Editing Apple III (section)

===Backward compatibility===
The Apple III has the built-in hardware capability to run Apple II software. In order to do so, an emulation boot disk is required that functionally turns the machine into a standard 48-kilobyte [[Apple II Plus]], until it is powered off. The keyboard, internal floppy drive (and one external Disk III), display (color is provided through the 'B/W video' port) and speaker all act as Apple II peripherals. The [[paddle (game controller)|paddle]] and [[serial port]]s can also function in Apple II mode, however with some limitations and compatibility issues.

Apple engineers added specialized circuitry with the sole purpose of blocking access to its advanced features when running in Apple II emulation mode. This was done primarily to discourage further development and interest in the Apple II line, and to push the Apple III as its successor. For example, no more than {{nowrap|48 KB}} of RAM can be accessed, even if the machine has {{nowrap|128 KB}} of RAM or higher present. Many Apple II programs require a minimum of {{nowrap|64 KB}} of RAM, making them impossible to run on the Apple III. Similarly, access to lowercase support, 80 columns text, or its more advanced graphics and sound are blocked by this hardware circuitry, making it impossible for even skilled software programmers to bypass Apple's lockout. A third-party company, Titan Technologies, sold an expansion board called the III Plus II that allows Apple II mode to access more memory, a standard game port, and with a later released companion card, even emulate the Apple IIe.

Certain Apple II slot cards can be installed in the Apple III and used in native III-mode with custom written SOS device drivers, including Grappler Plus and Liron 3.5 Controller.