A .COM file is a direct binary program image. The first byte in the .COM file coresponds to the first byte executed when it is loaded. The load address for a .COM file is always 100h, although the segment may be anything, and is allocated by the operating system. The only restraint on a .COM file is the fact that each segment is 64k, so a COM file which is larger than 64k may not load properly, the data after 64k in the file may just be ignored, or it may overwrite the data at the start of the segment (PSP, then program). So basically, try to keep com files smaller than 64k!