The AppleIIGS is the fifth and most powerful model of the Apple II family. It is a 16 bit computer using the 16-bit 65C816 Microprocessor. This processor gives direct access to megabytes of RAM, and the mouse. This enabled Apple to equip the computer with a color graphical user interface, and an Apple Desktop Bus interface that was used for Mouse, keyboard and other input devices.

The sound capabilities were also greatly improved using a wavetable synthesis chip from Ensoniq

The Apple IIGS came standard with 256KByte of memory, and was expandable to 8MByte. The computer supported a 12-bit palette in 320x200 or 640x200.

In 1986, a new version of ProDOS was introduced for the Apple IIgs. ProDOS 16 had the following characteristics:

• When the Apple IIgs was released in 1986, based on the 65C816 CPU, Apple created a 16-bit variant called ProDOS 16. This version ran on top of the 65C816 in native mode and served as the low-level disk/file management layer beneath the graphical GS/OS environment. ProDOS 16 preserved compatibility with ProDOS 8 volumes but added APIs that could take advantage of the wider registers and extended addressing space of the 65C816.
• Exploited the 65C816’s banked memory, allowing much larger buffers and data structures. The OS used high banks for system structures and left lower banks for application code and data. This allowed multitasking-like behavior under GS/OS, with multiple desktop programs coexisting and accessing the file system without being bound by the 64 KB wall.
• Retained compatibility with the ProDOS file system, so disks formatted under ProDOS 8 were usable. However, its system calls were extended to take advantage of 16-bit registers and larger buffers. The ProDOS 16 API became the basis for GS/OS’s File Manager, supporting more complex structures like resource forks and extended metadata while maintaining backward compatibility.
• Was not widely used standalone; it was effectively a transitional layer for the Apple IIgs. By the late 1980s, Apple replaced it with GS/OS, which incorporated ProDOS 16 services but added a full graphical desktop and richer APIs.

In short: ProDOS 8 was a lean, 8-bit OS for the 6502 world, while ProDOS 16 was the 65C816-aware evolution, built for the Apple IIgs and laying the groundwork for GS/OS’s multitasking, GUI-driven environment.

The W65C816, or 65816 for short, is an 8/16 bit microprocessor developed by the Western Design Center (WDC). The W65C816 is an enhanced version of the WDC 65C02 8-bit processor, which itself is an enhancement of the MOS 6502. The W65C816 was the CPU for the Apple II gs and, in slightly modified form, for the Super Nintendo Entertainment System.

The Processor has selectable 8- and 16-bit register sizes. In addition to the 16-bit registers, the processor also extended memory addressing to 24 bits, supporting memory sizes up to 16 MByte. Other improvements were an enhanced instruction set and a 16-bit stack pointer.

The W65C816 starts in emulation mode, in which it behaves like a 65C02. The processor can then be switched to native mode with a two instruction sequence, causing it to enable all the new features, while also maintaining backwards compatibility with most 65C02 code.

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