home *** CD-ROM | disk | FTP | other *** search
- Newsgroups: comp.sys.ibm.pc.hardware
- Path: sparky!uunet!usc!cs.utexas.edu!hermes.chpc.utexas.edu!news.utdallas.edu!corpgate!bnrgate!bmers95!bnr.ca!khor
- From: khor@bnr.ca (Kyle Hor)
- Subject: Re: Why does my 486 require 2 wait states?
- Message-ID: <1993Jan4.213815.28624@bmers95.bnr.ca>
- Sender: usenet@bmers95.bnr.ca
- Reply-To: khor@bnr.ca
- Organization: Bell-Northern Research Ltd.
- References: <1ia62bINN37o@savoy.cc.williams.edu>
- Date: Mon, 4 Jan 1993 21:38:15 GMT
- Lines: 35
-
- In article <1ia62bINN37o@savoy.cc.williams.edu>, 93gke@williams.edu (oldwanda) writes:
- |> I was wondering if anybody could help me with this. The docs for my
- |> computer say that at 33Mhz I need 2 wait states. Any other speed works
- |> at 0 wait states. Can someone explain this to me? I tried running at 0
- |> and 1 wait states, but I had memory problems. This sucks! Would I
- |> better off downgrading to 25Mhz??
- |>
- Let me try with my limited hardware knowledge.
-
- With 33MHz, you are accessing the memory at (1/33E6)secs = 30.3 ns.
- That is too fast as most DRAMs can only provide access reliablity
- of 70/80ns (older DRAMs -> 100ns).
-
- Hence 2 wait states slows memory access to 90.9 ns which is ok for the
- 70 and 80ns DRAMs.
-
- With 25MHz, it works out to 40ns and hence would need only 1 wait state.
- Not 0.
-
- It is true that 25MHz machines access memory faster than 33MHz
- machines (80 vs 90.9ns). But then the uP also performs other
- instructions as well. My rough estimation would be about 30% on
- memory fetches and 70% for other internal register manipulations.
- Which puts the 33MHz slightly ahead of the 25MHz.
-
- Also I'm waiting for the day when they come out with 60ns DRAMs. I can
- then reduce my 33MHz to 1 wait state and see it fly :).
-
- ............................................................................
- . Kyle Hor . / / _ . Email khor@bnr.ca .
- . Bell-Northern Research Ltd. . /__\ / / /_\ . Phone (613) 763-2488 .
- . Ottawa, Ontario, Canada . / \ \/ / \__ . ESN 393-2488 .
- ....................................__/.....................................
- ................ standard disclaimer, 2 cents opinion ....................
- ............. searching forever, for the perfect solution ................
-