ld.info: Output Section Fill

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Output section fill

   You can set the fill pattern for an entire section by using
`=FILLEXP'.  FILLEXP is an expression (*note Expressions::).  Any
otherwise unspecified regions of memory within the output section (for
example, gaps left due to the required alignment of input sections)
will be filled with the two least significant bytes of the value,
repeated as necessary.
   You can also change the fill value with a `FILL' command in the
output section commands; see *Note Output Section Data::.
   Here is a simple example:
     SECTIONS { .text : { *(.text) } =0x9090 }