300 Part 3 The instruction-set processor level: variations in the processor
Section 6 Processors with multiprogramming ability
Fig. 9. Load and store form main and secondary memory. (a) Instructions. (b) Addressing.
cal device in the system. In addition, a sequential file may be associated with a subroutine. Such a file is called a subroutine file, and the subroutine may thus be thought of as a "nonphysical" device. The subroutine file is defined by the address of a subroutine together with information indicating whether it is an input or an output file and whether it is word or character oriented. An input operation from a subroutine file causes the subroutine to be called. When it returns, the contents of the A register is taken to be the input requested. Correspondingly, an output operation causes the subroutine to be called with the word or character being output in A. The subroutine is completely unrestricted in the kinds of processing it can do. It may do further input or output and any amount of computation. It may even call itself if it preserves the old return address.
Recall that for sequential files the system transforms all information supplied by the user to the format required by the particular file; hence, the requirement that the user, in opening a subroutine file, must specify whether the file is to be character or word oriented. The system will thereafter do all the necessary packing and unpacking.
Subroutine files are the logical end-product of a desire to decouple a program from its environment. Since they can do arbitrary computations, they can provide buffers of any desired complexity between the assumptions a program has made about its environment and the true state of things. In fact, they make it logically unnecessary to provide an identical interface for all the input-output devices attached to the system; if uniformity did not exist, it could be simulated with the appropriate subroutine files. Considerations of convenience and efficiency, of course, militate against such an arrangement, but it suggests the power inherent in the subroutine file machinery.
Summary
The user machine described was designed to be a flexible foundation for development and experimentation in man-machine systems. The user has been given the capability to establish configurations of multiple processes, and the processes have the ability to communicate conveniently with each other, with central files, and with peripheral devices. A given user may, of course, wish only to use a subsystem of the general system (e.g., a compiler or a debugging routine) for his particular job. In the course of using the subsystem, however, he may become dissatisfied with it and wish to revise or even rewrite the subsystem. The features of the user machine not only permit this activity but make it easier.
References
BrigH64; ComfW65; ConwM63; CorbF65; DaleR65; DennJ66; ForgJ65; LampB65; LichW65; McCaJ63; McCuJ65; SaltJ66; SchwJ64