🚀 go-pugleaf

In article <16...@cc.usu.edu> SL...@cc.usu.edu (Roger Ivie) writes:

| Need we mention the DG NOVA/ECLIPSEs that had a PSW bit that limited
| indirect accesses to 16 levels? This was programmable so that you could
| go forever if you *really* wanted to.

The original Nova did not have a bound on the number of indirections.
The bound came in when the MMU was added with the Eclipse (I don't
remember if the high end Nova's also had it, and it was ancient
history when I joined DG in 1979).  In the ten years that I worked for
DG supporting compilers, I never encountered a function or fragment of
code that ever used more than a single indirect intentionally, though
there were rumors that one program used it in one spot.

For those of you who don't know what I'm talking about, 16-bit word
(and later also 32-bit double word in the MV) load and store
instructions had an indirect bit, which said to load up the word
(double word) at the location pointed to by the instruction, and use
that as the address to load/store into the register.  The word pointer
format did not use the top bit, so if the instruction specified
indirection, the top bit would indicate whether to loop back, using
the remaining bits as the next address to load, or to stop and finish
the instruction.

--
Michael Meissner	email: meis...@osf.org		phone: 617-621-8861
Open Software Foundation, 11 Cambridge Center, Cambridge, MA

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