MODE32 is a software product originally developed by Connectix for certain models of the Apple Macintosh. It was published in 1991 and originally cost $169, however on September 5, 1991, the software was made available free to customers under licensing terms with Apple Computer.
MODE32 effectively removes a ROM flaw in certain models of early 68020 and 68030 Apple Macintosh computers (see below), which were advertised by Apple to be 32-bit. Despite the machines' hardware being designed to accommodate for this, older, non 32-bit clean software in ROM forced these machines to run in 24-bit mode. The "dirty" software in ROMs crippled these otherwise high-end machines to support only 8 MB of RAM. Customer dissatisfaction likely prompted the licensing agreement with Connectix.
The software runs as a control panel in Macintosh System 7, enabling the 32-bit addressing option of the operating system. The option can be found in the memory control panel when MODE32 is enabled. Otherwise the option is not displayed.
For System 7.0 and 7.1, MODE32 version 1.2 is recommended. In the case of System 7.5, Apple specifically recommends MODE32 7.5 for compatibility purposes. If MODE32 1.2 was installed over 7.5, Apple recommends a complete reinstall of the operating system.
Generally, 32-bit addressing is only needed when large amounts of RAM are needed or when a piece of software requires it, which usually applies to old System 6 and earlier software. Running non-32-bit-clean software in 32-bit mode on any Macintosh can cause crashes and data corruption (see Low End Mac link below). Holding down the Esc key on the keyboard as soon as the machine is powered on temporarily enables 24-bit mode.
Mac OS 7.6 and later require 32-bit "clean" ROMs and do not require this utility. Connectix offered two similar products, Optima and Maxima for System 6; the former allowed addressing 14MB under 24-bit addressing, and the latter enables 32-bit addressing in System 6. A late version of System 6 for a few Macintoshes, called 6.0.8L, allowed addressing 10MB of RAM: otherwise, System 6 is typically limited to 8MB of RAM.
Other models do not need MODE32 because they either do not support 32-bit addressing at all or support it exclusively.