The Apple III Plus was a result of the discontinuation of the Apple III. The Apple III was in violation of FCC regulations, and the FCCC demanded the company change the name of the redesigned computer, to avoid any confusion.

The Apple III Plus has a built-in clock, video interlacing, standardized connections, 256KByte RAM and a redesigned keyboard that was more like the Apple IIe. Owners of the Apple III could purchase individual III+ upgrades such as the clock, and get a service replacement of hte new logic board. Apple also provided a keyboard upgrade kit, which had to be installed by an authorized technician.

Apple SOS (Sophisticated Operating System) was the disk operating system developed for the Apple III, released in 1980. Unlike Apple DOS on the Apple II, SOS was designed as a full-featured, modular operating system from the outset. It supported hierarchical directories, file attributes (read/write/locked, date/time stamps), and device-independent I/O through a standardized driver model. Written in 6502 assembly, SOS was structured with a kernel that provided file system services, a device driver layer, and an application interface based on system calls rather than patch tables. It could handle multiple file types, large volumes (up to 16 MB per disk), and offered random-access and record-oriented file operations, making it a much more business-oriented OS than its predecessor.

From a technical perspective, SOS treated all peripherals—including disks, screens, and serial devices—as logical files accessed through a uniform API. This abstraction allowed applications to be written in a hardware-agnostic way, simply opening and reading from devices as if they were files. Memory management was still bound by the 6502’s 64 KB address space, but SOS provided a well-defined application runtime environment with fixed entry points and system call conventions. Error handling and interrupt-driven device support were built in, ensuring higher stability than the patchwork approaches of Apple DOS. Although the Apple III hardware struggled with reliability, SOS itself was robust and forward-looking, influencing the later design of ProDOS and serving as Apple’s first real attempt at a professional-grade operating system.

The 6502 is an 8-bit MicroProcessor designed by MOS Technology. The team was led by Chuck Peddle and had also worked on the Motorola 6800. The 6502 is a simplified, but faster and cheaper design than the 6800.

The 6502 was introduced in 1975 and was the cheapest microprocessor on the market. Together with the Zilog Z80, the 6502 helped start the home computer revolution of the 1980s. The 6502 was used in a wide range of devices: the Atari 2600, the 8-bit Atari home computers, the Apple II, the Nintendo Entertainment System, the Commodore 64, the BBC Micro and many others. All used the 6502 or a variation of it.

The 6502 is a 1MHz design, while the 6502A is designed for 2MHz. The 6502A is 100% compatible with the original 6502.

Commodore soon bought MOS Technology, but conitnued to sell the microprocessor to competitors and licensed the design to other manufacturers.