Isaac Marino Bavaresco wrote:
> I=B4m using the PIC18 TCP/IP boot-loader from Microchip, and it indeed
> is almost transparent to the application developer.
>
> The only change to the code one must do is to reduce the length of the
> program FLASH segment in the linker-script, because the boot-loader
> resides at the end of the flash memory.
>
> The boot-loader then takes the first two instructions of the
> application, saves them in an area inside itself followed with a jump
> to the third instruction and replaces the original instructions with
> a jump to its own start.
>
> When calling the application, the boot-loader just jumps to the saved
> original instructions.

That seems silly for a production bootloader.  You're not trying to teach
students about how a bare PIC works, and you are in a environment where some
restrictions on the app code should be fine.

My TCP bootloader for the PIC 18 just specifies that execution of the app
starts at 4.  That leaves two instruction words before the first interrupt
vector.  That is enough for a GOTO instruction, which can jump to any
location in memory without being dependent on current state (unlike a PIC 16
GOTO which depends on PCLATH).  The bootloader owns locations 0-3 and a
chunk at the end of memory.  The bootloader only needs to ignore data sent
to it outside the app range.

This still makes it easy to test the app by itself.  The app code has two
NOP instructions at 0, followed by the GOTO.  When the app is loaded
directly with a programmer or the ICE, the PIC executes the two NOPs on
startup instead of the bootloader.  The same HEX file can either be
programmed directly or passed to the upload program to send to the
bootloader just like in the other case, but the bootloader itself is less
complex.


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