EXTENDED MEMORY SPECIFICATION  
Version 3.0 

January 1991 
 
Copyright ? 1988, Microsoft Corporation, Lotus Development Corporation, Intel Corporation, and AST Research, Inc. 
 

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The purpose of this document is to define the Extended Memory Specification (XMS) version 3.00 for MS-DOS. XMS allows DOS programs to utilize additional memory found in Intel's 80286 and 80386 based machines in a consistent, machine independent manner. With some restrictions, XMS adds almost 64K to the 640K which DOS programs can access directly. Depending on available hardware, XMS may provide even more memory to DOS programs. XMS also provides DOS programs with a standard method of storing data in extended memory. 

To be considered fully XMS 3.0 compliant, all calls except those associated with UMB support must be implemented. UMB functions 10h, 11h and 12h are optional for XMS 3.0 and may return the Function Not Implemented error code, 80h. 

DEFINITIONS
Extended Memory 
Memory in 80286 and 80386 based machines which is located above the 1MB address boundary. 
High Memory Area (HMA) 
The first 64K of extended memory. The High Memory Area is unique because code can be executed in it while in real mode. The HMA officially starts at FFFF:10h and ends at FFFF:FFFFh making it 64K-16 bytes in length. 
Upper Memory Blocks (UMBs) 
Blocks of memory available on some 80x86 based machines which are located between DOS's 640K limit and the 1MB address boundary. The number, size, and location of these blocks vary widely depending upon the types of hardware adapter cards installed in the machine. 
Extended Memory Blocks (EMBs) 
Blocks of extended memory located above the HMA which can only be used for data storage. 
A20 Line 
The 21st address line of 80x86 CPUs. Enabling the A20 line allows access to the HMA. 
XMM 
An Extended Memory Manager. A DOS device driver which implements XMS. XMMs are machine specific but allow programs to use extended memory in a machine-independent manner. 
HIMEM.SYS 
The Extended Memory Manager currently being distributed by Microsoft. 
Helpful Diagram: 

|                                                       |   Top of Memory
|                                                       |
|                                                       |
|                         /\                            |
|                        /||\                           |
|                         ||                            |
|                         ||                            |
|                                                       |
|                                                       |
|                                                       |
|             Possible Extended Memory Block            |
|                                                       |
|                                                       |
|                                                       |
|                         ||                            |
|                         ||                            |
|                        \||/                           |
|                         \/                            |
|                                                       |
|                                                       |
|       Other EMBs could exist above 1088K (1MB+64K)    |
|                                                       |
|                                                       |
|                                                       |   1088K
|                                                       |
|                                                       |
|               The High Memory Area                    |
|                                                       |
|                                                       |
|                                                       |   1024K or 1MB
|                                                       |
|                        /\                             |
|                       /||\                            |
|                        ||                             |
|                        ||                             |
|                                                       |
|                                                       |
|            Possible Upper Memory Block                |
|                                                       |
|                        ||                             |
|                        ||                             |
|                       \||/                            |
|                        \/                             |
|                                                       |
|       Other UMBs could exist between 640K and 1MB     |
|                                                       |
|                                                       |   640K
|                                                       |
|                                                       |
|                                                       |
|            Conventional or DOS Memory                 |
|                                                       |
|                                                       |
|                                                       |
|                                                       |
|                                                       |
+                                                       +   0K

DRIVER INSTALLATION
An XMS driver is installed by including a DEVICE= statement in the machine's CONFIG.SYS file. It must be installed prior to any other devices or TSRs which use it. An optional parameter after the driver's name (suggested name "/HMAMIN=") indicates the minimum amount of space in the HMA a program can use. Programs which use less than the minimum will not be placed in the HMA. See "Prioritizing HMA Usage" below for more information. A second optional parameter (suggested name "/NUMHANDLES=") allows users to specify the maximum number of extended memory blocks which may be allocated at any time. 

NOTE: XMS requires DOS 3.00 or above. 

THE PROGRAMMING API
The XMS API Functions are accessed via the XMS driver's Control Function. The address of the Control Function is determined via INT 2Fh. First, a program should determine if an XMS driver is installed. Next, it should retrieve the address of the driver's Control Function. It can then use any of the available XMS functions. The functions are divided into several groups: 

1. Driver Information Functions (0h) 
2. HMA Management Functions (1h-2h) 
3. A20 Management Functions (3h-7h) 
4. Extended Memory Management Functions (8h-Fh) 
5. Upper Memory Management Functions (10h-11h) 
DETERMINING IF AN XMS DRIVER IS INSTALLED
The recommended way of determining if an XMS driver is installed is to set AH=43h and AL=00h and then execute INT 2Fh. If an XMS driver is available, 80h will be returned in AL. 

Example: 

            ; Is an XMS driver installed?
            mov     ax,4300h
            int     2Fh
            cmp     al,80h
            jne     NoXMSDriver

CALLING THE API FUNCTIONS
Programs can execute INT 2Fh with AH=43h and AL=10h to obtain the address of the driver's control function. The address is returned in ES:BX. This function is called to access all of the XMS functions. It should be called with AH set to the number of the API function requested. The API function will put a success code of 0001h or 0000h in AX. If the function succeeded (AX=0001h), additional information may be passed back in BX and DX. If the function failed (AX=0000h), an error code may be returned in BL. Valid error codes have their high bit set. Developers should keep in mind that some of the XMS API functions may not be implemented by all drivers and will return failure in all cases. 

