Where should the type information be: in tags and descriptors 411

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Use of tagged descriptors for all array references, all references to code, and all references to stack in a uniform way can prevent jumping into code and can also prevent writing over arbitrary combinations of things. With appropriate lookaside memory and address forming logic, it need not cost much in the way of gates except for some of the more interesting pipelined cases).

Since computer programs often depend on data pbutted as parameters, and this data may not be anticipated at compile time (or library design time), the tagging can provide exceptions when the programmer has not done a good job of design or when a hacker is trying to exploit common Unixisms or when someone accidentally enters the wrong data.

Unfortunately, Unix and C provide a lousy semantic match to a tagged architecture and in order to "emulate" Unix and programs done in C (and some of its derivatives) you would loose most of the advantages of a properly designed tagged architecture. Which is one reason why only Univac in the A series still seem to be using tags and descriptors, e.g., Biin failed when they tried to do Unix and even the SPARC which originally had tags does not use them any more (as far as I know).

Ed

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