The Keil 8051 Compiler supports 24-bit addressing for the Philips MX, Dallas 390, Analog Devices ADuC812, and for any standard 8051.

For standard devices, you must have another 8 address lines coming from somewhere. They can come from an I/O port (P1 for example) or they can be memory mapped to a latch in XDATA memory space (0xFFFF for example).

To configure the compiler for variable access, you must create routines and read and write 1, 2, 3, and 4-byte sized objects. The tools do the rest. Variables are declared as far (for RAM vars) and far const (for ROM vars).

To configure the compiler for program access, you must define the method of bank switching that you will use (I/O port, XDATA port, or custom).

It's all pretty straight forward. Just remember that you must map your physical memory into the virtual memory space created by the compiler.

Jon Ward
Keil Software