The MEGA65 computer has a large 28-bit address space, which allows it to address up to 256MB of memory and memory-mapped devices. This memory map has several different views, depending on which mode the computer is operating in.Broadly, there are five main modes:(1) Hypervisor mode; (2) C64 compatibility mode; (3) C65 compatibility mode; (4) UltiMAX compatibility mode; and (5) MEGA65-mode, or one of the other modes, where the programmer has made use of MEGA65 enhanced features.

It is important to understand that, unlike the C128, the C65 and MEGA65 allow access to all enhanced features from C64-mode, if the programmer wishes to do so.This means that while we frequently talk about “C64-mode,” “C65-mode” and “MEGA65-mode,” these are simply terms of convenience for the MEGA65 with its memory map (and sometimes other features) configured to provide an environment that matches the appropriate mode. The heart of this is the MEGA65’s flexible memory map.

In this appendix, we will begin by describing the MEGA65’s native memory map, that is, where all of the memory, I/O devices and other features appear in the 28-bit ad- dress space. We will then explain how C64 and C65 compatible memory maps are accessed from this 28-bit address space.