24-bit addressing

24-bit addressing involves computers that can locate memory addresses that are 24-bits (3 bytes) wide, allowing for a theoretical maximum of 16 megabytes of memory, though logic board configurations may add other limitations.

History
The Motorola 68000 CPU in the Macintosh 128K had a 32-bit core, but only supported 24-bit addressing. The early compact Macs only had 22 of the 24 address lines connected, limiting them to 4 MB of RAM. The Macintosh Portable and PowerBook 100 supported the full 24-bit address space, making them the only 68000-based Macs to support more than 4MB of RAM.

Early Motorola 68020 and 68030-based computers had 32-bit address lines, but their ROMs were still designed for 24-bit addressing and were not "32-bit clean". In 24-bit mode, such computers were limited to 8 MB of RAM. Virtual memory or Connectix Maxima would allow access to up to 15 MB (minus 1MB for each installed NuBus card).

The first computer to support 32-bit addressing as shipped was the Macintosh IIci. Connectix Optima or Mode 32 could be used to patch the ROMs of older 68030-based systems to add such support. 68020-based systems also require the addition of a 68851 paged memory management unit to be able to support 32-bit addressing, as well as virtual memory.

Because applications from the 24-bit era that were not "32-bit clean" would crash in a 32-bit system, the Memory control panel in System 7 through 7.5.5 included a toggle switch to enable 24-bit support. However, Quadra AV and Power Macintosh computers, and Mac OS 7.6 do not support 24-bit addressing at all.