The Automatic Intercept System must maintain a large data base from which information can be obtained pertaining to telephone numbers on intercept in the area. The file complex provides the mass storage medium necessary for this system. This article describes this subsystem, its opera tion, and how it is maintained. files is accomplished independently over the peripheral bus system under control of signals from the central pulse distributor (CPD).Since the file subsystem is comprised of two identical units, much of the following hardware and software description considers just a single unit. II. HARDWARE DESCRIPTION-FILE CONTROL GeneralThe file controller is a wired logic machine consisting of several synchronous sequencers and register circuits. The sequencers are individually associated with major file functions. Collectively, they provide the ability to retrieve, store, and check the data on the discs.The file circuitry is composed of discrete-component, diode-transis tor-logic (DTL) circuit packs, originally designed for the No. 1 ESS. 2 The choice of this type of logic provided a wide variety of available circuits, compatible disc-logic speed, and a favorable cost factor.In the AIS, the file subsystem has duplicated files and each file operates independently of the other, allowing each to handle separate operations simultaneously. The data base on each disc, however, is the same with the exception of the recorded "call-counts" (the number of times the entry has been referenced) on the individual entries. This configuration provides both reliability and higher call throughput. If one file is out-of-service, the remaining file has the ability to handle all call processing requirements with minimum effect on service.Although a file complex is a synchronous machine (using clocks recorded on the disc), it is independent of the No. 2 ESS Control Unit (CU) and performs most of its actions autonomously. It requires only the initial data and an instruction to perform a particular func tion such as looking up the status of an intercepted number. This enables the CU to perform other tasks while lookups and other func tions are performed by the files. Fig. 1. Each CU has its own PUAB and SCAB but each file connects to both sets of busses, allowing either CU to communicate with either file. Control Unit-file communications A Control Unit communicates with a file over a 36-bit, ac, Peripheral Unit Address Bus (PUAB) and a 16-bit Scan Answer Bus (SCAB), as shown inA file receives orders and data from the online CU via its PUAB and returns data to both CUs over the SCABs. Instructions either to 108 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1974 FILE SUBSYSTEM 109 FILE SUBSYSTEM 113