>
> Wilf Melling <PICLIST@MITVMA.MIT.EDU> wrote:
>
> > I have made a start on my very first PIC project, it is to be a home
> > alarm system (hopefully). So here my very first plea for help, I
> > keep getting the following message. I have read the manuals but the
> > answer does not jump out at me.
> >
> > Message[302] C:\MPASM\HALARM.ASM 40 : Argument out of range.  Least
> > significant bits used.
> >
> > Code sample with line numbers:-
> > 39        Movlw   PORT_B_OUT      ;Define PORTB
> > 40        Movwf   TRISB           ;as output
> >
> > PORT_B_OUT   EQUates to b11111111
>
> Wilf:
>
> This has nothing to do with your problem, but if you want all the
> PORTB pins to be outputs, PORT_B_OUT must equate to 00000000.
>
> Ok...
>
> The MPASM message refers not to PORT_B_OUT but to TRISB.
>
> If you look at the instruction set for your PIC, you'll see that
> register-oriented instructions only have room for 7-bit register
> addresses, so they can only address 128 registers.  To accomodate
> more than 128 registers, PICs employ a banking scheme that uses a few
> bits of the STATUS register to select from multiple 128-register
> banks.
>
> If you try to directly access a register in any bank other than Bank
> 0, MPASM won't be able to fit the register's 8-bit address into the
> instruction.  Instead, it'll just use the low 7 bits of the
> register's address and generate the message you're seeing.
>
> TRISB is located at address 086H, which puts it in Bank 1.  To access
> it, you need to select bank 1 (by setting the RP0 bit in the STATUS
> register), then write to register 006H.  Before accessing registers on
> Bank 0, you'll need to switch back to that page by clearing the
> RP0 bit.  Your code should look like this:
>
>     BSF     STATUS,RP0
>
>         MOVLW   PORT_B_OUT
>     MOVWF   080H ^ TRISB
>
>     BCF     STATUS,RP0
>
> The "080H ^ TRISB" exclusive-ORs the TRISB address with 080H, thereby
> clearing the high bit of the address... It's equivalent to "07FH &
> TRISB", "TRISB - 128", and "006H".
>
> -Andy

That's a great explaination Andy. Now can I ask a dumb question?
The 16C84 Datasheet (and I assume the others too) state that the FSR
will utilize all 8 bits of the file address on indirection. So would
the following work?

   movlw     TRISB
   movwf     FSR
   movlw     .0     ; Note that 0 bits are required for output
   movwf     INDF

It's the same number of instructions but I find it slightly more straight-
forward personally.

BTW I just re-read the databook and in fact the IRP bit in the status
register is prepended to the FSR giving a nine bit address. So in fact
4 128 register pages are accessible using indirection. Cool.

BAJ