Path: utzoo!utgpu!attcan!uunet!husc6!uwvax!rutgers!rochester!pt.cs.cmu.edu!b.gp.cs.cmu.edu!Ralf.Brown@B.GP.CS.CMU.EDU
From: Ralf.Brown@B.GP.CS.CMU.EDU
Newsgroups: comp.sys.ibm.pc
Subject: Re: 286-based accelerator boards -- followup
Message-ID: <22edc16f@ralf>
Date: 27 Jul 88 11:41:35 GMT
Sender: netnews@pt.cs.cmu.edu
Lines: 36
In-Reply-To: <14742@shemp.CS.UCLA.EDU>

In article <14742@shemp.CS.UCLA.EDU>, wales@valeria.cs.ucla.edu writes:
}120-ns and 150-ns chips; yes, I know that 150-ns chips should theoret-
}ically not work at 8 MHz with 0 wait states, but apparently the toler-
}ances on these chips are such that they do work quite reliably (at
}least, I've never had any problems, and I run *everything* at 8 MHz).

In an XT, each memory access takes four clock cycles.  At 8 MHz 0 waits (I've
NEVER heard of an XT with waits!), the memory cycle time is 500ns, which
means 250ns memory chips will work just fine (memory cycle time = 2 * access).
Even the fastest 8088 machine I've seen advertised (12.8 MHz) would run just
fine on 150ns chips.

It's only on 286 machines that memory access time becomes important, because
a memory access on a 286 or 386 takes two clocks plus any wait states.

}    I would consider it unlikely to work at 10 MHz.  (Then again, are
}    video boards set up to use extra wait states to the video RAM, inde-
}    pendently of the wait states the CPU may or may not be using for the
}    regular RAM?)  I do plan to upgrade my video stuff eventually, but I

Most video boards insert loads of wait states because they can only let the 
CPU access the RAM when the video circuitry isn't.  This mean that for most
boards the CPU must wait an average of 1.1 microseconds to get access.

}In conclusion, I suspect that upgrading my system with a new motherboard
}would likely end up costing almost as much as a new system -- and that
}I'd be better off just waiting until I can afford to trade in my whole
}system for a "real" AT clone.

I've come to much the same conclusion about upgrading my AT to a 386.

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