#Option BootloaderHost

Syntax:

    #option bootloaderhost address

Explanation:

#Option Bootloader builds an application that runs above a bootloader: its reset and interrupt vectors are moved to address, address+8 and address+0x18. #option bootloaderhost builds the bootloader itself (the "host"):

  • it sits at the reset vector;
  • it does not use interrupts itself;
  • it forwards the chip’s interrupt vectors to the application at address.

Without either option, GCBASIC puts the reset vector at 0x0000 and the program’s own interrupt code at 0x0008 - so an application built separately and placed at address would never see its own interrupts fire, because the host’s vectors are still pointing at the host’s own code. #option bootloaderhost fixes this by placing two GOTO instructions at the chip’s real vector addresses, pointing into the application: 

;Vectors
    ORG    0
    goto    BASPROGRAMSTART
    ORG    8
    goto    <address>+0x08          ;high-priority interrupt: to the application
    ORG    24
    goto    <address>+0x18          ;low-priority interrupt: to the application
;Program_memory_page: 0
    ORG    28
BASPROGRAMSTART

address is the same value the application uses with #option bootloader address.

Currently supported on PIC18 (14-bit and other PIC families are not yet supported). AVR is not supported - the AVR boot section works differently and a bootloader built this way is unlikely to need it.

Rules and checks:

Rule Error if broken

The host cannot have its own interrupt handler (Sub Interrupt, On Interrupt) - its vectors belong to the application

#option bootloaderhost: interrupts are forwarded to the application; the bootloader cannot have an interrupt handler

The host’s own code (and tables) must end below the application address

#option bootloaderhost: program reaches address, the application area starts at address

#option bootloader and #option bootloaderhost cannot both be used in the same program

use either #option bootloader (an application) or #option bootloaderhost (a bootloader)

The address must be even, and above the host’s own vector area (greater than 0x001C)

#option bootloaderhost: invalid application address

Configuration words are emitted as normal - the host owns them. InitSys and #option StartupMethodsDisabled behave exactly as they do without this option.

Example:

    'The bootloader itself - sits at the reset vector, forwards both interrupt
    'vectors to an application that will be loaded at 0xA000
    #chip 18F4550, 20
    #option bootloaderhost 0xA000

    'bootloader code goes here - receive the application over a serial/USB
    'link and write it into flash starting at 0xA000, then jump there

    Do
    Loop

See Also:

  • #Option Bootloader — builds the application that runs at the forwarded address
  • #Option ReserveHighProg — an alternative way to protect a bootloader area already written with an external programmer
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