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 -- +------------------+ +------------------------------------------------------+ | Lance T Franklin | | Now accepting suggestions for clever, humourous or | | ltf@killer | | incredibly insightful .signature quote. Send Now! | +------------------+ +------------------------------------------------------+