🚀 go-pugleaf

From article <1hkqk3INNsns@transfer.stratus.com>, by dswartz@sw.stratus.com (Dan Swartzendruber):
> Here's a question for the hardware types: is there any reason either of
> design complexity or performance to not have *all* branches (conditional
> and unconditional) save their origin somewhere (probably in a fast
> dedicated register)? 

Can't speak to the hardware complexity, but it _has_ been done.

The Unisys (formerly Univac) 1100 series machines did this.   Various
models saved the last "n" jump locations ("n" being model dependent)
in part of the fast register stack.  I believe the minimum value
of n was 4.   Even this much "jump history" was quite useful in
debugging certain types of program failure.  (What is saved is the
value of the program counter when the jump is executed -- this is
usually one past the actual jump.  For conditional jumps, this
address is saved only if the jump is taken.)

I'm not quite sure what the specs are on the more recent models of the
2200 series machines (successors to the 1100's), but I think they 
save at least 8 previous jump locations. 

The Operating system also has the facility to detect "jump history
full" conditions and trap to a routine to save these values in main
memory.  This allows the OS to maintain a much longer jump history --
useful for OS debugging -- at the cost of lessened system performance.

Except for the "special" use of this in OS debugging, there was _no_
performance hit for the feature.  In fact, it was always "on" for
normal user code.  When the jump history filled, it "wrapped".   There
was a hardware status bit set to flag where the "oldest" of the 
history entries was, so one could make sense of them.

All in all, a _quite_ useful feature.  More machines should have it!

-- 
John G Dobnick                          ATTnet: (414) 229-5727
Computing Services Division             INTERNET: jgd@uwm.edu
University of Wisconsin - Milwaukee     UUCP: uunet!uwm!jgd

"Knowing how things work is the basis for appreciation,
and is thus a source of civilized delight."  -- William Safire