Step 8 of 11

Save and restore context

Goal

Preserve WREG and STATUS on ISR entry and restore them before RETFIE.

Course minimum sequence (uses temp_w and temp_status from step 3):

On entry: MOVWF temp_w → SWAPF STATUS,W → MOVWF temp_status

On exit (before RETFIE): SWAPF temp_status,W → MOVWF STATUS → MOVF temp_w,W

Without this, main code may break after the first interrupt.

Follow these steps

Between TMR0IF check and flag BCF, add W and STATUS save.
Before fin_isr, add restore in reverse order.
Build with no errors.
More detail: context guide and theory.


					isr_alta:
        BTFSS   INTCON, TMR0IF
        GOTO    fin_isr

        MOVWF   temp_w
        SWAPF   STATUS, W
        MOVWF   temp_status

        BCF     INTCON, TMR0IF

        SWAPF   temp_status, W
        MOVWF   STATUS
        MOVF    temp_w, W
fin_isr:
        RETFIE

Checklist before you continue

I save WREG and STATUS on ISR entry.
I restore WREG and STATUS before RETFIE.
Builds with no errors.

Tip: Never put code between restoring STATUS and RETFIE — STATUS must be correct on return.

I completed this step