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From: davidb@brad.inmos.co.uk (David Boreham)
Newsgroups: comp.arch
Subject: Re: "zero wait states"
Message-ID: <5353@ganymede.inmos.co.uk>
Date: 5 Apr 90 17:24:10 GMT
References: <1990Mar30.222138.13886@jarvis.csri.toronto.edu> <719@optis31.UUCP> <10758@portia.Stanford.EDU>
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Reply-To: davidb@inmos.co.uk (David Boreham)
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In article <10758@portia.Stanford.EDU> dhinds@portia.Stanford.EDU (David Hinds) writes:
>states.  However, I think a "x ns" DRAM takes "2x ns" for an access anyway,
>because the address lines are multiplexed and are strobed on two successive
>clock cycles.  So, with nothing special, you need 62 ns memory to get 0

Not quite. You are correct about the 2X access time for cycles. Not many
non-designers appreciate that. However, the reason is that the sense-amps
have to be precharged after every row access. Multiplexing the address is
not a significant speed hit until you get down to about 50ns access.

60ns DRAMs have been available for many years (first from INMOS) and have
very tight setup/hold times on the address inputs.

>incidentally.
>    On the subject of RAM chip specifications, there is something funny
>about quoting RAM speeds in multiples of 10 ns, with processor speeds in
>integral MHz numbers.  For example, I know that 100 ns memory is fast
>enough for 16MHz with 0 wait states when interleaved.  But this actually
>requires like 94 ns memory, by my calculations.  Is there some implicit
>tolerance in the RAM chip speeds, like are they always rounded up to the
>nearest 10 ns?

No. If the spec says 100ns then they can ship you 99.9ns parts. 
Of course the spec is for worst case temp and voltage margins and
includes a guard-band for tester innacuracy and device aging.
Because of pressures to extract the last ounce of performance,
PCs frequently overrun DRAMs. This is rather bad practice but
if you take the approach that the PC vendor can control the voltage
and so a certain extent the temperature, then mabe it's OK.
I would be surprised though that 100ns DRAMs would be overrun as
the major manufacturers have speed distributions centered on 80ns
therfore 80ns parts don't cost more than 100ns parts.

David Boreham, INMOS Limited | mail(uk): davidb@inmos.co.uk or ukc!inmos!davidb
Bristol,  England            |     (us): uunet!inmos.com!davidb
+44 454 616616 ex 547        | Internet: davidb@inmos.com