You can set the fill pattern for an entire section by using =fillexp. fillexp is an expression (see 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 value, repeated as necessary. If the fill expression is a simple hex number, ie. a string of hex digit starting with 0x and without a trailing k or M, then an arbitrarily long sequence of hex digits can be used to specify the fill pattern; Leading zeros become part of the pattern too. For all other cases, including extra parentheses or a unary +, the fill pattern is the four least significant bytes of the value of the expression. If the value is less than four bytes in size then it will be zero extended to four bytes. In all cases, the number is big-endian.

Fill Value     Fill Pattern
0x90           90 90 90 90
0x0090         00 90 00 90
144            00 00 00 90

You can also change the fill value with a FILL command in the output section commands; (see Output Section Data).

Here is a simple example:

SECTIONS { .text : { *(.text) } =0x90909090 }