Quoting Josh Koffman <joshybear@gmail.com>:

> ...
> I think I get the basic gist of what's going on, but I don't
> understand the use of constants in a couple of the commands,
> specifically:
>
>      MOVLW   0100H - 060H
> and
>      RETLW   0100H - 024H
>
> I'm not really sure why constants in an equation are used here rather
> than just the result of the equation.
>
> Also, am I correct in assuming SKPNC is "skip if no carry" and SKPDC
> is "skip if yes carry"? I've never really been able to find out
> exactly where those special instuctions come from.



This is what I figure is happening here... All math is being done in  
packed BCD format, so when the seconds go from 0x09, then next value  
should be 0x10.  But incrementing will take 0x09 to 0x0A, so he tests  
to see if that BCD-clock rollover point has reached the 'A' in the  
right digit by adding '6', which would take it to 0x10.  If adding 6  
does roll it over, the DC bit is set, and this is his indication to  
leave it alone now.  DC is actually Digit Carry, and gets set when the  
lower nibble overflows.  IIRC it does not work for incf, but for addwf  
it does.  The instruction set section of the datasheets tell you which  
instructions affect which status bits, and this explains those bits  
better... http://www.pictutorials.com/The_Status_Register.htm

Back to the code, if adding 6 does not cause the digit carry, then he  
undoes the 6-addition.

Now for the rightmost seconds digit, the test value is 6, but for  
leftmost digit of the seconds, the rollover point is when the '5' in  
0x59 changes to a '6' (as in 0x60).  To test for that, he temporarily  
adds whatever will cause 0x60 to rollover to 0x100 (to set the carry  
flag), and rather than doing the math, my guess is that it's somewhat  
self-commenting to use '0x100 - 0x60'.

Cheers,
-Neil.





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