Four?! Why not more?

[Matt Agajanian]

Hi all.

Although some of the 9800 series featured a three-level stack, what was the reason the handhelds were made with a four register stack? Outside of memory, why not five or more registers?

[mfleming]

LastX is often used as a "fifth register" in programs. Does that count?

[Steve Simpkin]

There were some practical reasons to only have a 4 level stack. The first two generations of HP RPN desktop calculators only had a 3 level stack. Adding more levels would have required more hardware (magnetic core or semiconductor) which was expensive. The HP-35 added a 4th level that was shared as a temporary register during trig operations. The 4th level, and the single memory register, was likely judged to be enough to solve most problems and the number of transistors inside the ICs (around 6,000) was already pushing the limits of available technology. Also with only a single level display, keeping track of more than 4 levels would have been deemed harder. This last factor (and tradition) probably kept HP's later calculator models at 4-levels. The addition of a Last X register starting with the HP-45 also helped.

When memory became cheaper still and multi-line displays became available, RPN evolved into RPL with its stack size only limited by memory.

[rprosperi]

Well, after X, Y, Z and T for TOP were used, there was no more room; you can't have a register past the top, there's just no room left.

[Matt Agajanian]

I don’t see it as a matter of alphabet semantics, besides, if it were and T comes before X, why couldn’t it be:


T (top, as you say)
Z
Y
X
W
V—viewable (display)

or
T (top, as you say)
Z
Y
X
W (working register/display)

?

[Garth Wilson]

I suspect it was because the early ones were not programmable, and it's hard to mentally keep track of more than four in manual calculations.

[Kostas Kritsilas]

Like Steve Simpkin said, if you want a bigger/deeper stack, get an RPL machine.

In the earlier days, transistor count was critical as it would determine if a chip, and by extension, a calculator could be built. Later on, as chip fabrication improved and more complex chips could be produced economically, you could get units with bigger/steeper stacks. I think it has been proven, that aside from the most complex of equations, that a 4 level stack is adequate.

I also believe that you could emulate a deeper stack by using registers, or by writing programs to do so on the programmable machines (at least some of them).

[rprosperi]

(06-14-2021 03:13 AM)Matt Agajanian Wrote:  I don’t see it as a matter of alphabet semantics, besides, if it were and T comes before X, why couldn’t it be...

My reply was intended as a small joke, but apparently far too small.

[Steve Simpkin]

Of course if you want to have an RPN (not RPL) calculator with a stack limited only by memory, you can turn on the Big Stack option in Thomas Okken"s amazing Free42 (V3.0 or newer).

"Free42 3.0 introduces the option of using a dynamic stack, also known as Big Stack. In this mode, the stack can grow arbitrarily deep, limited only by available memory. This is similar to the stack in the HP-28/48/49/50 series calculators."

https://thomasokken.com/free42/#doc

[KeithB]

There is probably some report somewhere discussing the optimum number of registers. As a practical matter, has anyone ever needed more than 4?

[hth]

(06-14-2021 01:43 PM)KeithB Wrote:  There is probably some report somewhere discussing the optimum number of registers. As a practical matter, has anyone ever needed more than 4?

Back in the day I would say once. Today I am not as good at it like I used to be, so I often take help using data registers.

In programs I often could have used more than four, typically when doing nested subroutines.

[Matt Agajanian]

(06-14-2021 10:45 AM)rprosperi Wrote:  

(06-14-2021 03:13 AM)Matt Agajanian Wrote:  I don’t see it as a matter of alphabet semantics, besides, if it were and T comes before X, why couldn’t it be...

My reply was intended as a small joke, but apparently far too small.

Now I know (insert laugh track).

[Matt Agajanian]

(06-14-2021 01:21 AM)Steve Simpkin Wrote:  <Large Snip>

This last factor (and tradition) probably kept HP's later calculator models at 4-levels. The addition of a Last X register starting with the HP-45 also helped.

<End Snip>

Well, since including a Last X register began with the 45 in the Classic series, the tech was already there. So,
why not include the Last X register beginning with the 21?

[Steve Simpkin]

(06-14-2021 06:56 PM)Matt Agajanian Wrote:  

(06-14-2021 01:21 AM)Steve Simpkin Wrote:  <Large Snip>

This last factor (and tradition) probably kept HP's later calculator models at 4-levels. The addition of a Last X register starting with the HP-45 also helped.

<End Snip>

Well, since including a Last X register began with the 45 in the Classic series, the tech was already there. So,
why not include the Last X register beginning with the 21?

The Last X feature required one of the 56-bit registers from an external RAM chip to store the Last X value. The HP-21 did not have an external RAM chip so this feature could not be offered on that model.

http://www.brouhaha.com/~eric/hpcalc/memory_size.html

[OlidaBel]

(06-14-2021 02:55 AM)rprosperi Wrote:  Well, after X, Y, Z and T for TOP were used, there was no more room; you can't have a register past the top, there's just no room left.

X,Y,Z,T,C
C: ceiling.

(busy digging the HP-15C manual. some archeology :-) )

[ChanTran]

While it's possible to run out of the 4 level stack but I don't remember if I ever did. So 4 is enough most of the time and of course those machines didn't have that much memory besides with 1 line display it's hard to keep track of more.

[Massimo Gnerucci]

(06-14-2021 08:45 PM)OlidaBel Wrote:  

(06-14-2021 02:55 AM)rprosperi Wrote:  Well, after X, Y, Z and T for TOP were used, there was no more room; you can't have a register past the top, there's just no room left. :)

X,Y,Z,T,C
C: ceiling.

(busy digging the HP-15C manual. some archeology :-) )

Well then why not X, Y, Z, T, O (over the top)? :D

[Steve Simpkin]

Demosthenes reminded me that the WP 34S (and WP 31S) have an optional 8-level stack mode. For that option the stack levels are labeled as follows:
D
C
B
A
T
Z
Y
X

[twoweims]

I seem to remember an unreleased model that provided a stack large enough for complex architectural calculations.
It had a Stack like this:

R - Roof
A - Attic
P - Penthouse
M - Mezzanine
T
Z
Y
X
B - Basement
S - Sub Basement

Yes, the bottom two levels sum it up.... B.S. ;-)

[teenix]

Don't forget the extra ROM requirements to implement the larger stack. This would have required 1 extra ROM chip to be installed on all of the early models as they were all full up.

cheers

Tony

(06-14-2021 06:56 PM)Matt Agajanian Wrote:  

(06-14-2021 01:21 AM)Steve Simpkin Wrote:  <Large Snip>

This last factor (and tradition) probably kept HP's later calculator models at 4-levels. The addition of a Last X register starting with the HP-45 also helped.

<End Snip>

Well, since including a Last X register began with the 45 in the Classic series, the tech was already there. So,
why not include the Last X register beginning with the 21?