Overview of Memory-Management Functionality in MS-DOS

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This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.


This article contains an overview of how expanded memory that conforms to the Expanded Memory Specification (EMS) and extended memory that conforms to the Extended Memory Specification (XMS) is created and managed in MS- DOS versions 5.0 and later by the device drivers HIMEM.SYS and EMM386.EXE. How MS-DOS is loaded into the high memory area (HMA) and manages upper memory blocks (UMBs) are also discussed.

NOTE: Before reading this article, you may want to review the sections about memory management, HIMEM.SYS, and EMM386.EXE in your printed MS-DOS documentation. If you are using MS-DOS 6.0, 6.2, 6.21, or 6.22, refer to the online help. For example, type help emm386.exe at the MS-DOS command prompt.

Information in this article is organized as follows:

  • Old Extended Memory Allocation Methods: Interrupt 15h and VDisk Headers
  • HIMEM.SYS Creates XMS Memory
  • A20 and the HMA
  • DOS=HIGH Asks MS-DOS to Run in the HMA
  • EMM386.EXE [RAM | NOEMS] Uses XMS to Create EMS and/or UMBs
  • DOS=UMB Asks MS-DOS to Manage UMBs
  • DEVICEHIGH=[<drive>:][\<path>\]<filename>
  • MEM Reports on Memory MS-DOS Is Managing
  • How MS-DOS Uses Extended or Expanded Memory
  • Available References
NOTE: Only the section "How MS-DOS Uses Extended or Expanded Memory" applies to MS-DOS versions 3.x and 4.x.

More Information

Old Extended-Memory Allocation Methods: Interrupt 15h & VDisk Headers

In the past, there were two ways for programs to allocate extended memory: top-down (using Interrupt 15) and bottom-up (using VDisk headers).

Interrupt 15h is a ROM BIOS service that includes several extensions to the original PC ROM BIOS, including the means to find out how much RAM (conventional plus extended) is on the system. A program uses this service to find out how much extended memory there is, then "hooks" Interrupt 15h and reports to any other programs that there is <n>K less memory available, effectively slicing <n>K of extended memory off the top. By doing this, the program has allocated its own extended memory from the top of the memory pool.

Bottom-up memory allocation works by checking for a header at the start of extended memory stating that <n>K of RAM is in use. If a header is there, the program checks <n>K further for another header. If no header exists, the program puts in its own header. These headers are called VDisk headers because the original IBM DOS RAM drive utility (VDISK.SYS) uses this method.

A drawback to these methods is that after memory has been allocated to a program, deallocating it is usually not possible. The XMS was designed to make allocating and deallocating extended memory easier for all involved.

HIMEM.SYS Creates XMS Memory

HIMEM.SYS implements all of the XMS except the optional UMB portion. HIMEM.SYS versions earlier than 3.0 are XMS 2.0 compliant and recognize up to 16 MB of RAM; versions 3.0 and later (first included with Windows 3.10 and MS-DOS 6.0) are XMS 3.0 compliant and recognize up to 4 GB.

On loading, HIMEM.SYS determines the amount of extended memory available. Unless it was loaded with the /INT15= option, HIMEM.SYS sets out to allocate all available extended memory for use as XMS memory (note the actual allocation of all the available extended memory does not occur until a program makes an XMS function call). HIMEM.SYS reserves the HMA with a VDisk header and hooks Interrupt 15h. Programs that want to use extended memory (other than the HMA) without using XMS can do so until a program actually requests XMS.

After a program actually asks for XMS memory, HIMEM.SYS uses its Interrupt 15h hook to notify programs that only the amount of extended memory specified by /INT15=xxxx, minus the HMA (64K), is available for use. Memory available through /INT15=xxxx is physically located above the HMA; XMS- managed RAM is physically located above any /INT15=xxxx RAM.

A20 and the HMA

The HMA is defined as FFFF:0010-FFFF:FFFF on 80286 and higher systems that have physical addressable RAM at these addresses. This area can be addressed in real mode (8086 emulation) on 80286 and higher systems if the 21st address line (A20) is enabled, which produces 64K-16 bytes of additional usable RAM. On an 8086 or an 80x86 with the A20 line disabled, FFFF:0010 "wraps around" and is the same as 0000:0000.

Turning this line on and off is accomplished using the keyboard port and is hardware dependent. HIMEM.SYS includes a number of "A20 handlers" for different machines. The XMS allocates and deallocates the HMA as one block, that is, only one program can use it at a time. MS-DOS 5.0 and later can run in the HMA if DOS=HIGH is in the CONFIG.SYS file.

