On Fri, 15 Feb 2002 00:59:40 -0800 Ray Gallas <rgallas@MYBC.COM> writes:
> 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?
>

        When the assembler finds a call subroutine instruction, it inserts the
CALL instruction with the address of the subroutine. The existing program
counter is put on the stack and the address of the subroutine is put in
the program counter. When the return instruction is encountered by the
processor in the subroutine, it pulls the return address from the stack
and continues execution with the instruction after the call.
        With a macro, on the other hand, the actual code of the macro is
inserted into the program EVERY TIME THE MACRO IS "INVOKED." So, using a
macro invocation (sounds religious, doesn't it...) results in a larger
program. However, it is a little faster (no return address storage and
recovery) and results in a less deep stack (which can really be a concern
on a PIC... How many times have you been deep in mainline code subroutine
calls and then have an interrupt cause a stack overflow?).
        What I REALLY like about macros is the use of big arguments and multiple
arguments. For example, the code below calls a subroutine that puts a
message up on the lcd, but I don't have to deal with loading the high
byte and low byte of the pointer into the message. Just call it as
MsgToLcd(SomeMessage) and it happens!


MsgToLcd macro msg
; Set up pointers and send a messag to LCD
        movlw   high(msg)
        movwf   MsgHi
        movlw   low(msg)
        movwf   MsgLo
        call    LcdMsg
        endm


        I also have macros for setting banks (bank0, bank1, bank2, etc.) so I
don't have to remember all those rp bits. I also have macros for
saveContext and RestoreContext for use in my interrupt service routine.


Harold

FCC Rules Online at http://hallikainen.com/FccRules
Lighting control for theatre and television at http://www.dovesystems.com

________________________________________________________________
GET INTERNET ACCESS FROM JUNO!
Juno offers FREE or PREMIUM Internet access for less!
Join Juno today!  For your FREE software, visit:
http://dl.www.juno.com/get/web/.

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body