The Universe of Discourse

An anecdote about backward compatibility

A long time ago I worked on a debugger program that our company used to debug software that it sold that ran on IBM System 370. We had IBM 3270 CRT terminals that could display (I think) eight colors (if you count black), but the debugger display was only in black and white. I thought I might be able to make it a little more usable by highlighting important items in color.

I knew that the debugger used a macro called WRTERM to write text to the terminal, and I thought maybe the description of this macro in the manual might provide some hint about how to write colored text.

In those days, that office didn't have online manuals, instead we had shelf after shelf of yellow looseleaf binders. Finding the binder you wanted was an adventure. More than once I went to my boss to say I couldn't proceed without the REXX language reference or whatever. Sometimes he would just shrug. Other times he might say something like “Maybe Matthew knows where that is.”

I would go ask Matthew about it. Probably he would just shrug. But if he didn't, he would look at me suspiciously, pull the manual from under a pile of papers on his desk, and wave it at me threateningly. “You're going to bring this back to me, right?”

See, because if Matthew didn't hide it in his desk, he might become the person who couldn't find it when he needed it.

Matthew could have photocopied it and stuck his copy in a new binder, but why do that when burying it on his desk was so much easier?

For years afterward I carried around my own photocopy of the REXX language reference, not because I still needed it, but because it had cost me so much trouble and toil to get it. To this day I remember its horrible IBM name: SC24-5239 Virtual Machine / System Product System Product Interpreter Reference. That's right, "System Product" was in there twice. It was the System Product Interpreter for the System Product, you see.

Anyway, I'm digressing. I did eventually find a copy of the IBM Assembler Product Macro Reference Document or whatever it was called, and looked up WRTERM and to my delight it took an optional parameter named COLOR. Jackpot!

My glee turned to puzzlement. If omitted, the default value for COLOR was BLACK.

Black? Not white? I read further.

And I learned that the only other permitted value was RED, and only if your terminal had a “two-color ribbon”.

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