DOS=HIGH Asks MS-DOS to Run in the HMA

If DOS=LOW or no DOS= command is in the CONFIG.SYS file, MS-DOS and its data are initialized and loaded into their final place in low memory before the DEVICE= and DEVICEHIGH= commands are processed.

If the DOS=HIGH command is in the CONFIG.SYS file, MS-DOS data (which must remain low for compatibility) is loaded into its final place in conventional memory. After each device driver is initialized, a check is made to determine if an XMS driver has been installed. If so, and if the HMA is available, MS-DOS is moved into the HMA. If not, MS-DOS keeps checking after each DEVICE[HIGH]= command, and then begins processing the INSTALL= commands.

If a DOS=HIGH command exists but MS-DOS hasn't loaded high (if no XMS driver was loaded or the HMA wasn't available), MS-DOS reports "HMA not available/loading DOS low" and loads itself into conventional memory above all the installable device drivers and/or terminate-and-stay-resident (TSR) programs loaded with INSTALL=.

The transient portion of COMMAND.COM remains in conventional memory whether MS-DOS is loaded high or low. Most of the COMMAND.COM resident portion, any software code pages, and the disk buffers (usually) also load high if DOS=HIGH is in the CONFIG.SYS file.

EMM386.EXE [RAM | NOEMS] Uses XMS to Create EMS and/or UMBs

EMM386.EXE is a device driver for 80386 and higher systems with XMS memory. EMM386.EXE uses XMS memory to create and manage EMS memory and/or XMS upper memory blocks (UMBs). EMS is available to programs through the EMS 4.0 interface; UMBs are available through the XMS interface. When providing UMBs, EMM386.EXE answers only requests to allocate or deallocate UMBs; all other XMS memory is managed by HIMEM.SYS.

On loading, EMM386.EXE shows a report of its activity. Among other things, this report includes:

  • Whether EMS memory is being provided, and, if so:

    • Amount of EMS available
    • Address of EMS page frame segment
  • If UMBs are being provided, the following is also shown:

    • Amount of UMBs available through the XMS
    • Largest UMB available through the XMS
    • Address of upper memory starting segment
This report is also available at the MS-DOS command prompt by running EMM386.EXE.

NOTE: EMM386.EXE versions 4.45 and later do not display this information at startup UNLESS the /VERBOSE switch is used.

DOS=UMB Asks MS-DOS to Manage UMBs

The DOS=UMB command asks MS-DOS to allocate any UMBs available through the XMS to itself. MS-DOS then makes UMBs available through its own memory- management services in Interrupt 21h, including:

Function Description Version
48h Allocate memory 2.0
49h Free allocated memory 2.0
4Ah Set memory block size 2.0
5800h Get allocation strategy 2.0
5801h Set allocation strategy 2.0
5802h Get upper-memory link status 5.0
5803h Set upper-memory link status 5.0
If DOS=UMB is in the CONFIG.SYS file, EMM386.EXE reports that 0 (zero) UMBs are available from the MS-DOS command prompt, and any program that attempts to use UMBs through the XMS services is unable to find them.

Users can determine whether MS-DOS has any UMBs available by using the MEM /C command. Programs can use Interrupt 21h, Function 5803h to determine if UMBs exist.


The DEVICEHIGH= command asks that the device driver file be loaded into an MS-DOS UMB if there is one available that is big enough. If there isn't, the driver is loaded into conventional memory and executed; no error is displayed by MS-DOS.


The LH (or LOADHIGH) command tells MS-DOS to load the program file into an MS-DOS UMB if there is one available that is big enough. If there is not, the program is loaded into conventional memory and executed; no error is displayed by MS-DOS.


If you are using MS-DOS version 6.0, 6.2, 6.21, or 6.22, you can use the /L and /S switches to control the memory regions into which a device driver or program is loaded.

DEVICEHIGH [[/L:region1[,minsize1][;region2[,minssize2] [/S]]=
The /L: switch specifies the memory region(s) a program can be loaded into. If the /L: switch is not specified, MS-DOS loads the program into the largest free UMB and all other UMB regions are available to the program. Use the MEM /F command to view the available memory regions. (Region 0 is conventional memory.)

The /S switch shrinks the UMB to the minimum size (minsize) while the program is loading. This switch can only be used in conjunction with the /L: switch and affects only UMBs for which a minimum size was specified.

