How do I fill unused program memory with a known value using the XC8 compiler?

The --FILL driver option allows you to fill the unused program memory with a known value. 
You can access this option in MPLAB X IDE, XC8 Project Properties, XC8 Linker, Fill Flash Memory selection in the drop-down menu; and then enter the appropriate argument in the Fill field.

This option utilizes the HEXMATE application to fill unused locations, thus it will only work for HEX files — it will not fill unused locations in binary file outputs.

The usage of the driver option is:

Executing the following command will fill the unused locations in hex file with 0x3FFF

hexmate.exe Code.hex --fill=0x3FFF

fill_expr can use the syntax (where const and increment are n-byte constants):

  • const fill memory with a repeating constant.
    For example: --FILL=0xBEEF becomes 0xBEEF, 0xBEEF, 0xBEEF, 0xBEEF
  • const+=increment fill memory with an incrementing constant.
    For example: --fill=0xBEEF+=1 becomes 0xBEEF, 0xBEF0, 0xBEF1, 0xBEF2
  • const-=increment fill memory with a decrementing constant.
    For example: --fill=0xBEEF-=0x10 becomes 0xBEEF, 0xBEDF, 0xBECF, 0xBEBF
  • const,const,…,const fill memory with a list of repeating constants.
    For example: --FILL=0xDEAD,0xBEEF becomes 0xDEAD,0xBEEF,0xDEAD,0xBEEF

The options following fill_expr result in the following behavior:

  • @unused (or nothing) fill all unused memory with fill_expr.
    For example: --FILL=0xBEEF@unused fills all unused memory with 0xBEEF.
  • @address fill a specific address with fill_expr.
    For example: --FILL=0xBEEF@0x1000 puts 0xBEEF at address 1000h
  • @address:end_address fill a range of memory with fill_expr.
    For example: --FILL=0xBEEF@0:0xFF puts 0xBEEF in unused addresses between 0 and 255
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