If the .MODEL directive is preceded by the .386 or .486 directive (or their privileged counterparts), the assembler uses 32-bit segments. If the .386 or .486 directive follows the .MODEL directive, the default 16-bit segments are used. The flat memory model uses 32-bit segments and must be preceded by a .386 or .486
directive.
32-bit segments are available only on the 80386/486 processors. They are not supported by any version of MS-DOS or by OS/2 before version 2.0.