32-bit Addressing on Older Macs

Dan Knight - March 1998, updated Dec. 2001

I upgraded my memory past 8 MB, but a bunch of memory seems to be missing or used by the OS.

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) and the Mac OS prior to System 7 ran only in a 24-bit addressing mode. To maintain compatibility, and because the operating system didn't support 32-bit addressing until System 7, 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 support 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, and a few as much as 256 MB.

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, Mode32 (search the page for "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; they can only see 10 MB of RAM - even if you have 12 MB installed.

Troubleshooting Your Mac Articles

Join us on Facebook!

Links for the Day

Mac of the Day: Mac IIfx, introduced 1990.03.19. This 'wicked fast' 40 MHz Mac trumped the 33 MHz DOS world.
Support Low End Mac

Recent Deals

Go to the Troubleshooting Your Mac index.

About LEM Support Usage Privacy Contact

Low End Mac is an independent publication and has not been authorized, sponsored, or otherwise approved by Apple Inc. Entire Low End Mac website copyright ©1997-2012 by Cobweb Publishing, Inc. unless otherwise noted. All rights reserved. Apple, the Apple logo, Macintosh, iPad iPhone, iMac, iPod, MacBook, MagSafe, Mac Pro, Apple TV, and AirPort are registered trademarks of Apple Inc. Additional company and product names may be trademarks or registered trademarks and are hereby acknowledged.
Advice is presented in good faith, but what works for one may not work for all. Please report errors to .
LINKS: We allow and encourage links to any public page as long as the linked page does not appear within a frame that prevents bookmarking it.
Email may be published at our discretion unless marked "not for publication"; email addresses will not be published without permission, and we will encrypt them in hopes of avoiding spammers. Letters may be edited for length, context, and to match house style.
PRIVACY: We don't collect personal information unless you explicitly provide it. For more details, see our Terms of Use.

Custom Search

Follow Low End Mac on Twitter
Join Low End Mac on Facebook

Low End Mac Reader Specials

TypeStyler 11 is now in the Mac App Store!! -- Special Introductory Price of $59.95!! -- To Buy From The Mac App Store Click Here Now!! Or buy direct from Strider Software.

Don't install Parallels to play poker online! Poker Mac will show you how to download and install a native Mac poker and Mac Casino applications in minutes.

Affiliates

Amazon.com
The Apple Store
The iTunes Store
PC Connection Express
GainSaver
Parallels Desktop for Mac
eBay