The World of Computer Software/The World of Computer Software.iso/xmm12.zip/XMS30.DOC

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eXtended Memory Specification (XMS), version 3.0


January 1991


Copyright (c) 1988, Microsoft Corporation, Lotus Development
Corporation, Intel Corporation, and AST Research, Inc.

Microsoft Corporation
Box 97017

One Microsoft 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
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.












Extended Memory
(Other EMBs could exist above 1088K (1MB+64K)



Top of Memory
High Memory Area (HMA)
1088K


Upper Memory Area


1024K






Conventional or DOS memory





640K












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.

Query A20 (Function 07h):

    ARGS:    AH = 07h

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

    ERRS:    BL = 00h if the function succeeds
        BL = 80h if the function is not implemented
        BL = 81h if a VDISK device is detected

This function checks to see if the A20 line is physically enabled. It
does this in a hardware independent manner by seeing if "memory wrap"
occurs.


Query Free Extended Memory (Function 08h):

    ARGS:    AH = 08h

    RETS:    AX = Size of the largest free extended memory block in K-bytes
        DX = Total amount of free extended memory in K-bytes

    ERRS:    BL = 80h if the function is not implemented
        BL = 81h if a VDISK device is detected
        BL = A0h if all extended memory is allocated

This function returns the size of the largest available extended memory
block in the system.

NOTE: The 64K HMA is not included in the returned value even if it is
not in use.


Allocate Extended Memory Block (Function 09h):

    ARGS:    AH = 09h
        DX = Amount of extended memory being requested in K-bytes

    RETS:    AX = 0001h if the block is allocated, 0000h otherwise
        DX = 16-bit handle to the allocated block

    ERRS:    BL = 80h if the function is not implemented
        BL = 81h if a VDISK device is detected
        BL = A0h if all available extended memory is allocated
        BL = A1h if all available extended memory handles are in use

This function attempts to allocate a block of the given size out of the
pool of free extended memory. If a block is available, it is reserved
for the caller and a 16-bit handle to that block is returned.  The
handle should be used in all subsequent extended memory calls. If no
memory was allocated, the returned handle is null.

NOTE: Extended memory handles are scarce resources. Programs should try
to allocate as few as possible at any one time. When all of a driver's
handles are in use, any free extended memory is unavailable.

Free Extended Memory Block (Function 0Ah):

    ARGS:    AH = 0Ah
        DX = Handle to the allocated block which should be freed

    RETS:    AX = 0001h if the block is successfully freed, 0000h otherwise

    ERRS:    BL = 80h if the function is not implemented
        BL = 81h if a VDISK device is detected
        BL = A2h if the handle is invalid
        BL = ABh if the handle is locked

This function frees a block of extended memory which was previously
allocated using Function 09h (Allocate Extended Memory Block). Programs
which allocate extended memory should free their memory blocks before
exiting. When an extended memory buffer is freed, its handle and all
data stored in it become invalid and should not be accessed.


Move Extended Memory Block (Function 0Bh):

    ARGS:    AH = 0Bh
        DS:SI = Pointer to an Extended Memory Move Structure (see below)

    RETS:    AX = 0001h if the move is successful, 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 = A3h if the SourceHandle is invalid
        BL = A4h if the SourceOffset is invalid
        BL = A5h if the DestHandle is invalid
        BL = A6h if the DestOffset is invalid
        BL = A7h if the Length is invalid
        BL = A8h if the move has an invalid overlap
        BL = A9h if a parity error occurs

Extended Memory Move Structure Definition:

        ExtMemMoveStruct    struc
            Length              dd  ?   ; 32-bit number of bytes to transfer
            SourceHandle        dw  ?   ; Handle of source block
            SourceOffset        dd  ?   ; 32-bit offset into source
            DestHandle          dw  ?   ; Handle of destination block
            DestOffset          dd  ?   ; 32-bit offset into destination block
        ExtMemMoveStruct    ends

This function attempts to transfer a block of data from one location to
another. It is primarily intended for moving blocks of data between
conventional memory and extended memory, however it can be used for
moving blocks within conventional memory and within extended memory.

NOTE: If SourceHandle is set to 0000h, the SourceOffset is interpreted
as a standard segment:offset air which refers to memory that is directly
accessible by the processor.  The segment:offset pair is stored in Intel
DWORD notation. The same is true for DestHandle and DestOffset.


   SourceHandle and DestHandle do not have to refer to locked memory
   blocks.
   Length must be even.
   Although not required, WORD-aligned moves can be significantly faster
   on most machines. DWORD aligned move can be even faster on 80386
   machines.
   If the source and destination blocks overlap, only forward moves
   (i.e. where the source base is less than the destination base) are
   guaranteed to work properly.
   Programs should not enable the A20 line before calling this function.
   The state of the A20 line is preserved.

This function is guaranteed to provide a reasonable number of  interrupt
windows during long transfers.


Lock Extended Memory Block (Function 0Ch):

    ARGS:    AH = 0Ch
        DX = Extended memory block handle to lock

    RETS:    AX = 0001h if the block is locked, 0000h otherwise
        DX:BX = 32-bit physical address of the locked block

    ERRS:    BL = 80h if the function is not implemented
        BL = 81h if a VDISK device is detected
        BL = A2h if the handle is invalid
        BL = ACh if the block's lock count overflows
        BL = ADh if the lock fails

This function locks an extended memory block and returns its base
address as a 32-bit physical address.  Locked memory blocks are
guaranteed not to move.  The 32-bit pointer is only valid while the
block is locked.Locked blocks should be unlocked as soon as possible.

NOTE: A block does not have to be locked before using Function 0Bh (Move
Extended Memory Block). "Lock counts" are maintained for EMBs.


