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From: dennisg@fritz.UUCP
Newsgroups: comp.sys.mac
Subject: Re: How to speed up the MAC ??
Message-ID: <4032@fritz.felix.UUCP>
Date: Fri, 27-Mar-87 21:00:47 EST
Article-I.D.: fritz.4032
Posted: Fri Mar 27 21:00:47 1987
Date-Received: Sun, 29-Mar-87 07:48:45 EST
References: <3053@ihlpg.ATT.COM>
Sender: root@felix.UUCP
Reply-To: dennisg@fritz.UUCP (Dennis Griesser)
Organization: FileNet Corp., Costa Mesa, CA
Lines: 25
Keywords: DTACK, waitstates, DRAM

In article <3053@ihlpg.ATT.COM> jccf@ihlpg.ATT.COM (Hartong) writes:
>I've upgraded my mac with some 256 chips, with an access time of 120 ns.
>My question is: How can I speed up the performance of the mac when
>executing program's or fetching data in RAM. Since the mac adds a wait 
>cycle for every memory cycle, it should be possible to change the 
>wiring of the  68000 DTACK signal in such a way, that waitcycles are
>not added any more in RAM.

It's not that easy.  Those wait states are there for a reason, and it
is not really slow RAM chips.

RAM access is subject to wait states because the video output circuitry
also needs to access the RAM.  No wait states on the ROM because the video
never needs to read there.  So the access is interleaved in a simple way
that ends up being tough to change.  [I believe that the RAM access runs at
full speed during horizontal and vertical blanking.]

If you really want to speed up the Mac, your choices include:
   o increasing CPU clock speed (while keeping video clock the same)
   o adding more memory in a bank with independent arbitration
Neither scheme is for the faint of heart.  Increasing the CPU clock would
be especially tricky.

Details of the mac timing can be had in "IM Underground", a set of schematics,
diagrams, and dis-assemblies on a disk.  If these guys are still around.