MASM provides a SEGMENT directive. The directive takes several parameters. The use parameter can take a value FLAT. It's unclear to me what this value does.
The Microsoft docs specify it as an accepted value but make no attempt to describe it:
use
USE16, USE32, FLAT
The book The Art of Assembly Language Programming available online mentions it but calls it out of scope and recommends reading the MASM Programmer's Guide:
The
use32andflatoperands tell MASM to generate code for a 32 bit segment. Since this text does not deal with protected mode programming we will not consider these options. See the MASM Programmer's Guide for more details.
In the MASM 6.1 Programmer's Guide from Microsoft, in the section describing the SEGMENT directive, the FLAT value is mentioned but its effects are never described:
The size attribute can be USE16, USE32, or FLAT.
What are the effects of the FLAT operand to the SEGMENT directive?