MODE32

MODE32 is a software utility developed by Connectix for a small number of early Macintosh models. It was released in June 1991 and originally cost 169. However, it was licensed by Apple Computer and became freely available on September 5, 1991.

Description
MODE32 patches the ROM code in certain models of early Macintosh computers with Motorola 68020 and 68030 processors (see Affected models below) which had been advertised by Apple as supporting 32-bit memory management. Despite the machines' hardware being designed to support 32-bit addressing, the lack of a "32-bit clean" Memory Manager in ROM forced these machines to run in 24-bit addressing mode, which crippled these otherwise high-end machines to support only up to 8 MB of physical RAM, regardless of how much was installed. Prior to licensing MODE32, Apple had been subject to complaints from individuals over this limitation not being fixed, even with the release of System 7.

Usage
32-bit addressing is generally only necessary when access to over 8 MB of physical RAM are needed. Launching non-32-bit clean software (such as early versions of Adobe Illustrator prior to 3.0) in a Macintosh running in 32-bit mode will lead to a system error and potential. With MODE32 installed, holding down the ESC key on the keyboard during bootup will cause the system to run in 24-bit mode for the current session.

Versions
Early versions of MODE32 (up to 1.2) included a control panel that lets the user enable or disable MODE32 under Macintosh System 7. The option to enable 32-bit addressing can be found in the Memory control panel when MODE32 has been successfully installed. Otherwise, the option is not displayed on the affected models. The first release of MODE32 is known to have issues when virtual memory is turned on in System 7.1. MODE32 version 1.2 was recommended for users of System 7.0 to 7.1.2.

Connectix specifically recommends MODE32 7.5 for users of System 7.5.x in order to avoid problems with file corruption, especially if Thread Manager is loaded. If MODE32 1.2 was mistakenly installed on System 7.5, Connectix recommends a complete reinstall of the operating system. MODE32 only supports up to Mac OS 7.5.5, as Mac OS 7.6 and later require newer models with "32-bit clean" ROMs, regardless of the use of this utility. The only other option would be to replace the original ROM with a compatible aftermarket version.

Connectix offered two similar products, Maxima and Optima for System 6; the former implemented a RAM disk and allowed access to up to 14 MB under 24-bit addressing; the latter enabled 32-bit addressing in System 6, though for only one application at a time as MultiFinder was not 32-bit clean.

Affected models

 * Macintosh II
 * Macintosh IIx
 * Macintosh IIcx
 * Macintosh SE/30

Other models do not need MODE32 because they either do not support 32-bit addressing at all, or support it entirely.

Articles

 * 32-bit Addressing on Older Macs by Daniel Knight at Low End Mac (2015-12-26)