> The other registers have similar overlapping low byte addresses.
> So...going into my ISR I might not be in the correct bank

More than likely the only 'bank' you have to worry about is which
RAM block is being pointed to by BSR. And if you aren't changing
BSR in the ISR then it's not a concern

BTW, the point of BSR is to speed up RAM access. Simply put, a
12-bit address is used for RAM, which can be up to 16 blocks of
256. BSR is the most significant 4 bits (a value of 0 to 15), which is
the start address of a block, and the faster-fetched least significant 8
bits is the RAM address within that block

I prefer to use 'block' here rather than 'bank' because it's less
confusing

Note that MOVFW, MOVWF have an 8-bit address and can
access only the RAM block pointed to by BSR, which you would
call the current bank. MOVFF uses a 12-bit address, allowing
you to copy across blocks, including copying an SFR (block 15)
into RAM and vice versa. Similarly the FSRs

> W_Temp     0x080
> PORTA       0xF80

It is true that the LSB address is common. However, the instruction

CLRF LATA

is compiled to

CLRF 0xF89,ACCESS

because MPLAB recognises, via the .inc file, LATA

0xF89 is in Bank15. It's an SFR with a fixed address. To access it
directly you would need to set BSR = 15. W_Temp has a user-
defined (that's you) address

For example, you might set aside a scratch block/bank of RAM

mc_area  = 0x0200             ;message compilation block

Now, you can move data in and out of that block to other blocks or
within the same block using the FSRs or MOVFF, without touching
BSR, because of the wider addressing range as mentioned above

If you're wanting to go through W though, you'd have to change BSR
to point to first the source and then the target

Perhaps one location (called m_num) in that message text needs to be
sent to a display routine with its variables in bank 0 RAM, one of which
is called message_number

MOVLB 2 ;set RAM pointer to 0x200 - 0x2FF
MOVF m_num,W ;get the contents of m_num into WREG
MOVLB 0 ;set RAM pointer to 0x000 - 0x0FF
MOVWF message_number

In reality MOVFF m_num,message_number would be quicker, but I
think it illustrates the point

In effect BSR works like banksel, so you would use it to work with
the RAM in a particular block. There is not the ambiguity though of
SFRs sharing addresses in different banks

eg (18F)

         movlb   .1
         movlw   0x04
         movwf   lata

versus (16F)

         bank0
         movlw   0x01
         movwf   porta

         bank1
         movlw   0x01
         movwf   porta

> I suppose I could use a lot of banksel commands

'banksel' the pseudo-op isn't applicable on the 18F. SFRs are in
contiguous order. If you look at a Data Memory Map, they are
bunched down at the end of data memory. So there's no instance of
PORTA in Bank0 at the same position as TRISA in Bank1, but
'banks' in the 16F sense don't exist on the 18F anyway as SFRs
don't co-exist with GPRs in a block like they do on a 16F, except in
Bank15, but even then the addresses are quite distinct. The whole
memory structure is much more user-friendly and less prone to errors

wbr
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