EPOC16, originally simply named EPOC, was the operating system developed by Psion in the late 1980s and early 1990s for Psion's "SIBO" (SIxteen Bit Organisers) devices. All EPOC16 devices featured an 8086-family processor and a 16-bit architecture. EPOC16 was a single-user preemptive multitasking operating system, written in Intel 8086 assembler language and C and designed to be delivered in ROM. It supported a simple programming language called Open Programming Language (OPL) and an integrated development environment (IDE) called OVAL. SIBO devices included the: MC200, MC400, Series 3 (1991–98), Series 3a, Series 3c, Series 3mx, Siena, Workabout and Workabout mx. The MC400 and MC200, the first EPOC16 devices, shipped in 1989.
In the late 1990s, the operating system was referred to as EPOC16 to distinguish it from Psion's then-new EPOC32 OS.
The first version of EPOC32, Release 1 appeared on the Psion Series 5 ROM v1.0 in 1997. Later, ROM v1.1 featured Release 3 (Release 2 was never publicly available.) These were followed by the Psion Series 5mx, Revo / Revo plus, Psion Series 7 / netBook and netPad (which all featured Release 5).
The EPOC32 operating system, at the time simply referred to as EPOC, was later renamed Symbian OS. Adding to the confusion with names, before the change to Symbian, EPOC16 was often referred to as SIBO to distinguish it from the "new" EPOC. Despite the similarity of the names, EPOC32 and EPOC16 were completely different operating systems, EPOC32 being written in C++ from a new codebase with development beginning during the mid-1990s.
EPOC32 was a pre-emptive multitasking, single user operating system with memory protection, which encourages the application developer to separate their program into an engine and an interface. The Psion line of PDAs come with a graphical user interface called EIKON which is specifically tailored for handheld machines with a keyboard (thus looking perhaps more similar to desktop GUIs than palmtop GUIs ). However, one of EPOC's characteristics is the ease with which new GUIs can be developed based on a core set of GUI classes, a feature which has been widely explored from Ericsson R380 and onwards.
EPOC32 was originally developed for the ARM family of processors, including the ARM7, ARM9, StrongARM and Intel's XScale, but can be compiled towards target devices using several other processor types.
During the development of EPOC32, Psion planned to license EPOC to third-party device manufacturers, and spin off its software division as Psion Software. One of the first licensees was the short-lived Geofox, which halted production with less than 1,000 units sold. Ericsson marketed a rebranded Psion Series 5mx called the MC218, and later created the EPOC Release 5.1 based smartphone, the R380. Oregon Scientific also released a budget EPOC device, the Osaris (notable as the only EPOC device to ship with Release 4).