Low End Mac • Troubleshooting

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Troubleshooting Your Mac: 32-bit Addressing I upgraded my memory past 8 MB, but a bunch of it seems to be missing. It's there, but you need to switch from 24-bit to 32-bit addressing in the Memory control panel. The oldest Macs (68000-based) ran only in a 24-bit addressing mode. To maintain compatibility, and because the operating system didn't support 32-bit addressing, later Macs defaulted to 24-bit addressing. Apple didn't abandon 24-bit addressing as an option until the Centris 660av and Quadra 840av shipped in July 1993. Power Macs only operate in 32-bit mode. System 7 was Apple's first operating system to allow 32-bit addressing. This broke through the 8 MB memory barrier on all Macs not based on the 68000 CPU. With 32-bit addressing enabled, many models could see 64-128 MB of memory. But without enabling 32-bit addressing, the Mac OS only has access to 8 MB of memory. Anything beyond that is reported as used by the system. One more problem is that some of the earlier Macs (Mac II, IIx, SE/30, IIcx) didn't ship with 32-bit "clean" ROMs, meaning they couldn't operate in 32-bit mode without some help. Connectix created a program called Mode32, which let Macs with "dirty" ROMs operate in 32-bit mode. Apple licensed the software, allowing any Mac user who needs it to download a copy for free. Another related problem comes with the LC II and Color Classic, which can only see 10 MB of RAM -- even if you have 12 MB installed. <back to Troubleshooting Your Mac>   MacPromote Xchange Low End Mac established April 7, 1997. This page, the entire Low End Mac site, and all subsites of <http://lowendmac.net/> copyright ©1997, 1998, 1999 by Dan Knight unless otherwise noted. All rights reserved. Apple, the Apple logo, Macintosh, and iMac are registered trademarks of Apple Computer, Inc. Additional company and product names may be trademarks or registered trademarks of the individual companies and are hereby acknowledged.