8.5 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 register and STATUS register. This has to be implemented
in software.
The action of saving information is commonly referred to as “PUSHing,” while the action of restoring
the information before the return is commonly referred to as “POPing.” These (PUSH, POP)
are not instruction mnemonics, but are conceptual actions. This action can be implemented by a
sequence of instructions. For ease of code transportability, these code segments can be made
into MACROs (see MPASM Assembler User’s Guide for details on creating macros).
Example 8-1 stores and restores the STATUS and W registers for devices with common RAM
(such as the PIC16C77). The user register, W_TEMP, must be defined across all banks and must
be defined at the same offset from the bank base address (i.e., W_TEMP is defined at 0x70 -
0x7F in Bank0). The user register, STATUS_TEMP, must be defined in Bank0, in this example
STATUS_TEMP is also in Bank0.
The steps of Example 8-1:
1. Stores the W register regardless of current bank.
2. Stores the STATUS register in Bank0.
3. Executes the Interrupt Service Routine (ISR) code.
4. Restores the STATUS (and bank select bit register).
5. Restores the W register.
If additional locations need to be saved before executing the Interrupt Service Routine (ISR)
code, they should be saved after the STATUS register is saved (step 2), and restored before the
STATUS register is restored (step 4).