444 Part 5 The PMS level

Secondary storage unit. Associated with the secondary computer is the secondary storage unit which consists of 60 storage locations containing 16-bit words. Sixteen of these locations can be used as base registers by the primary computer and may be selected by the primary computer according to the a-, b-, c-, and d-digits in the primary instruction word. The contents of the registers selected by the primary computer in this way are automatically added to the address numbers specified in the primary computer instruction word. The secondary storage unit is also capable of being addressed directly by the primary computer. The fifteen 4-word blocks of the secondary storage are identified by 15 special primary address numbers. Other addressable registers associated with the secondary storage hold the address numbers of current and next instruction words in the primary program.

Program control unit. The secondary computer program operates with a 2-address instruction system, the addresses referring to words in the secondary storage unit, including the base registers. See Table 4. From time to time the primary instruction program may order the insertion of a new instruction into the secondary instruction register or may order the transfer of data in either direction between the primary storage units and the secondary storage unit. The secondary computer program may also cause data to be transferred into the secondary storage unit from the primary instruction register and can also cause information to be transferred into the primary instruction register from a location in the main memory.

Using these facilities, the secondary computer can inspect each instruction word in the primary program as it is selected from the primary store and, acting upon specifications written into the secondary program, can cause the primary instruction either to be executed as written or to be replaced by a new instruction word from a memory location determined by the secondary. Other types of discrimination can be effected by the secondary that depend upon the result of a primary operation, such as an overflow, jump, etc. These features facilitate the use of interpretive programming methods.

Concurrent input-output trunks. The concurrent input-output trunks have the function of controlling the transfer of information in either direction between the internal memory and the external storage units. All input-output transfers are initiated by a single internally programmed instruction, and are carried out by the trunk units with the aid of automatic interlocks similar to those used in the inter-memory transfer trunk for preventing interference with the progress of the computing program. The size of the block of data that is transferred may range from a single word to the entire contents of the memory and may be directed to any addresses. Using two such trunks, it is possible to maintain two continuous streams of data simultaneously flowing between the internal memory and any two external storage units without interrupting the progress of the computations.

Format controller. Data that are passing in and out of the internal storage system via the input-output trunks are subject to further concurrent processing by the format controller. The format controller is an independent internally-programmed data-processing unit specially designed for carrying out general-purpose editing, inspecting, and format-modifying operations on incoming or outgoing data. Programs for the format controller are stored on removable plugboards, and the primary computer program is able to direct the format controller to select whichever particular format program may be appropriate from among the small library of format programs contained on the boards currently attached to the machine. Among the typical kinds of programs that the format controller can carry out are: (1) searching of magnetic tapes for words bearing identifying addresses or other coded labels specified by the internal program, with selective input or output of data at these selected tape locations, (2) insertion of incoming data for the internal storage units of the system into address locations specified by the incoming data itself, (3) conversion and rearrangement of data that are stored on external units in formats not compatible with the formats used in the internal units; e.g., binary-decimal character conversion, adjustment of word-length modules, etc.

External storage in the initial installation of the system will consist mainly of magnetic tape units. Because of the flexibility of the format controller, it will be possible to supplement these tape units later with a wide variety of other types of external units without making any significant changes in the existing equipment.