This sections is a part of the .bss section. What makes the .noinit section special is that variables which are defined as such:

int foo __attribute__ ((section (".noinit")));

will not be initialized to zero during startup as would normal .bss data.

Only uninitialized variables can be placed in the .noinit section. Thus, the following code will cause avr-gcc to issue an error:

int bar __attribute__ ((section (".noinit"))) = 0xaa;

It is possible to tell the linker explicitly where to place the .noinit section by adding -Wl,--section-start=.noinit=0x802000 to the avr-gcc command line at the linking stage. For example, suppose you wish to place the .noinit section at SRAM address 0x2000:

$ avr-gcc ... -Wl,--section-start=.noinit=0x802000 ...

Alternatively, you can write your own linker script to automate this. [FIXME: need an example or ref to dox for writing linker scripts.]