org 100h is because .com files are loaded into a 64kb segment,
and the first 100h bytes is the Program Segment Prefix. I believe
masm's ORG directive actually moves the point where the assembled
code is emitted, so you can use it to overwrite "old" code.

I'm more fond of nasm's org, it only changes the "the code is
suppose to run at this address", but it doesn't change the address
the code is emitted to. This can be very useful for creating overlays
(of one kind or another).
Posted on 2001-11-28 15:57:36 by f0dder
org 0100h is used because .com files begin at this address. But you can use org with any memory offset you want; another org very used is org 7C00h.
Posted on 2001-11-28 16:06:21 by CodeLover