HP-35/45 Simulator for PDP-8

[Vincent Slyngstad]

From: Doug Ingraham: Tuesday, September 13, 2016 1:38 PM
> This struck me as a pretty clever bit of code.  My first cut used 23 words
> including 2 masks and 3 temporaries.

Thanks!!

> This is the working version of Vince's original.  It uses 18 words
> including 1 mask and 2 temporaries.
> I have done some edge testing and it appears to work.
> 
>   30              / THIS VERSION BY VINCE SLYNGSTAD
>   31 00210  0000  BSWI,   .-.             /ENTRY POINT
>   32 00211  3175          DCA SAVEAC
>   33 00212  7420          SNL             /REMEMBER LINK STATE
>   34 00213  7040          CMA             /AS A -1 FOR ISZ TEST
>   35 00214  3176          DCA SAVEL
>   36 00215  7100          CLL             /NEED THE LINK CLEARED

Ok, I just don't see why one should need to clear link here.

>   37 00216  1175          TAD SAVEAC      /GET 0 XXX XXX YYY YYY
>   38 00217  0177          AND C7700       /    0 XXX XXX 000 000

These two lines, the comment would have "L" instead of their first "0".

>   39 00220  1175          TAD SAVEAC      /    X XXX XX0 YYY YYY

Poof!  There goes the prior value of link!  And the "0" in the comment 
is correct.

>   40 00221  7006          RTL             /    X XXX 0YY YYY YXX
>   41 00222  7006          RTL             /    X X0Y YYY YYX XXX
>   42 00223  7006          RTL             /    0 YYY YYY XXX XXX
>   43 00224  2176          ISZ SAVEL       /WAS LINK SET?
>   44 00225  7020          CML             /YES, RESTORE LINK
>   45 00226  5610          JMP I BSWI      /RETURN
>   46              $
> 
> 
> As Klemens Krause points out you do need the CLL (line 36) somewhere before
> the second TAD SAVEAC or it will flip
> what was the original leftmost bit (AC 0).

Someone help me understand this claim.

> I have thought about this a bunch and have come up with an improved
> version.  It also uses 18 words including 2 constants
> two temporaries.  This is one less instruction executed.
> 
>   32 00210  0000  BSWI,   .-.             /ENTRY POINT
>   33 00211  3174          DCA SAVEAC
>   34 00212  7430          SZL             /REMEMBER LINK STATE
>   35 00213  1177          TAD C0100       /PRE ROTATE LINK POSITION
>   36 00214  3175          DCA SAVEL
>   37 00215  7100          CLL             /NEED THE LINK CLEARED
>   38 00216  1174          TAD SAVEAC      /GET 0 XXX XXX YYY YYY
>   39 00217  0176          AND C7700       /    0 XXX XXX 000 000
>   40 00220  1174          TAD SAVEAC      /    X XXX XX0 YYY YYY
>   41 00221  1175          TAD SAVEL       /    X XXX XXL YYY YYY
>   42 00222  7006          RTL             /    X XXX LYY YYY YXX
>   43 00223  7006          RTL             /    X XLY YYY YYX XXX
>   44 00224  7006          RTL             /    L YYY YYY XXX XXX
>   45 00225  5610          JMP I BSWI      /RETURN
> 
> Not bad but I realized even more was possible.
> 
>   31 00210  0000  BSWI,   .-.             /ENTRY POINT
>   32 00211  3174          DCA SAVEAC
>   33 00212  7430          SZL             /REMEMBER LINK STATE
>   34 00213  1176          TAD C0100       /PRE ROTATE LINK POSITION
>   35 00214  7100          CLL             /NEED THE LINK CLEARED
>   36 00215  1174          TAD SAVEAC      /GET 0 XXX XXX YYY YYY
>   37 00216  0175          AND C7700       /    0 XXX XXX 000 000
>   38 00217  1174          TAD SAVEAC      /    X XXX XX0 YYY YYY
>   39 00220  7006          RTL             /    X XXX LYY YYY YXX
>   40 00221  7006          RTL             /    X XLY YYY YYX XXX
>   41 00222  7006          RTL             /    L YYY YYY XXX XXX
>   42 00223  5610          JMP I BSWI      /RETURN

Nice!

    Vince