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The 3B20D Processor has been developed to meet the need for very reliable, real‐time control of a variety of Bell System applications. To achieve its high‐reliability goals, most of the major subsystems within the processor are duplicated, including the Central Processing Unit (CPU). The CPU uses a 32‐bit architecture throughout, including the memory and input/output buses. Extensive self‐checking logic is employed. The 3B20D CPU is microprogrammed to select dynamically up to four instruction sets. The microstore uses a 64‐bit word with up to 16K words of high‐speed bipolar PROM or RAM available. This rich emulation capability makes the 3B20D Processor ideal for emulating existing instruction sets and porting existing software. Peripheral units are connected to the CPU via the Direct Memory Access unit (DMA). The DMA controllers provide direct memory transfers between the main store and peripheral devices, reducing the load placed on the central control to process input/output requests.

The memory hierarchy of the 1A Processor consists of call/program stores, file stores, and an auxiliary data system utilizing magnetic tape. The call/program store is a 65,536‐word, 26‐bit‐per‐word memory system using a two‐wire, coincident‐current, ferrite‐core array as the memory medium, and is capable of operating with a 1.4‐μs read/write cycle time. The file store (fs) with its disk‐file memory provides the 1A Processor with a high‐performance bulk‐storage system. Each fs controller can control from one to four disk files to provide a maximum storage of 2.56 × 106 words. Two fss make up a file‐store community of 5.12 × 106 words of duplicated storage. The auxiliary data system (ads) provides a means for efficiently transferring data between the 1A Processor call‐ or program‐store memories and magnetic tape. The tape medium is used for inputting generic programs, office data, and trouble‐location information and for out putting billing, traffic, and error‐analysis data. The ads hardware consists of data unit selectors (duss) and tape frames (tfs).

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