DOS Flat Mode from The Tech ARP BIOS Guide

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DOS Flat Mode from The Tech ARP BIOS Guide

DOS Flat Mode

Common Options : Enabled, Disabled

 

Quick Review of DOS Flat Mode

The DOS Flat Mode BIOS feature controls the BIOS’ built-in extended memory manager.

When enabled, DOS programs can run in protected mode without the need of an extended memory manager.

When disabled, DOS programs require an extended memory manager to run in protected mode.

It is recommended that you enable DOS Flat Mode if you use the MS-DOS operating system and run protected mode DOS programs.

However, if you use a newer operating system that supports protected mode (for example, Windows XP, disable DOS Flat Mode.

 

Details of DOS Flat Mode

In real mode, MS-DOS (Microsoft Disk Operating System) cannot address more than 1 MB. It is also restricted to memory segments of up to 64 KB in size.

These limitations can be circumvented by making use of extended memory managers. Such software allow DOS programs to make use of memory beyond 1MB by running them in protected mode. This mode also removes the segmentation of memory and creates a flat memory model for the program to use.

Protected mode refers to a memory addressing mode supported since the Intel 80286 was introduced. It provides the following benefits over real mode :

  • Each program can be given its own protected memory area, hence, the name protected mode. In this protected memory area, the program is protected from interference by other programs.
  • Programs can now access more than 1 MB of memory (up to 4 GB). This allows the creation of bigger and more complex programs.
  • There is virtually no longer any segmentation of the memory. Memory segments can be up to 4 GB in size.

As you can see, running DOS programs in protected mode allows safer memory addressing, access to more memory and eliminates memory segmentation. Of course, the DOS program must be programmed to make use of protected mode. An extended memory manager, either within the program or as a separate program, must also be present for the program to run in protected mode. Otherwise, it will run in real mode.

Please note that even with an extended memory manager, MS-DOS does not actually run in protected mode. Only specifically-programmed DOS programs will run in protected mode. The processor switches between real mode and protected mode to handle both MS-DOS in real mode and the DOS program in protected mode.

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This is where Gate A20 becomes critical to the computer’s performance. For more information on the effect of Gate A20 on the memory mode switching performance of the processor, please consult the Gate A20 Option BIOS feature.

The DOS Flat Mode BIOS feature controls the BIOS’ built-in extended memory manager.

When enabled, DOS programs can run in protected mode without the need of an extended memory manager.

When disabled, DOS programs require an extended memory manager to run in protected mode.

It is recommended that you enable DOS Flat Mode if you use the MS-DOS operating system and run protected mode DOS programs.

However, if you use a newer operating system that supports protected mode (for example, Windows XP), disable DOS Flat Mode.

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