Editing Motorola 68000 (section)

===Address bus===
The 68000 has a 24-bit external address bus and two byte-select signals "replaced" A0. These 24 lines can therefore address 16 MB of physical memory with byte resolution. Address storage and computation uses 32 bits internally; however, the 8 high-order address bits are ignored due to the physical lack of device pins. This allows it to run software written for a logically flat 32-bit [[address space]], while accessing only a 24-bit physical address space. <!-- "By modern definition (ref?) this meant that the 68000 was a 32-bit microprocessor." That means an 8080 or 6809 is a 16-bit processor? Most definitions of bitness use internal bus size or instruction types.  The 68000 used unparalleled 16 bit internal bus and 3 16-bit ALU: 2 ALUs for Data Registers and 1 ALU for Addresses.--><!-- The 68000 could be described as 32/32-bit CPU, having 32-bit registers and a 32-bit logical address space; likewise, the 8080 is an 8/16-bit (or 8/16/16-bit) CPU, and the 6502/6800/6809 are 8/16-bit CPUs. --> Motorola's intent with the internal 32-bit address space was forward compatibility, making it feasible to write 68000 software that would take full advantage of later 32-bit implementations of the 68000 instruction set.<ref name="Starnes" />

However, this did not prevent programmers from writing forward incompatible software. "24-bit" software that discarded the upper address byte, or used it for purposes other than addressing, could fail on 32-bit 68000 implementations. For example, early (pre-7.0) versions of Apple's [[Classic Mac OS|Mac OS]] used the high byte of memory-block master pointers to hold flags such as ''locked'' and ''purgeable''. Later versions of the OS moved the flags to a nearby location, and Apple began shipping computers which had "[[32-bit clean]]" ROMs beginning with the release of the 1989 Mac IIci.

The 68000 family stores multi-byte integers in memory in [[endianness|big-endian]] order.