The System360 Model 20 Wasn't As Bad As All That 3861

On Thu, 06 Jul 2006 02:06:23 -0500, Charles Richmond

My original defense of the Model 20, such as it was, was a reaction to some articles I read that took the view that its incompatibility with the regular System-360, and, more specifically, the limitations on the data types it could work with, made it nearly useless.

In fact, its instruction set, because it did include the decimal instructions, was large enough to do useful work, particularly for people who had been using a 1401 or related computer before.

And the other major incompatibility - the availability of an instruction format that avoided using a base register - wasn't a bad idea, and, in fact, could have been used to good effect on some of their other models that had maximum memory capacities of 32 Kbytes or less. (The Model 25 would support switching into "Model 20 mode", in fact.)

The whole idea behind System-360 was that people could invest in a smaller model, and then easily upgrade as their needs grew. Microprogramming a big-computer instruction set certainly wasn't the cheapest way to build a small-size computer, and that was a valid price-performance criticism of all the lower-end models in the 360 family. But IBM's chief customers were businesses, which had both growing data processing requirements, and which put a value on convenience and help using the computer.

Computer-savvy engineers could switch over from a PDP-15 with a floating-point attachment to a PDP-10 when they needed to. They didn't need to pay PDP-11 prices for a box with PDP-8 performance pretending to be a PDP-10.

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