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. In all cases, the number is big-endian.
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 }