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Space requirements for computer programs

Published:01 July 1972Publication History
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Abstract

While core is becoming relatively inexpensive, there are applications where program size is important. A project is described using a PDP 8 which demonstrates that significant programs can be written with a minimum of main memory.In learning to program, the students at many universities have access to a large central computer. For the most part, they can ignore time and space requirements in their programs. Usually this is a valid assumption since computing time is becoming less expensive; however, with the proliferation of minicomputers, there are many applications where space is at a premium, and the programmer must consider this limitation in his design.At the University of Maryland a Digital Equipment Corporation PDP 8 computer was made available to graduate students at the Computer Science Center. It was a minimum configuration of 4K 12 bit memory and a teletype. Students were forced to program for a machine with little main memory and no mass storage device. One such exercise, which demonstrated the amount of programming which can be accomplished in a small memory, will now be described.The PDP 8 memory is divided into 32 pages of 128 words each. By convention the top page is not used by any DEC software other than the binary loader, a program which is used to load all other software into the machine. This lead (sic) to the project of having students write debugging programs which fit into this top page. Since a locally written binary loader was written in 74 words, that meant 54 words were available in which the student had to write an interesting program.The results of that assignment clearly demonstrated that significant programs could be written in a minimum of space. Some of the programs which resulted were: a dump program which dumped core onto the teletype at 8 words per line, a modify program which displayed the contents of a memory location onto the teletype and enabled the user to change the contents of a location via the teletype, and a punch program which punched a paper tape of specified core segments in a format which enabled it to be reloaded by the binary loader at a later time. These programs, in a very crude manner, approximated the facilities available in the standard DEC debugging program ODT (1), however, each one was significantly shorter, and only took 10 seconds to load from a teletype - thus it was useful for debugging programs since no high speed I/O device was available.This example has some important consequences. While core is becoming less expensive, there are still applications where size is important. Instructors of programming courses should not completely lose sight of that fact. While one shouldn't stress size over all other considerations, the above exercise shows that small programs can do relatively powerful things, even on a relatively simple PDP 8 computer.

References

  1. Digital Equipment Corporation, Introduction to Programming, 1970 pp7.51--7.54Google ScholarGoogle Scholar

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  1. Space requirements for computer programs

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    • Published in

      cover image ACM SIGCSE Bulletin
      ACM SIGCSE Bulletin  Volume 4, Issue 2
      July 1972
      20 pages
      ISSN:0097-8418
      DOI:10.1145/873721
      Issue’s Table of Contents

      Copyright © 1972 Author

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      Association for Computing Machinery

      New York, NY, United States

      Publication History

      • Published: 1 July 1972

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