The following is an example:

This command loads PROGRAM.EXE and restricts it to load into memory regions 1 and 2. The program has access to only 12,194 bytes in region 1 and only 34,213 bytes in region 2.

NOTE: Rather than attempting to manually configure the memory regions programs load into, MS-DOS 6.0, 6.2, 6.21, and 6.22 users are encouraged to run the MemMaker memory optimization program. To run MemMaker, type
memmaker at the MS-DOS command prompt.

MEM Reports on Memory MS-DOS Is Managing

If MS-DOS is managing UMBs (DOS=UMB), MEM /C or MEM /D includes the UMA in its report. Any areas not being managed by MS-DOS are labeled "SYSTEM" in this report. For UMB information, type mem /c | more at the MS-DOS command prompt. (Note that if you are using MS-DOS 6.0 or later, you can type mem /c /p to view the memory report one screen at a time.)

If MS-DOS is not managing the UMBs, MEM does not report on the UMA. Other UMB managers typically have some means to determine what is loading high. Check the documentation for your UMB manager for details.

MEM also includes information about extended memory (determined using the Interrupt 15/VDisk header interface), the XMS (reported through the XMS 2.0 or 3.0 interface), and the EMS (reported through the EMS 3.2 or 4.0 interface) for your convenience.

How MS-DOS Uses Extended or Expanded Memory

As a whole, MS-DOS does not use extended or expanded memory for general usage or for loading applications.

However, some MS-DOS utilities and drivers use extended or expanded memory for data areas. In addition, MS-DOS versions 5.0 and later load most of the MS-DOS kernel, command interpreter, code pages (if used), and disk buffers into the HMA, which is the first 64K of extended memory.

The table below outlines which MS-DOS utilities use extended memory without an extended memory driver, extended memory through an XMS driver, or expanded memory through a Lotus-Intel-Microsoft (LIM) EMS driver.

These utilities load into conventional memory but use nonconventional memory for data areas. Because different versions of many utilities that ship with MS-DOS also ship with Microsoft Windows, the Windows versions are included in this table. Microsoft recommends that you do not mix versions of these drivers that are included with different products.

Conv. Extended XMS LIM 3.2/4.0 EMS
RAMDRIVE.SYS 3.3 Yes Yes No Yes
RAMDRIVE.SYS 4.x Yes Yes No Yes
RAMDRIVE.SYS Win30 Yes No Yes Yes
RAMDRIVE.SYS 5.x Yes No Yes Yes
RAMDRIVE.SYS Win31 Yes No Yes Yes
RAMDRIVE.SYS 6.0-6.22 Yes No Yes Yes

SMARTDRV.SYS 4.x No Yes No Yes
SMARTDRV.SYS Win30 No No Yes Yes
SMARTDRV.SYS 5.x No No Yes Yes

HIMEM.SYS (1) All No Yes Provider No

EMM386.SYS(2) 4.x No Yes No Provider
EMM386.SYS(3) Win30 No No Yes Provider
EMM386.EXE(3) All No No Yes(4) Provider

(1) Uses extended memory to provide XMS memory
(2) Uses extended memory to provide EMS memory
(3) Uses XMS memory to emulate EMS memory
(4) MS-DOS 5.0 and later EMM386.EXE can also be configured to
provide UMBs according to the XMS. This causes EMM386.EXE
to be a provider of the UMB portion of the XMS.
In MS-DOS versions 4.x, you can place the disk buffers in expanded memory by using the /X switch; however, this is not recommended because of problems that may arise. SMARTDRV can provide much of the same performance enhancements and is preferable to using BUFFERS /X. For more information about BUFFERS /X, query in the Microsoft Knowledge Base on the following words:
buffers and ems and /x
Loading the disk buffers in EMS memory is not supported in MS-DOS versions 5.0 and later. These versions support loading the MS-DOS kernel and, if there is room, the disk buffers into the HMA. MS-DOS accesses the HMA through the XMS protocol.

Available References

The official LIM EMS specification, "Lotus/Intel/Microsoft Expanded Memory Specification Version 4.0," is available from Intel by calling (800) 538- 3373.

The official XMS specification, "Extended Memory Specification Version 3.0," is available free from Microsoft. To obtain the specification, see the "Instructions for Downloading" section below.

The official MS-DOS program interface documentation, "Microsoft MS-DOS Programmer's Reference," is available from Microsoft Press (by calling [800] 677-7377), or internationally through Penguin Books.


Article ID: 95555 - Last Review: May 12, 2003 - Revision: 1