Where should the type information be: in tags and descriptors 414

Typical implementations require a single memory bit for a fixed number of memory bytes to indicate whether an item is "special" (Usualy on 8 or 16 byte boundaries), i.e.,. If that bit is on then further interpretation is required. Special operations set the bit or permit the bit to remain on. Other operations turn the bit "off". Accesses to data are performed through a path of descriptors which provide primitive types that the hardware uses or escapes to "mixed types".

Many optimizations are possible. The implementation will determine what is suitable for a particular application and what is not.

what is an internal overrun?

These require array descriptors that point to elements of fixed size that can be descriptors or data. This is the same kind of problem that one has with a capability based architecture. See Levy for example.

And how often do we see that this has not been done in practice? If the programmer has done this, fine. If he-she has forgotten something then how do we get a safe system? One of the major recent that they were not used is that the SPARC exception handling require so many cycles! If you want to do tagged architecture with the possibility of extended types, you must implement fast exception handling. Of course this presents many problems for pipelined architecture. Ed Feustel

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