Date: Sun, 28 Sep 1997 11:16:14 +0200 (CEST) From: hm@kts.org (Hellmuth Michaelis) To: tony@dell.com (Tony Overfield) Cc: tlambert@primenet.com, hackers@FreeBSD.ORG Subject: Re: INB question Message-ID: <m0xFFSY-00023ZC@bert.kts.org> In-Reply-To: <3.0.2.32.19970928015842.006ce080@bugs.us.dell.com> from Tony Overfield at "Sep 28, 97 01:58:42 am"
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Tony Overfield wrote:
> Port 0x84 is an unused DMA page register, just like 0x80, 0x85, 0x86
> and others. Each of these unused registers is read/write and should
> have no effect, except to cause an 8-bit ISA I/O cycle.
This might be on some (or even the majority) motherboards, but not on
all: some existing chipsets seem to detect accesses to "unused"
register addresses and prevent the i/o cycle to happen.
I had exactly this problem in pcvt, and for the majority of cases the
i/o read to 0x84 ran without problems but on some platforms you'll
get strange behaviours (see /usr/src/sys/i386/isa/pcvt/pcvt_hdr.h,
around line 1364, "#if PCVT_PORTIO_DELAY").
hellmuth
--
Hellmuth Michaelis hm@kts.org Hamburg, Europe
"Those who can, do. Those who can't, talk.
And those who can't talk, talk about talking." (B. Shaw)
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