Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!mit-eddie!ll-xn!ames!pasteur!ucbvax!decwrl!spar!snjsn1!bilbo!greg From: greg@bilbo (Greg Wageman) Newsgroups: comp.sys.atari.st Subject: Re: Making Magic Sac compatible with 128K Roms Message-ID: <318@snjsn1.SJ.ATE.SLB.COM> Date: 13 Jul 88 21:40:51 GMT References: <1988Jul12.201345.24692@gpu.utcs.toronto.edu> <4132@saturn.ucsc.edu> Sender: news@SJ.ATE.SLB.COM Reply-To: greg%sentry@spar.slb.com (Greg Wageman) Organization: Schlumberger ATE, San Jose, CA Lines: 32 In article <4132@saturn.ucsc.edu> koreth@ucscb.UCSC.EDU (Froot-Loops Addict) writes: >In article <1988Jul12.201345.24692@gpu.utcs.toronto.edu> lharris@gpu.utcs.toronto.edu (Leonard Harris) writes: >>Does anyone have any info on patching magic sac to the 128K roms. >[deleted] >>Technically there should be no problem. > >I believe there is a technical problem, actually. Isn't the cartridge's >address bus only 16 bits wide? If so, that places a 64K limit on the size >of cartridge software, unless some fancy footwork (a la OSS's SuperCartridges >on the 8-bit) is done to increase the effective space. And said footwork >isn't trivial, as far as I know. Of course, I'm not a hardware person, so >this could be totally bogus. 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. Greg Wageman ARPA: greg%sentry@spar.slb.com Schlumberger Technologies BIX: gwage 1601 Technology Drive CIS: 74016,352 San Jose, CA 95110 GEnie: GWAGEMAN (408) 437-5198 UUCP: ...!decwrl!spar!sentry!greg ------------------ The opinions expressed herein are solely the responsibility of the author.