First released on the Oregon Scientific Osaris, this is Psion's 32 bit operating system for handheld computers.

The first device that Psion itself released which used EPOC was the Psion series 5, which used EPOC Release 3.

The GeoFox One was the only device to use EPOC Release 4, and EPOC Release 5 was used by the Psion series 5mx, Psion Revo and Psion series 7 and netBook, as well as the sonic blue/Diamond Mako, the Ericsson MC218 handheld computer, the Ericsson R380 smartphone, and some versions of the Nokia Communicator.

EPOC Release 6 is used by the Nokia 92x0 series of Nokia Communicators, and Release 7 is about to emerge on the SonyEricsson P800 and P802.

EPOC devices still outsell both Palm OS and Pocket PC devices in the UK and the rest of Europe.

One reason for their popularity is the plethora of application software that comes built-in including a Microsoft Excel-compatible spreadsheet and a Microsoft Word compatible word-processor. Another is the platforms usability, and yet another is the platforms extreme stability.

Typically, EPOC devices have provided much more screen real estate than competitor products, and Psion's own EPOC devices have provided very useful keyboards in addition to the touch-screen facilities.

For a significant period of Microsoft's hand-held marketing campaigns, based around the 'compatibility' of Pocket Excel and Pocket Word with their desk-top counterparts, the EPOC software provided better interoperability with the desktop Microsoft Office software.

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