📄 xms.txt
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eXtended Memory Specification (XMS), ver 2.0
July 19, 1988
Copyright (c) 1988, Microsoft Corporation, Lotus Development
Corporation, Intel Corporation, and AST Research, Inc.
Microsoft Corporation
Box 97017
16011 NE 36th Way
Redmond, WA 98073
LOTUS (r)
INTEL (r)
MICROSOFT (r)
AST (r) Research
This specification was jointly developed by Microsoft Corporation,
Lotus Development Corporation, Intel Corporation,and AST Research,
Inc. Although it has been released into the public domain and is not
confidential or proprietary, the specification is still the copyright
and property of Microsoft Corporation, Lotus Development Corporation,
Intel Corporation, and AST Research, Inc.
Disclaimer of Warranty
MICROSOFT CORPORATION, LOTUS DEVELOPMENT CORPORATION, INTEL
CORPORATION, AND AST RESEARCH, INC., EXCLUDE ANY AND ALL IMPLIED
WARRANTIES, INCLUDING WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE. NEITHER MICROSOFT NOR LOTUS NOR INTEL NOR AST
RESEARCH MAKE ANY WARRANTY OF REPRESENTATION, EITHER EXPRESS OR
IMPLIED, WITH RESPECT TO THIS SPECIFICATION, ITS QUALITY,
PERFORMANCE, MERCHANTABILITY, OR FITNESS FOR A PARTICULAR PURPOSE.
NEITHER MICROSOFT NOR LOTUS NOR INTEL NOR AST RESEARCH SHALL HAVE ANY
LIABILITY FOR SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES ARISING
OUT OF OR RESULTING FROM THE USE OR MODIFICATION OF THIS
SPECIFICATION.
This specification uses the following trademarks:
Intel is a registered trademark of Intel Corporation, Microsoft is a
registered trademark of Microsoft Corporation, Lotus is a registered
trademark of Lotus Development Corporation, and AST is a registered
trademark of AST Research, Inc.
Extended Memory Specification
=============================
The purpose of this document is to define the Extended Memory Specifica-
tion (XMS) version 2.00 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.
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
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 2.00 of the specification.
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