doesn't apply to >more modern machines. It affects all machines with ISA busses, not just early x86. The ed driver goes to great pains, however, to attempt to work around this problem by putting 16bit cards into 16bit memory mode only while actually accessing them. The rest of the time they are left in 8bit mode. So you should be able to mix them in the same 128K region in this specific case, but GENERALLY you cannot. -DG David Greenman Co-founder/Principal Architect, The FreeBSD Project To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hardware" in the body of the message
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