External RAM booter.
This 68HC11 bootstrap program acts as a simple server for uploading a program in external memory. It supports only two commands:
Write the memory at given address.
Jump to program at given address.
This bootstrap program is uploaded by GTAM to be able to upload larger program in external RAM.
- This program must not be linked with a startup file. It implements the startup entry point.
_start function must be at beginning of this file. By doing so, it will be mapped at address 0 by the linker and we avoid to have some jump to call it (since the boot will jump there).
- It must be compiled with -mshort -Os -fomit-frame-pointer to make sure it fits in less than 240 bytes (keep a few bytes at end for the stack).
- It must be compiled with -msoft-reg-count=0 or =1 to reduce the use of soft-registers (since they are mapped in RAM in .page0).
- The pointer used to write the memory is marked volatile to prevent GCC from optimizing memory accesses. We rely on this for the good computation of the CRC.
- Before scanning the memory, we switch the bus in the expanded mode. We do not switch back to single chip when jumping to the program.
- The soft registers must be located at beginning of the .page0. This will clobber the beginning of the
_start function. We don't care because this clobbers some initialisation part.
Source file: booter.c
- Another booter program is necessary to upload in EEPROM.
- It is not possible to write in the 68HC11 RAM. We don't check for this and this will probably crash.