I think the biggest reason is code space. With a subroutine there is only
one instance of that subroutine in the code, all calls to it branch to that
address. With a macro you are using something called 'inline code', every
time you "call" the macro a copy of that code gets added, so 20 "calls" to
the macro, 20 copies of that code. Now whether this is a bad thing or not is
up to you and your circumstance. A macro consisting of 1 or 2 lines of code
is mostly better then a subroutine, a macro 30 lines long is probably best
done as a subroutine, but it's up to you. In timing critical applications it
MAY be better to use a macro since you don't have the overhead of the
subroutine call (which admitidly on a PIC is only two extra cycles, but that
might be important in certain circumstances, in other micro's calling a
subroutine can hit you with a relatvily large cycle penalty). TTYL

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Ray Gallas
> Sent: Friday, February 15, 2002 04:00
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [PIC]: Basic question about macros
>
>
> This is a beginner question, so I apologise if it seems dumb --
> I've been studying various assembly programs and I've noticed that many
> of them invoke an assembler directive, "MACRO".
>
> Now from what I can see (and perhaps I'm missing something here), they
> seem to fill essentially the same role as a subroutine.  That is, they're
> a predefined set of commands, which at some point in your program, you
> would have to, well, *define*.  It seems to me that doesn't really save
> you much work over just calling a subroutine, which you also write out.
>
> Obviously, MACROs must have some good reason for existing, otherwise,
> why would anyone use them?  So my question is, what's the difference
> between a MACRO, and just using a subroutine?  Why would one choose one
> over the other in a given situation?
>
> Thanks for any clues!
>
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