This document describes the core software that resides in an Input/Output Controller (IOC), one of the major components of EPICS. EPICS consists of a set of software components and tools that Application Developers use to create a control system. The basic components are: OPI--Operator Interface. This is a UNIX based workstation which can run various EPICS tools; IOC--Input/Output Controller. This is a VME/VXI based chassis containing a processor, various I/O modules and VME modules that provide access to other I/O buses such as GPIB; and LAN--Local Area Network. This is the communication network which allows the IOCs and OPIs to communicate. EPICS provides a software component, Channel Access, which provides network transparent communicati...