If you need complete control over segments, you can fully define the segments in your program. This section explains segment definitions, including how to order segments and how to define the segment types.
If you write a program under DOS without .MODEL and .STARTUP, you must initialize registers yourself and use the END directive to indicate the starting address. Under OS/2 you do not have to initialize registers. Section 2.3.2, “Controlling the Segment Order,” describes typical start-up code.