On Sun, 12 Sep 1999, leo van Loon wrote:

> I meant:
> toggle GP4 and GP5 to drive the piëzo and keep GP0-GP2 unchanged.
> So I will try your second solution.
> 
> But, being a beginner,
> 
> What is meant with:
> MASK    equ     GP4 | GP5

You'd probably be better off reading the MPASM documentation on the 'equ'
directive. But I'll give a very brief summary.

First, GP4 and GP5 have to be defined because the assembler has no idea
what they are. Most constants are defined with the equ (short for equate)
directive. So I'd probably define GP4 and GP5 like this:

GP4    equ   1<<4
GP5    equ   1<<5

This associates the value 1<<4 or 0x10 with the label GP4 and 1<<5 or 0x20
with the label GP5. Once this is done, you can use the constants GP4 and
GP5 and the assembler will replace them with thier equivalent numerical
values.

So, 

MASK   equ   GP4 | GP5

will assign the logical OR (that's what the '|' means) of GP4 and GP5 to
MASK. Since GP4 and GP5 are equal to 0x10 and 0x20, MASK will become 0x10
| 0x20 which is 0x30. Is that clear?

Now you could've just said:

MASK  equ 0x30

or simpler yet:

   movlw  0x30
instead of
   movlw  MASK

and everything would've worked just fine. However, 6 months down the road
when you have to go and fix a bug in this code you're going to need every
bit of help you can get. These symbols will make the code more readable
and comprehensible. It doesn't slow the execution down at all. So use them
where ever possible!