some time ago, someone said me that 'rb' reserves already nullified byte, and 'db' no. But now im wondering how it can be not nullified. Compiler adds 00 to output file.

times 256 db 'a'

or -

db 512 dup "b"

or as a last resort after every other avenue has been tried and failed -

read the fasm documentation

rb and db are mostly the same. rb will reserve bytes with a value of 0. But the main difference is when fasm writes the file, the reserved spaces at the end of each section are not written to disk, thus saving disk space. Above code won't put 1meg of zeros on the disk. Compare to: The difference here is that fasm must put that last byte on to disk so it will also put the 1meg of zeros before it onto the disk file.