What is flat memory model?

Answer 1

A program written in native 32 bit Windows format is created in what is called FLAT memory model that has a single segment, which contains both code and data. Such programs must be run on a 80386 or higher processor.

Differing from earlier 16-bit code that used combined segment and offset addressing with a 64 Kb segment limit, FLAT memory model works only in offsets and has a range of 4 Gigabytes. This makes assembly code easier to write and the compiled (assembled) code is generally a lot faster than the equivalent 16-bit code.

All segment registers are automatically set to the same value with the flat memory model. This means that segment / offset addressing must NOT be used in 32-bit programs that run in 32-bit Windows operating systems.

For programmers who have written code in DOS, a 32-bit Windows PE (executable) file is similar in some respects to a DOS COM file - they have a single segment that can contain both code and data and they both work directly in offsets. That is, neither uses segment / offset addressing.

Flat-model assembler code defaults to NEAR code addressing and NEAR data addressing within the range of 4 gigabytes.

The FS and GS segment registers are rarely (if ever) used in application programs but may be used in some instances by the operating system itself.