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(nasm.info)Section 7.2.2

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7.2.2. Using the `obj' Format To Generate `.COM' Files
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   If you are writing a `.COM' program as more than one module, you may
wish to assemble several `.OBJ' files and link them together into a
`.COM' program. You can do this, provided you have a linker capable of
outputting `.COM' files directly (TLINK does this), or alternatively a
converter program such as `EXE2BIN' to transform the `.EXE' file output
from the linker into a `.COM' file.

   If you do this, you need to take care of several things:

   * The first object file containing code should start its code
     segment with a line like `RESB 100h'. This is to ensure that the
     code begins at offset `100h' relative to the beginning of the code
     segment, so that the linker or converter program does not have to
     adjust address references within the file when generating the
     `.COM' file. Other assemblers use an `ORG' directive for this
     purpose, but `ORG' in NASM is a format-specific directive to the
     `bin' output format, and does not mean the same thing as it does
     in MASM-compatible assemblers.

   * You don't need to define a stack segment.

   * All your segments should be in the same group, so that every time
     your code or data references a symbol offset, all offsets are
     relative to the same segment base. This is because, when a `.COM'
     file is loaded, all the segment registers contain the same value.