Date: Sun, 19 Nov 1995 00:24:23 +1100 From: Bruce Evans <bde@zeta.org.au> To: bde@zeta.org.au, hm@altona.hamburg.com Cc: freebsd-hackers@freebsd.org, hosokawa@mt.cs.keio.ac.jp Subject: Re: DELAY's in syscons Message-ID: <199511181324.AAA25738@godzilla.zeta.org.au>
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>> DELAY(n) only delays n-20 usec on an infinitely fast machine so it >If not told otherwise, pcvt does NOT use DELAY() for exactly the reason >you just describe. >Instead it uses dummy reads from port 0x84 which execute in a defined time >of ~= 1,25us. (IF they ever do execute - i was told that some implementations >do know that there is nothing to read from port 0x84 and somehow don't let >this read though to the bus, the result is, that these reads do execute very >fast). I think this only works reliably for 8MHz ISA buses. Otherwise I would have used it in DELAY(). The only way it can be reliable is if all buses know it is special and put something there that inserts wait states to extend the i/o time to 1.25us. Bruce
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