A note on Program Debugging in an On-Line Environment

It is commonly asserted that one of the main benefits of an on-line system is that it simplifies the task of getting a program to work: indeed, this has become part of the gospel of multi-access. This note examines this assertion, and suggests that it is true, though not for the reasons usually given. The obvious advantages of a multi-access system are twofold: a filing system that allows programs to be stored and readily altered, and almost instant access to the machine for test runs. Anyone who has used such a system for program development will testify that these are great benefits; however, comparable benefits can be obtained at less cost. Programs on punched cards can be modified fairly easily, and a full-scale filing system can be maintained quite separately from a remote console system. The same goes for access to the machine: given a ten-minute turnround on test runs most programmers would be happy, and it is certain that many of the people in universities who ask for multi-access really want rapid turnround. Though most conventional systems provide a turnround measured in hours or even days, this should not obscure the fact that rapid turnround can be achieved without a full-scale remote console system/ ' However, supposing that we have a multi-access system available, with a filing system and instant turnround, what do we pay (in programmer convenience, not machine efficiency) for these benefits ? The most significant trade-off is that since we are communicating with the machine through a narrow bandwidth channel, we are restricted in what we get from the machine to what can be printed in a reasonable time at ten characters per second. No listings, no core dumps: with a compiler that was written for off-line use, this makes life difficult. The minimal facility necessary is the ability to record a core-dump in a file, for later examination. If on-line debugging is to be effective one needs an elaborate interrogation program which can access the core dump and