On Sat, 20 Mar 2010, PICdude wrote:

> 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.
> >

> 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'.
> 

Yes it is common practice to use expressions that show how a constant is 
derived rather than the derived constant itself. This helps understand the 
code (self-commenting) and makes it MUCH easier to identify and change 
constants while you are debugging it or when you need to look at it at a 
later date.

e.g.
	; 5 is the number of slots available
	; 2 is the number of reserved slots
	addlw	5-2

is easier to understand than

	; 5 is the number of slots available
	; 2 is the number of reserved slots
	addlw	3

Regards
Sergio Masci

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