GCBASIC documentation
Compiler Preprocessor Options

Syntax:

    #option NoContextSave

Explanation:

Interrupts can occur at almost any time, and may interrupt another command as it runs. To ensure that the interrupted command can continue properly after the interrupt, some temporary variables (the context) must be saved. Normally GCBASIC will do this automatically, but in some cases it may be necessary to prevent this - for example, when porting existing assembly code to GCBASIC, or when creating a bootloader using GCBASIC that will call another program.

NoContextSave can be used to prevent the context-saving code from being added automatically.

Be very careful using this option - it is very easy to cause random corruption of variables. If creating your own context-saving code, you may need to save several variables. These are:

Other variables may also need to be saved, depending on what commands are used inside the interrupt handler. Everything that is saved will also need to be restored manually when the interrupt handler finishes.

Example:

    ' This shows an example that could be used by a bootloader to call some application code.

    ' The application code must deal with context save and restore
    ' Suppose that application code starts at location 0x100, with interrupt vector at 0x108

    'Chip model
    #chip 18F2620

    'Do not save context automatically
    #option NoContextSave          ' <<< disabling GCBASIC's automatic interrupt context save/restore

    'Main bootloader routine
    Set PORTB.0 On
    'Do other stuff to make this an actual bootloader and not a trivial example
    'Transfer control to application code
    goto 0x100

    'Interrupt routine - this will be placed at the interrupt vector
    Sub Interrupt
        'If any interrupt occurs, jump straight to application interrupt vector
        goto 0x108
    End Sub

Key line: #option NoContextSave — here the interrupt handler does not use W, STATUS, or any other register before jumping away, so no context needs saving; had it done any real work first, this option would require the handler to manually save and restore every register listed above before returning.

See Also: