Man and Supermini

To the Editors:

In Jeremy Bernstein’s review of Tracy Kidder’s book on the Data General machine, there is one substantive error on the significance of 32 bits. And the significance of the “supermini” is also passed over.

The current IBM mainframes, the 360/370/303X line, have a 32-bit word, but only a 24-bit address. They are restricted to addressed 16 million characters. Since 16 million is more than 65,000, the limit of a 16-bit machine, this is the reason that DEC, Prime, Interdata (Perkin-Elmer), Data General, and others have brought out 32-bit machines.

But 24 bits do not make nearly as large a number as 32 bits, and this is the significance of the VAX. The DEC VAX has a 32-bit address, and can make available one billion characters of storage, if the supporting facilities are there. This is the significance of 32 bits, as opposed to 16 bits, or 32 bits with a 24-bit address, which is the IBM standard.

The other important point to note is that the DEC VAX, the Data General Eagle, and the other machines in the category, are really minis. They have a bus structure, they do time-sharing naturally, without an additional software system, and they have a very complete hardware facility for handling a large number of interrupts from different devices. This means that they give mainframe space and power, while still keeping the time-sharing and peripheral-handling capability of minis. This is what the term “supermini” means.

Donald Larner

Kansas City, Missouri

Jeremy Bernstein replies:

I am grateful to Mr. Larner for his comments. Perhaps I can translate them a little to make them more accessible. He points out that while the large IBM machines do process in 32-bit words, only 24 of the 32 bits can be used to give locations in the memory. This is a little like requiring users of a telephone book to use part of each listing to give the subscriber’s age, which might be interesting but would not contribute to the listing of the telephone numbers. His point is that the VAX can use all of its 32 bits to give memory locations and thus many more locations can be specified. His other point is essentially that the superminis have built into the hardware functions that in other small machines would have to be programmed. Thus the superminis can function like their bigger brothers, the so-called “mainframe” machines. One of the pleasures of this field is the “jargon”—once one gets used to it.