history of the 4-level stack

[axd1967]

Hello,

I'm trying to find out what lead HP to evolve to a 4-stack layout.

The 9100 [1] started with X (keyboard), Y (accu). Browsing the 9100B manual (p. 10) tells me Z is a kind of "temporary" storage (used for 'temporary' storage; the number is stored in the Z register while arithmetic operations are performed on two other numbers in the X and Y registers, then the number in Z can be returned to Y and included once again in the calculation . Use of the Z register saves using the program and data storage registers.), not very convincing.

[2] reads "Experienced HP calculator users have determined that by starting every problem at its innermost number or parentheses and working outward, just as you would with paper and pencil, you maximize the efficiency and power of your HP calculator." Which is not a lie, but does not admit that not all expressions will be solvable with 4 levels... The solution is registers (Section 4).

Likewise, [3] reads "Automatic storage of intermediate results is the reason that the HP-41 C makes solution of even the most complex equations simple." Again, not very honest, seems to me.

Why not more, or less? (Especially when expressions exist that cannot be evaluated with 4 levels.)

Why did they evolve from 3 to 4 levels? I can't believe it is only to be able to provide the T trick (endless provision of a constant for non-commutative ops, [4]).

One hypothesis of mine was that this is somehow related to the three types of operations: sums, products, powers.

Someone hinted that maybe "Architecturally, addressing a fourth thing is pretty cheap, and costs only a few transistors. Adding a FIFTH register, on the other hand, would be a massive change, adding an extra wire to the bus, an extra bit to the microcode field, ...".

I can't recall exactly where someone wrote that T was related to trig calculations.

I started bundling related articles at [5].

rgds,

-alex-

ps - having a 48SX and a swiss micro 15C clone (horrible keys), browsing the 9100 user manual does not give me confidence I would be able to correctly operate a 9100...

[1] http://www.hpmuseum.org/rpnvers.htm#3level
[2] HP-67 Owners Handbook, p. 66 (Automatic stack)
[3] HP-41 C/CV Owner's hb, Section 3 (Automatic stakc)
[4] HP-41 CX owners manual vol 2, p177
[5] https://delicious.com/axd/HP,stack