Example: 

            ; Get the address of the driver's control function
            mov     ax,4310h
            int     2Fh
            mov     word ptr [XMSControl],bx        ; XMSControl is a DWORD
            mov     word ptr [XMSControl+2],es

            ; Get the XMS driver's version number
            mov     ah,00h
            call    [XMSControl]    ; Get XMS Version Number

NOTE: Programs should make sure that at least 256 bytes of stack space is available before calling XMS API functions. 

API FUNCTION DESCRIPTIONS
The following XMS API functions are available: 

0h) Get XMS Version Number 
1h) Request High Memory Area 
2h) Release High Memory Area 
3h) Global Enable A20 
4h) Global Disable A20 
5h) Local Enable A20 
6h) Local Disable A20 
7h) Query A20 
8h) Query Free Extended Memory 
9h) Allocate Extended Memory Block 
Ah) Free Extended Memory Block 
Bh) Move Extended Memory Block 
Ch) Lock Extended Memory Block 
Dh) Unlock Extended Memory Block 
Eh) Get Handle Information 
Fh) Reallocate Extended Memory Block 
10h) Request Upper Memory Block 
11h) Release Upper Memory Block 
12h) Realloc Upper Memory Block 
88h) Query any Free Extended Memory 
89h) Allocate any Extended Memory Block 
8Eh) Get Extended EMB Handle 
8Fh) Realloc any Extended Memory 
Each is described below. 

Get XMS Version Number (Function 00h)
ARGS: 

AH = 00h 
RETS:

AX = XMS version number 
BX = Driver internal revision number 
DX = 0001h if the HMA exists, 0000h otherwise 
ERRS: 

None 
This function returns with AX equal to a 16-bit BCD number representing the revision of the DOS Extended Memory Specification which the driver implements (e.g. AX=0235h would mean that the driver implemented XMS version 2.35). BX is set equal to the driver's internal revision number mainly for debugging purposes. DX indicates the existence of the HMA (not its availability) and is intended mainly for installation programs. 

NOTE: This document defines version 3.00 of the specification. 

Request High Memory Area (Function 01h)
ARGS:

AH = 01h 
If the caller is a TSR or device driver, 
DX = Space needed in the HMA by the caller in bytes 
If the caller is an application program, 
DX = FFFFh 
RETS:

AX = 0001h if the HMA is assigned to the caller, 0000h otherwise 
ERRS:

BL = 80h if the function is not implemented 
BL = 81h if a VDISK device is detected 
BL = 90h if the HMA does not exist 
BL = 91h if the HMA is already in use 
BL = 92h if DX is less than the /HMAMIN= parameter 
This function attempts to reserve the 64K-16 byte high memory area for the caller. If the HMA is currently unused, the caller's size parameter is compared to the /HMAMIN= parameter on the driver's command line. If the value passed by the caller is greater than or equal to the amount specified by the driver's parameter, the request succeeds. This provides the ability to ensure that programs which use the HMA efficiently have priority over those which do not. 

NOTE: See the sections "Prioritizing HMA Usage" and "High Memory Area Restrictions" below for more information. 

Release High Memory Area (Function 02h)
ARGS:

AH = 02h 
RETS:

AX = 0001h if the HMA is successfully released, 0000h otherwise 
ERRS:

BL = 80h if the function is not implemented 
BL = 81h if a VDISK device is detected 
BL = 90h if the HMA does not exist 
BL = 93h if the HMA was not allocated 
This function releases the high memory area and allows other programs to use it. Programs which allocate the HMA must release it before exiting. When the HMA has been released, any code or data stored in it becomes invalid and should not be accessed. 

Global Enable A20 (Function 03h)
ARGS:

AH = 03h 
RETS:

AX = 0001h if the A20 line is enabled, 0000h otherwise 
ERRS:

BL = 80h if the function is not implemented 
BL = 81h if a VDISK device is detected 
BL = 82h if an A20 error occurs 
This function attempts to enable the A20 line. It should only be used by programs which have control of the HMA. The A20 line should be turned off via Function 04h (Global Disable A20) before a program releases control of the system. 

NOTE: On many machines, toggling the A20 line is a relatively slow operation. 

Global Disable A20 (Function 04h)
ARGS:

AH = 04h 
RETS:

AX = 0001h if the A20 line is disabled, 0000h otherwise 
ERRS:

BL = 80h if the function is not implemented 
BL = 81h if a VDISK device is detected 
BL = 82h if an A20 error occurs 
BL = 94h if the A20 line is still enabled 
This function attempts to disable the A20 line. It should only be used by programs which have control of the HMA. The A20 line should be disabled before a program releases control of the system. 

NOTE: On many machines, toggling the A20 line is a relatively slow operation. 

Local Enable A20 (Function 05h)
ARGS:

AH = 05h 
RETS:

AX = 0001h if the A20 line is enabled, 0000h otherwise 
ERRS:

BL = 80h if the function is not implemented 
BL = 81h if a VDISK device is detected 
BL = 82h if an A20 error occurs 
This function attempts to enable the A20 line. It should only be used by programs which need direct access to extended memory. Programs which use this function should call Function 06h (Local Disable A20) before releasing control of the system. 

NOTE: On many machines, toggling the A20 line is a relatively slow operation. 

Local Disable A20 (Function 06h)
ARGS:

AH = 06h 
RETS:

AX = 0001h if the function succeeds, 0000h otherwise 
ERRS:

BL = 80h if the function is not implemented 
BL = 81h if a VDISK device is detected 
BL = 82h if an A20 error occurs 
BL = 94h if the A20 line is still enabled 
This function cancels a previous call to Function 05h (Local Enable A20). It should only be used by programs which need direct access to extended memory. Previous calls to Function 05h must be canceled before releasing control of the system. 

NOTE: On many machines, toggling the A20 line is a relatively slow operation.