Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!killer!ltf
From: ltf@killer.UUCP (Lance Franklin)
Newsgroups: comp.sys.atari.st
Subject: Re: Making Magic Sac compatible with 128K Roms
Message-ID: <4793@killer.UUCP>
Date: 14 Jul 88 08:53:27 GMT
References: <1988Jul12.201345.24692@gpu.utcs.toronto.edu> <4132@saturn.ucsc.edu> <318@snjsn1.SJ.ATE.SLB.COM>
Reply-To: ltf@killer.UUCP (Lance Franklin)
Organization: The Unix(R) Connection BBS, Dallas, Tx
Lines: 39

In article <318@snjsn1.SJ.ATE.SLB.COM> greg%sentry@spar.slb.com (Greg Wageman) writes:
>True, the address bus is only 16 bits wide (15, actually, with Upper
>Data Strobe and Lower Data Strobe providing the High/Low byte info, as
>well as doubling as the Data Strobe).  However, there are two
>additional lines called "ROM Select 3" and "ROM Select 4" which become
>active when the addresses 0xFB000-0xFBFFFF and 0xFA0000-0xFAFFFF,
>respectively, are read.  This gives access to 64K bytes x 2 = 128K
>bytes, just enough for the ROMS, but no room for the clock as in the
>64K byte Magic Sac.

The clock should not be a problem...Dallas Semiconductor makes a nice little
clock that sits under a ROM socket (or a separate 16 pin package for use with
systems not using JEDEC 28-pin packages) that sits kinda "phantom" on the
address bus and awakens when a specific sequence of addresses comes across
the address bus, then passes the time serially across a data line during
subsequent reads.  So, unless the clock on the Magic Sac is required to be
functionally identical to the one on the Mac, a clock is certainly possible,
at a fairly low cost.

Incidently, an additional chip is available from the same source that, using
a similar setup, can bankswitch up to 16 banks of memory...so a single
cartridge could access up to 16 banks of 128k, or 2 megs.  Finally, a third
chip uses the same addressing scheme to implement a simple serial port (very
simple...software clocked, in fact, but probably sufficient to drive a
printer)...of course, the bankswitch could probably switch in/out some
memory-mapped i/o, like a Zylog SIO chip to drive an AppleTalk port.

At any rate, if you get a chance, take a look in the Dallas Semiconductor data
book...they have the most unusual chips in the industry.  The company I work
for uses their KeyRing Electronic Key for its AutoCAD-related product (only
for the European/Far Eastern Versions though).  It's probably the least 
objectionable form of copy-protection available on the market these days.

Lance

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