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Chapter 36 D825-a multiple-computer system for command and control 453

read in. A check is then made for the occurrence of additional external requests. Finally, the computer restores the saved register contents and returns in normal mode to the interrupted program.

AOSP control of I/O activity. As mentioned above, control of all I/O activity is also within the province of the AOSP. Records are kept on the condition and availability of each I/O device. The locations of all files within the computer system, whether on magnetic tape, drum, disc file, card, or represented as external inputs, are also recorded. A request for input by file name is evaluated, and, if the device associated with this name is readily available, the action is initiated. If for any reason the request must be deferred, it is placed in a program queue to await conditions which permit its initiation. Typical conditions which would cause deferral of an I/O operation include:

1 No available I/O control module or channel.

2 The device in which the file is located is presently in use.

3 The file does not exist in the system.

In the latter case, typically, a message would be typed out on the supervisory printer, asking for the missing file.

The I/O complete interrupt signals the completion of each I/O operation. Along with this interrupt, an I/O result descriptor is deposited in an AOSP table. The status relayed in this descriptor indicates whether or not the operation was successful. If not successful, what went wrong (such as a parity error, or tape break, card jams, etc.) is indicated so that the AOSP may initiate the appropriate action. If the operation was successful, any waiting I/O operations which can now proceed are initiated.

AOSP control of program scheduling. Scheduling in the D825 relies upon a job table maintained by the AOSP. Each entry is identified with a name, priority, precedence requirements, and equipment requirements. Priority may be dynamic, depending upon time, external requests, other programs, or a function of many variable conditions. Each time the AOSP is called upon to select a program to be run, whether as a result of the completion of a program or of some other interrupt condition, the job table is evaluated. In a real-time system, situations occur wherein there is no system program to be run, and machine time is available for other uses. This time could be used for auxiliary functions, such as confidence routines.

The AOSP provides the capability for program segmentation at the discretion of the programmer. Control macros embedded in the program code inform the AOSP that parallel processing with two or more computers is possible at a given point. In addition, the programmer must specify where the branches indicated in this manner will join following the parallel processing.

Computer module. The computer modules of the D825 system are identical, general-purpose, arithmetic and control units. In determining the internal structure of the computer modules, two considerations were uppermost. First, all programs and data had to be arbitrarily relocatable to simplify the storage allocation function of the AOSP; secondly, programs would not be modified during execution. The latter consideration was necessary to minimize the amount of work required to pre-empt a program, since all that would have to be saved to reinstate the interrupted program at a later time would be the data for that program and the register contents of the computer module running the program at the time it was dumped.

The D825 computer modules employ a variable-length instruction format made up of quarter-word syllables. Zero-, one-, two-, or three-address syllables, as required, can be associated with each basic command syllable. An implicitly addressed accumulator stack is used in conjunction with the arithmetic unit. Indexing of all addresses in a command is provided, as well as arbitrarily deep indirect addressing for data.

Each computer module includes a 128-position thin-film memory used for the stack, and also for many of the registers of the machine, such as the program base register, data base register, the index registers, limit registers, and the like.

The instruction complement of the D825 includes the usual fixed-point, floating-point, logical, and partial-field commands found in any reasonably large scientific data processor.

Memory module. The memory modules consist of independent units storing 4096 words, each of 48 bits. Each unit has an individual power supply and all of the necessary electronics to control the reading, writing, and transmission of data. The size of the memory modules was established as a compromise between a module size small enough to minimize conflicts wherein two or more computer or I/O modules attempt access to the same memory module, and a size large enough to keep the cost of duplicated power supplies and addressing logic within bounds. It might be noted that for a larger modular processor system, these trade-offs might indicate that memory modules of 8192 words would be more suitable. Modules larger than this-of 16,384 or 32,768 words, for example-would make construction of relatively small equipment complements meeting the requirements set forth above quite

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