Path: utzoo!attcan!uunet!zephyr.ens.tek.com!gvgpsa!gold!grege
From: grege@gold.GVG.TEK.COM (Greg Ebert)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: How long to hold the IOCHRDY (IO Channel Ready) signal ?
Keywords: IOCHRDY, AT-BUS, ISA
Message-ID: <1528@gold.GVG.TEK.COM>
Date: 2 Oct 90 17:03:57 GMT
References: <188@alderan.uucp>
Organization: Grass Valley Group, Grass Valley, CA
Lines: 24

In article <188@alderan.uucp> bsteh@alderan.uucp (Herbert Bsteh) writes:

>Is there any good reason for restricting the "hold-time" to not longer
>than 2.5 us.  and what problems could arise if we go up to the limit
>of 15 us ?
>

2 problems. First, the floppy drive. If the floppy is running, a DMA xfer
occurs every 16 or so us, and the transfer itself can take up to 2usec if
you include bus acquisition/release times. Secondly, if you have a 386 or
486, a 32 bit write into an 8-bit device (such as a video card) can
require 4 bus cycles. Each cycle can be extended by the video card via
IOCHRDY.

So.... if you have the scenario where:

	1) Your floppy is running
	2) Your application is doing 32-bit writes to an 8-bit video card
	3) Your device is yanking on IOCHRDY
	4) [Possibly] it's a full moon
	5) .....

All of this stuff is supposed to be done within the refresh period. Although
the probability is low, it can STILL happen.