MS-DOS Memory Management: Conventional, Upper, Expanded

MS-DOS Memory Management

The memory management depends on the version of MS-DOS we use and the CPU that we have.

Processor Memory Limits

Processors have different address limits:

• 8088 / 8086 (XT): handle up to 1 MB
• 286 (AT): handle up to 16 MB
• 386: handle up to 4096 MB

Conventional Memory

Conventional memory is where programs run; you must manage it properly and release memory when possible.

0 Kb

S.O.

Contains the interrupt table, computer specification, etc.

2 KB

Core MS-DOS Files

The core files include (examples by version):

• IO.SYS (depending on the version)
• MSDOS.SYS (system)
• Other variable core components

Order File (CONFIG.SYS)

Configuration directives and settings found in CONFIG.SYS:

• BUFFERS
• FCBS
• FILES
• LASTDRIVE
• STACKS

Device Drivers

Device drivers loaded via CONFIG.SYS include:

• DEVICE=...

Some drivers commonly loaded:

• ANSI.SYS
• HIMEM.SYS
• EMM386.EXE
• Ramdrive.sys

COMMAND.COM (Command Processor)

COMMAND.COM — command processor or shell.

4.8 Kb

Resident Programs and AUTOEXEC.BAT

Resident programs loaded at startup (for example via AUTOEXEC.BAT):

• KEYB
• DOSKEY
• FastOpen
• PRINT

Free Memory and User Applications

Free memory: here are loaded user applications.

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.

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640 Kb

The core MS-DOS and COMMAND.COM files can be moved out of conventional memory to leave more space within the first 640 KB and allow more user applications. These files can be loaded into upper memory or into extended memory areas when supported by the system.

Upper Memory

Upper memory stores routines and control programs.

640 Kb

Video Memory

Here is where information to be represented by the graphics card is stored; this matches the memory chips of the graphics card.

Expanded Memory (EMS)

Expanded memory is a memory-management technique that addresses the 1 MB limitation of MS-DOS. Through it, we access memory above 1024 KB by creating page frames in upper memory. A page frame occupies 64 KB and is divided into 4 pages of 16 KB.

ROM BIOS

Here are placed the instructions for the BIOS (Basic Input/Output System).

1024 Kb (1 Mb)

Extended Memory

MS-DOS does not natively work with addresses above 1 MB, so using expanded/extended memory techniques allows that limit to be exceeded.

1024 Kb

High Memory Area (HMA)

Using commands like DOS=HIGH, the core MS-DOS can be loaded into the High Memory Area. Depending on how COMMAND.COM and CONFIG.SYS are structured, drivers can also be uploaded there. This frees conventional memory for loading programs.

1088 kb

By the command Ramdrive.sys logical drives are available in RAM as a removable, volatile disk.