"Aubrey McIntosh, Ph.D." wrote:
>
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> I was looking at page 16 of 30292c.pdf, the PIC 16F87x Data Sheet.  Figure
> 2-4, the 16F874/73 register map shows two banks of General Purpose
> registers, 0x20 -- 0x7F and 0xA0 -- 0xFF.  These same banks are also
> addressed when the addresses 0x120 and 0x1A0 are addressed.


It's every man for himself as far as context saving goes in this chip.

If you have W_temp at address 0x7F, it appears in all RAM pages
regardless of the bank select bits. (see data sheet)

S_temp  equ 20h

W_temp  equ 0x7F


Now, if an interrupt occurs, just save W at W_temp


    org 0x0004

    movwf W_temp

Whatever registers your interrupt routine changes, you should save them
as well, including STATUS, PCLATH, FSR etc.

To preserve the Z bit in the STATUS register use the SWAPF instruction.

   swapf STATUS,W
   clrf STATUS         ; ensure BANK 0 where S_temp is located
   movwf S_temp

   ; execute interrupt code

   swapf S_Hold,W      ; restore S register
   movwf STATUS        ; BANK bits will be restored

Use the SWAPF instruction twice to recover the W data to avoid changing
the STATUS Z bit.

   swapf W_Hold
   swapf W_Hold,W
   retfie


regards

Tony

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