medge <medge@SONOMETRICS.COM> wrote:
> Can someone tell me the difference between the "#define" and "equ"
> directives?  I have seen them used in the AN's but they both seem to serve
> the same purpose.

In principle, #define is a preprocessor directive.  However, as far as I
can tell, Microchip didn't actually implement it that way, so I'm not sure
whether all of the examples I give below will actually work.

#define can define a symbol which will be substituted for any string.  The
string does not have to be a numeric expression.  Thus you can say

        #define foo     "bar"
        #define somebit portb.7
        #define subwl   sublw   ; some people claim the mnemonic sublw
                                ; is backwards
        #define plus1   +1

in addition to the more mundane

        #define rstcmd  010h

These definitions let you do things like:

        msgtab: addwf   pcl
        msg1    equ     $-(msgtab+1)
                dt      "This is a message about",foo,".",0dh,0ah,0

or
                bcf     somebit

or

                subwl   72      ; subtract W from literal 72

In contrast, equ is a pseudo-op.  It can only be used to define names
for numeric constants.  So you can do this:

        stopcmd equ     011h

You should be able to combine uses of #defines and equ.  A silly example:

        playcmd equ     gocmd plus1
        ffcmd   equ     playcmd plus1

In general, I use equ any time that I want to use a symbol for a fixed
numeric constant, and #define for any other kind of substitution.

Eric