I apologize in advance if this question has been asked and answered previously. I did read the fine manual, and attempted to search the archives, but could not find the answer. The question is, is it intentional behavior that register L is not updated when applying the INC function to the value in the stack X register. In other words, this happens:

Keystroke	X display

1		1

ENTER		1

2		2

+		3

INC X		4

RCL L		2 (equals the “2” used in the plus operation above, which was not the last value in X)

Instead of this:

Keystroke	X display

1		1

ENTER		1

2		2

+		3

INC X		4

RCL L		3 (equals the “3”, which was the last value in X prior to the INC X function)

Thanks.

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If I'd document it the way it works (i.e. not loading L), it would be a feature ;-) After all, LASTx is primarily for error recovery IMHO, so not loading L in INC doesn't hurt, it's just not documented. Or is there any *need* for loading L here?

Well, I "needed" it when I determined that it did not operate that way :-)

LASTx’s primary use may be error recovery, but seeing as how functions like RCL+L, RCL-L, X<>L, etc. are allowed, it is also useful for general data manipulation. So should INC X, which is after all a function that could be called “X+1”, load L just like any* other function operating on X?

* - I did not do an exhaustive search to determine if there are any other situations where something is done to the value in X that does not load L.

...

This behaviour is intentional. Likewise for DEC. The only register it might make sense for is X and if we did it then, we'd have an exception which is bad. Better for these to just not touch L unless it is their argument.

- Pauli

One of these days I intended to use INC X in a program, but I quit because it didn't update the L register. I shifted to the HP-42S instead and used NOT, even though this is equivalent to 1 + +/- (for integers in the valid range). I think both DEC and INC should update the L register, but only when they act on X, if this is not difficult to implement.

What does the 41C/42S do with "STO+ ST X" and the LastX register?

This is the same case.

- Pauli

I see your point. Fortunately on the HP-42S we have "RCL+ST X", which behaves differently regarding the LastX register :-)

Gerson.

On the 34S RCL register arithmetic sets L appropriately too :-) This makes sense since X is always changes. For STO arithmetic (and INC and DEC), X isn't always changed so L is left alone. There is some logic here :-)

- Pauli

I see your points. Probably a trivial issue, one that will never impact me again. I just wanted to make sure it was intentional.

In this case, I was to trying create x(x+1) without disturbing the stack by the following steps:

INC X

RCLxL

But the following does the job:

X^2

RCL+L

...

Edited: 3 Oct 2012, 7:40 a.m.

Quote:

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This behaviour is intentional. Likewise for DEC...

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I certainly wouldn't argue about such a minor point but my SandMath's INCX and DECX functions do store X in L.

If you consider Inc() and Dec() to be -trivial- functions then x is their argument.

However DSE/ISG ST X do not save it, so that's supporting your approach.

Either way is good, a consistent implementation is better... but "consistency is in the eye of the beholder"

Cheers,
'AM

Remember the 34S's INC and DEC take a register argument and increment or decrement that. In fact, the share code with DSE/ISG/DSZ/iSZ/DSL/ISE.

If we'd implemented an explicit INCX/DECX, it would have saved X into L.

Additionally, my experience with INC/DEC supports the don't touch L viewpoint. I've written plenty of programs where leaving L intact was a benefit. Of course, most programs really don't care either way.

- Pauli

To add to Pauli's comment, what about INC->nn where nn contains 100? Updating LastX would be dependent on the data, not only on the command alone. Looks like a Pandoara's box to me which should be better left unopened.

Quote:

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Than it should be stated in section "last x behaviour" if one exists. Similar to stack lift/disable characteristic of different functions. Intuitively I would think DEC would load L with X.

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We should also get sections for carry and overflow in integer mode like the 16c manual has.

- Pauli

For carry please see p. 55 :-)

That's seven of the carry altering functions. See the 16c manual page 98 for some more. That is still not a comprehensive list.

- Pauli

Literally copying old manuals is little fun :-( So I may point to that manual instead ...

I find 55 commands that change carry and/or overflow. The 16c manual only lists 16 commands.

      Command           Carry    Overflow            

      +                   X         X                   

      -                   X         X                   

      times                         X                   

      /                   X       zero              

      LCM                           X                   

      X^2                           X                   

      X^3                           X                   

      +/-                           X                   

      ABS                           X                   

      DBL[times]                  zero              

      DBL/                X       zero              

      DBLmod                      zero              

      DBLrmdr                     zero              

      SL                  X                             

      SR                  X                             

      ASR                 X                             

      RR                  X                             

      RL                  X                             

      RRC                 X                             

      RLC                 X                             

      SQRT                X                             

      y^x                 X         X                   

      LG2                 X                             

      2^x                 X         X                   

      LOG                 X                             

      10^x                          X                   

      (-1)^x                        X                   

      FIB                           X                   

      RECV1                         X                   

      LN                  X         X                   

      E^x                 X         X                   

      CUBERT              X         X                   

      x!                  X         X                   

      GAMMA               X         X                   

      LOGXY               X         X                   

      COMB                X         X                   

      PERM                X         X                   

      XROOT               X         X                   

      INC                 X         X                   

      DEC                 X         X                   

      ISZ                 X         X                   

      DSZ                 X         X                   

      ISG                 X         X                   

      DSE                 X         X                   

      ISE                 X         X                   

      DSL                 X         X                   

      STO+                X         X                   

      STO-                X         X                   

      STO[times]                    X                   

      STO/                X       zero              

      RCL+                X         X                   

      RCL-                X         X                   

      RCL[times]                    X                   

      RCL/                X       zero              

- Pauli

Edited: 4 Oct 2012, 5:41 a.m.