Jinx,
      Soon after sending my question I've realized I have to play with MPLAB
and its memory map view. Anyway, your detailed explanation is revealing:o)
      Thank you very much.


On 8/31/07, Jinx <joecolquitt@clear.net.nz> wrote:
>
> Hi Carol,
>
> > I think this has to do with the fact I'm still not so sure about PIC
> > memory
>
> Each datasheet has a memory map. In your editor (MPLAB ?) you
> can see where instructions are put after compilation. Select View /
> Program Memory
>
> > and where or in which order set the ISR, inicialisation part,
> > main program and then the subrrutines.
>
> At the bottom of memory are the jump vectors. For the F87x there
> are just two ; Reset, at location 00 and Interrupt, at location 04.
> Memory for the user's program starts at 05
>
> Say you have this (it's not how you might normally do it but just
> as an example)
>
> org 0x000
> goto main
>
> og 0x004
> goto isr
>
> org 0x080 ; interrupt service routine starts at location 0x080
> isr code
>
> org 0x100 ; main body starts at location 0x100
> main code
>
> After compilation of the above you'll see in memory
>
> 000 2900 GOTO MAIN ; jump to MAIN after a reset
> (blank, 3FFFs)
> 004 2880 GOTO ISR ; jump to ISR on an interupt
> (blank, 3FFFs)
> 080 ISR code
>
> 100 MAIN code
>
> Say you didn't include orgs, and had just a Reset jump and some ISR
> code before MAIN
>
> GOTO MAIN
> irq movwf    w_temp
>     swapf    STATUS,W
>     clrf        STATUS
>     movwf   status_temp
>     retfie
> main GOTO    INIT
> Loop BSF       PORTA,0
>      CALL      delay
>      GOTO     Loop
>
> Would compile as
>
> 000 GOTO MAIN
> 001 MOVWF W_TEMP ; IRQ
> 002 SWAPF STATUS,W
> 003 CLRF STATUS
> 004 MOVWF STATUS_TEMP
> 005 RETFIE
> 006 GOTO INIT ; MAIN
> 007 BSF PORTA,0
> 008 CALL DELAY
> 009 GOTO 007
>
> Program code is in the 0x004 vector. Which will do no harm if
> interrupts are not being used
>
> With (the recommended) orgs
>
> org 0x000
> goto main
> org 0x004
> goto irq
>
> actual code would compile into memory directly after the vectors (at
> 0x005)
>
> 000 GOTO MAIN
> 001 3FFF
> 002 3FFF
> 003 3FFF
> 004 GOTO IRQ
> 005 MOVWF W_TEMP ; IRQ
> 006 SWAPF STATUS,W
> 007 CLRF STATUS
> 008 MOVWF STATUS_TEMP
> 009 RETFIE
> 00A GOTO INIT ; MAIN
> 00B BSF PORTA,0
> 00C CALL DELAY
> 00D GOTO 00B
>
> As for where routines can go, that's largely a matter of personal
> taste. Some people put the ISR at the bottom of memory, mine
> are usually in the body of the code. 'Library' routines such as delays
> or display drivers are at the end of code, out of the (my) way. It
> can sometimes be helpful to put data in the same page as the calling
> routine so you don't need to worry about PCLATH. Mostly issues
> like that are a minor inconvenience though
>
> > However, in other examples,  I  try to do it this way -subrrutines
> > at the last part of the whole code- and compiler do not make its job
> > saying I'm  "Overwriting previous address contents (0000)"
>
> Possibly you're putting an org too high in memory for the size of the
> routines and the code is wrapping around to the start address
>
> > May be you know some tutorial I can read about this issue.
>
> You could try MPLAB's Help / Topics / MPASM Assembler. For
> example the differences between absolute and relocatable code, org,
> memory useage, RAM bank access, interrupt handling etc
>
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