Date   : Tue, 12 Mar 1985 15:26:29 GMT
From   : john chapman <jchapman%watcgl.uucp@BRL-TGR.ARPA>
Subject: Re: Dynamic Memory

> > The lesson to be learned is that if you have a disk controller that
> > does DMA don't buy dynamic memory. 
> 
> Please don't tell my Intersystems Box! I'd hate for the DMA to quit working
> after 5 years [4 MHz z80, no wait states].
> 
>   <mike
no johnny he's too fast! out of my way reed....
FLAME ON!
 The above response is not very useful - "it worked for me...
 must be a law of nature or something I guess :-) "

 The problem is much more complicated than that, simplistic replies
 contribute little.
FLAME OFF!


  I think the point really is that on S100 systems you won't
 know if dynamic memory will  work with your system until you try it;
 then if you change some component sometime down the road everything
  may not be so peachy keen.  As an example some z80 cards bring out
 the z80 refresh signal to the bus which the memory then uses to do
 refresh cycles without causing wait states; if your memory depends
 on this then you will not be able to change cpu's (ever). Another
 aspect is that the term "IEEE/696 compatible" sems to be a *very*
 flexible term in some manufacturers minds so maybe you get a system
 where the cpu or a dma interface isn't quite within specs. A system
 with static memory will probably run with no problems but the weird
 contortions that dynamic memory boards must go through in order to
 maintain refresh while minimizing/eliminating wait states will very
 probably cause it to fail.
 
 Another example : some systems try for an overall io speedup by
 buffering whole disk tracks. An st506 disk will require >3.2 ms
 to transfer just two 1k sectors; the refresh period of the rams
 is 2ms, so unless your disk controller is willing to allow the
 memory to have wait states (which it may very well do - however it
 is *not* to be taken for granted) or the memory can sneak refresh
 cycles in while the interface isn't looking (some try to sync up
 with bus activity and do their refresh cycles after a memory
 access, which because of the speed of an st506, would work in this
 case) then your system will not work.
 
 Another example : because of the way static rams are set up you
 can leave the chips enabled all the time and feed in the address
 bus continually to the chips; when the control circuitry on the
 board decides the board is actually being addressed for a memory
 cycle it can gate the data lines to/from the chips and set the
 read/write enable appropriately.  Beacuse of the timing of most
 of the micros there is a significant delay between the address
 appearing on the bus and the control/status information describing
 the cycle appearing on the bus - using the above technique (which
 is implemented on a board I own) can decrease board access time
 by significant amounts (50-100 ns).  The way that addresses are
 gated into drams (and cycles initiated) precludes doing this.
 
 and on and on and on and on.......
 
 Opinion: do not buy a dram board unless you know (or someone you
          trust tells you) that it will work in the configuration
          in which you plan to use it; if you buy it be prepared
          to have to throw it out if you ever want to change any
          part of your system that does dma (including/especially
          the cpu).
 
 Opinion: if you buy everything from the same manufacturer it will
          probably all work but eventually you will have the same
          problem as above (unless the same manufacturer also makes
          the new board as well).

 Opinion: I think that one of the big reasons for the decline of
          S100 popularity is dynamic memory - for what it costs
          to buy one 64k static board for an s100 you can get
          256k dynamic boards (that work) for pc clones; I mean
          geez, for the price of one Compupro 256k static board
          you can get a complete sanyo mbc, with disk drive,
          monitor, software, and 256k.  I think this is a crying
          shame - s100 systems are flexible and do not commit
          you to a single manufacturer, the bus interface is
          actually very simple and the entire s100 concept
          encourages proliferation of ideas, products and concepts.
          Micros today seem to be heading towards a very mainframe
          mentality - medicority dominates.
 
 Opinion: There is no reason why a good s100 dynamic board could
          not be made for <500 with 1mbyte of no wait state (and
          refesh states <5% of the time) memory, given the LSI
          dram controllers available and the current price of
          256k chips. Just keep to the standard.
   and on and on and on ......
 
 Sorry to be so lengthy,
 
        John Chapman
 
 ...!watmath!watcgl!jchapman