Datasheet

MCP19111
DS22331A-page 98  2013 Microchip Technology Inc.
15.4 Context Saving During Interrupts
During an interrupt, only the return PC value is saved
on the stack. Typically, users may wish to save key
registers during an interrupt (e.g., W and STATUS
registers). This must be implemented in software.
Temporary holding registers W_TEMP and
STATUS_TEMP should be placed in the last 16 bytes
of GPR (see Figure 11-2). These 16 locations are
common to all banks and do not require banking. This
makes context save and restore operations simpler.
The code shown in Example 15-1 can be used to:
• Store the W register
• Store the STATUS register
• Execute the ISR code
• Restore the Status (and Bank Select Bit register)
• Restore the W register
EXAMPLE 15-1: SAVING STATUS AND W REGISTERS IN RAM
Note: The MCP19111 device does not require
saving the PCLATH. However, if
computed GOTOs are used in both the ISR
and the main code, the PCLATH must be
saved and restored in the ISR.
MOVWF W_TEMP ;Copy W to TEMP register
SWAPF STATUS,W ;Swap status to be saved into W
;Swaps are used because they do not affect the status bits
MOVWF STATUS_TEMP ;Save status to bank zero STATUS_TEMP register
:
:(ISR) ;Insert user code here
:
SWAPF STATUS_TEMP,W ;Swap STATUS_TEMP register into W
;(sets bank to original state)
MOVWF STATUS ;Move W into STATUS register
SWAPF W_TEMP,F ;Swap W_TEMP
SWAPF W_TEMP,W ;Swap W_TEMP into W