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(nasm.info)Section 7.2.1

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7.2.1. Using the `bin' Format To Generate `.COM' Files
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   `.COM' files expect to be loaded at offset `100h' into their segment
(though the segment may change). Execution then begins at `100h', i.e.
right at the start of the program. So to write a `.COM' program, you
would create a source file looking like

             org 100h
     
     section .text
     
     start:
             ; put your code here
     
     section .data
     
             ; put data items here
     
     section .bss
     
             ; put uninitialised data here

   The `bin' format puts the `.text' section first in the file, so you
can declare data or BSS items before beginning to write code if you
want to and the code will still end up at the front of the file where it
belongs.

   The BSS (uninitialised data) section does not take up space in the
`.COM' file itself: instead, addresses of BSS items are resolved to
point at space beyond the end of the file, on the grounds that this
will be free memory when the program is run. Therefore you should not
rely on your BSS being initialised to all zeros when you run.

   To assemble the above program, you should use a command line like

     nasm myprog.asm -fbin -o myprog.com

   The `bin' format would produce a file called `myprog' if no explicit
output file name were specified, so you have to override it and give
the desired file name.