Bin and other short question

Hi.
I wont to create bin file with using gas haw can i do
it. 
( fir example in nasm its enough to compile with -f
bin
nasm -f bin -o * *.asm )

Scund question is about closing and opening sections.
If forexample i want to change some thing in section i
have to create it with "w" index ... is it posible to
change this state inside section ? for example wfter
writing what i want to change i wont to close it ...
just for case ( interupt 125 do something like this ,
when i used it in .text section it didn'y made any
changes i mean if i try to write sometning iside i
recived segmentation error)

Lukas

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