Path: utzoo!bnr-vpa!bnr-fos!bnr-public!mlord
From: mlord@bnr-public.uucp (Mark Lord)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Extended memory board and RAM speed...
Message-ID: <731@bnr-fos.UUCP>
Date: 10 Jul 89 20:02:38 GMT
References: <5876@rpi.edu>
Sender: news@bnr-fos.UUCP
Reply-To: mlord@bnr-public.UUCP (Mark Lord)
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Organization: Bell-Northern Research, Ottawa, Canada
Lines: 40

In article <5876@rpi.edu> spierk@turing.cs.rpi.edu (Kevin Spier) writes:
>I am looking into purchasing an extended / expanded memory board for
>my Epson Equity III+, and I have no idea what speed RAM I need.  The
>machine runs at 12MHz with 1 wait state. Can I use 80ns chips or do I
>need faster? And can I just buy 70ns chips anyway; ie.  is better to
>have faster chips even if you don't need them or does it cause
>problems? Also can Windows 286 v2.11 make use of extended memory
>besides just using it for swapping??? Any help would be greatly
>appreciated.
>
>Thanks,
>Kevin L. Spier
>spierk@turing.cs.rpi.edu
>
>Kevin L. Spier
>spierk@turing.cs.rpi.edu

80ns ought to be more than fast enough.  A simple, non-interleaved non-cached 12-Mhz system deals with 83.3ns per cycle.  At one wait state, a memory access is
therefore allowed 83.3+83.3=166.6ns to complete.  Dynamic rams require their
"access time" (such as 80ns or 70ns in your query) to respond with data, plus
an allowance for other circuitry to generate the chip selects and strobe the
addresses.  This extra overhead varies from machine to machine, and about 30ns
is probably a reasonable allowance.  80ns+30ns = 110ns = plenty of time.

BUT.. dynamic rams also have a requirement for resting between accesses (hard
work..), typically slightly less than their access times.  In this example, 
it might be 70ns or 60ns, although this depends on the manufacturer.  When
figuring out this tolerance, the 30ns overhead need not be considered in most
cases, so as long as the time available (166.6ns) is more than the chip "access
time" (80ns or 70ns) plus the resting time (70ns or 60ns), it will probably
be okay (depending on other system variables as mentioned).  

In this case, 80+70ns or 70+60ns are both less than the 166.6 cycle time for the
12mhz+1ws machine, so 80ns rams are just peachy.  Most of the 12Mhz systems I
have seen use 100ns chips.  Perhaps this is because an extra cycle is used 
by the processor (2 cycles for memory accesses?) allowing plenty of time. 
I don't have a h/w manual handy to see exactly how many cycles the 80286 really
uses when going to memory, but the above calculations are worst case.

-Mark