Always Mount A Scratch Monkey

Computer Science Teacher
Computer Science Teacher - Thoughts and Information from Alfred Thompson

Always Mount A Scratch Monkey

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Do today’s students know what a scratch disk or a scratch tape it? Most of them probably do not. Back in the early days of computers when magnetic tape was in wide use and hard drives had removable disks people used to actually try to fix computers rather than replace them. One tool that was used was diagnostic programs and some of these would exercise the read and write capabilities of tapes or disks. Now you didn’t want to write test data all over your good important data so before running a diagnostic you would replace the production disk (or tape) with what we called a scratch disk (or scratch tape). This was to make sure you didn’t lose anything important while allowing the tool to do a thorough job of testing. Recently a friend of mine reminded me via Facebook of the scratch monkey story. There are two versions of this story (one here and one here) What the stories boil down to is a technician starts to perform a diagnostic without realizing that the computer is controlling a system attached to a monkey. Activity happens on the computer and bad things happen to the monkey. (In some versions of the story a monkey dies.)

Are these stores true? I don’t know. One of them is told by someone I know personally and whose skepticism is legendary so I suspect it may be. But the truth of the story is not as important as the lessons learned. A good story, and who can forget a story of a test run on a computer changing he life of a monkey, is a great teaching tool. In this case there are two lessons. One is that people should be doubly careful about what they do to other people’s computers. The other lesson is that people should protect their resources with documentation.

The technician in this story didn’t understand the ramifications of his actions. He did not know what the computer was doing – and how could he? Well he should have talked to someone who did know. Assuming things is a dangerous thing to do. It is seldom obvious what a computer , for example, is doing just by looking at it. The console could be dark and the computer still be processing away. Blade computers and other rack mounted computers give very little indication of what they are up to. Someone usually has a way of knowing and one is a lot better off finding that person than assuming an idle computer.

The other lesson is that when something is doing something important, especially if that something is not obvious – like controlling the environment for a monkey on a different floor – some protection needs to be made and it should be unambiguous. In one of  the scratch monkey stories a disk is set to read only. In a world of removable disks that often means don’t write to the particular disk. It is not obvious that the drive is a front for some other activity not associated with  the drive itself. An explanation might have saved a monkey’s life!

Students are great at making assumptions. They tend not to look too far past the obvious and the jump to conclusions. So this story may be a helpful lesson. On the other hand for teachers, and anyone trying to get across something important, it is also a reminder that stories can be powerful tools for teaching. After all some of the most influential people in history presented their messages through stories.



  • "Scratch disk": One that could be scratched (and likely would be by the tests you were doing).

    The equivalent of "scratch monkey" story that I heard involved the system control machine (fill in the blank if it were an EKG or an assembly line computer) that would always drop off around 9:30pm for about 10 minutes. Investigations showed that this is the time the night janitor came and, needing to plug in his vacuum, unplugged the control machine.

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