🚀 go-pugleaf

Out of respect to a much admired friend, I should point out that Jacob
Schwartz is more famous for his great contributions to mathematics and
computer science than for a old programming trick.  And, come to think of
it, I'm not sure that he's the one responsible for the trick; I heard it
that way once, but I now recall someone telling me I had it wrong.

If you haven't heard this one, you're probably wondering what it's about.
On 6600's you loaded and stored X registers by loading A registers with
the addresses; if you loaded A0, X0 got stored; if you loaded A1 to A4,
the corresponding X got loaded.  To store an A register, you moved it to
a free X register; to load it, you got the address into an X register.
So X0 (I may have which registers did what a bit wrong; it's been ten years)
mostly had to be kept empty.

Now suppose you're writing an interrupt routine (not common; the
PP's took care of the I/O); you need to save the registers.  But you have
no right to assume that any register is free.  Later 6600's had the exchange
jump (XJ) instruction to deal with that case, but not the one at Courant.
How do you get X0 free so you can save registers?  Answer:

	Branch to A1 on X0 negative
	store 0 at location c1
	Branch to B1
A1	store 1 at location c1
B1	Shift X0 left
	Branch to A2 on X0 negative
	store 0 at location c2
	Branch to B2
A2	store 1 at location c2
..... 58 more times.
	Save the rest of the registers.
	Reconstruct X0 from c1-c60.
	Store it.

This is my memory of what I was told by various friends.  Anyone with more
direct knowledge, please correct me where I'm wrong.

Mark