Unlock Extended Memory Block (Function 0Dh):

    ARGS:    AH = 0Dh
        DX = Extended memory block handle to unlock

    RETS:    AX = 0001h if the block is unlocked, 0000h otherwise

    ERRS:    BL = 80h if the function is not implemented
        BL = 81h if a VDISK device is detected
        BL = A2h if the handle is invalid
        BL = AAh if the block is not locked

This function unlocks a locked extended memory block.  Any
32-bitpointers into the block become invalid and should no longer be
used.


Get EMB Handle Information (Function 0Eh):

    ARGS:    AH = 0Eh
        DX = Extended memory block handle
    RETS:    AX = 0001h if the block's information is found, 0000h otherwise
        BH = The block's lock count
        BL = Number of free EMB handles in the system
        DX = The block's length in K-bytes

    ERRS:    BL = 80h if the function is not implemented
        BL = 81h if a VDISK device is detected
        BL = A2h if the handle is invalid

This function returns additional information about an extended memory
block to the caller.

NOTE: To get the block's base address, use Function 0Ch (Lock Extended
Memory Block).


Reallocate Extended Memory Block (Function 0Fh):

    ARGS:    AH = 0Fh
        BX = New size for the extended memory block in K-bytes
        DX = Unlocked extended memory block handle to reallocate

    RETS:    AX = 0001h if the block is reallocated, 0000h otherwise

    ERRS:    BL = 80h if the function is not implemented
        BL = 81h if a VDISK device is detected
        BL = A0h if all available extended memory is allocated
        BL = A1h if all available extended memory handles are in use
        BL = A2h if the handle is invalid
        BL = ABh if the block is locked

This function attempts to reallocate an unlocked extended memory block
so that it becomes the newly specified size.  If the new size is smaller
than the old block's size, all data at the upper end of the old block is
lost.


Request Upper Memory Block (Function 10h):

    ARGS:    AH = 10h
        DX = Size of requested memory block in paragraphs

    RETS:    AX = 0001h if the request is granted, 0000h otherwise
        BX = Segment number of the upper memory block
        If the request is granted,
        DX = Actual size of the allocated block in paragraphs otherwise,
        DX = Size of the largest available UMB in paragraphs

    ERRS:    BL = 80h if the function is not implemented
        BL = B0h if a smaller UMB is available
        BL = B1h if no UMBs are available

This function attempts to allocate an upper memory block to the caller.
If the function fails, the size of the largest free UMB is returned in
DX.

NOTE: By definition UMBs are located below the 1MB address boundary.
The A20 Line does not need to be enabled before accessing an allocated
UMB.

   UMBs are paragraph aligned.
   To determine the size of the largest available UMB, attempt to
   allocate one with a size of FFFFh.
   UMBs are unaffected by EMS calls.


Release Upper Memory Block (Function 11h):

    ARGS:    AH = 11h
        DX = Segment number of the upper memory block

    RETS:    AX = 0001h if the block was released, 0000h otherwise

    ERRS:    BL = 80h if the function is not implemented
        BL = B2h if the UMB segment number is invalid

This function frees a previously allocated upper memory block.  When an
UMB has been released, any code or data stored in it becomes invalid and
should not be accessed.


Reallocate Upper Memory Block (Function 12h)

    ARGS:    AH = 12h
        BX = New size for UMB in paragraphs
        DX = Segment number of the UMB to reallocate

    RETS:    AX = 1 if the block was reallocated, 0 otherwise

    ERRS:    BL = 80h if the function is not implemented
                BL = B0h if no UMB large enough to satisfy the request
                         is available.
                         In this event, DX is returned with the size of
                         the largest UMB that is available.
                BL = B2h if the UMB segment number is invalid

This function attempts to reallocate an Upper Memory Block to a newly
specified size. If the new size is smaller than the old block's size,
all data at the upper end of the block is lost.


SUPER EXTENDED MEMORY SUPPORT

These changes are intended to provide support for extended memory pools
up to 4 Gb in size. The current XMS API, since it uses 16-bit values to
specify block sizes in Kb, is limited to 64 Mb maximum block size.
Future machines are expected to support memory above 64 MB.

This support is implemented in the form of extensions to existing
functions, rather than entirely new entry points, to allow for more
efficient implementations.

Programs should generally use the existing functions, instead of these
extended ones, unless they have an explicit need to deal with memory
above 64 Mb.

Query Any Free Extended Memory (Function 88h)

    ARGS:    AH = 88h

    RETS:    EAX = Size of largest free extended memory block in Kb.
        BL = 0 if no error occurs, otherwise it takes an error code.
        ECX = Highest ending address of any memory block.
        EDX = Total amount of free memory in Kb.

    ERRS:    BL = 80h if the function is not implemented.
        BL = 81h if a VDISK device is detected.
        BL = A0h if all extended memory is allocated.

This function uses 32-bit values to return the size of available memory,
thus allowing returns up to 4GByte. Additionally, it returns the highest
known physical memory address, that is, the physical address of the last
byte of memory.  There may be discontinuities in the memory map below
this address.

The memory pool reported on is the same as that reported on by the
existing Query Free Extended Memory function.  If the highest memory
address is not more than 64 Mb, then these two functions will return the
same results.

Because of its reliance on 32-bit registers, this function is only
available on 80386 and higher processors.  XMS drivers on 80286 machines
should return error code 80h if this function is called.

If error code 81h is returned, the value in ECX will still be valid.  If
error code A0h is returned, EAX and EDX will be 0, and ECX will still be
valid.


Allocate Any Extended Memory (Function 89h)

    ARGS:    AH = 89h
        EDX = Amount of extended memory requested, in Kb.

    RETS:    AX = 1 if the block is allocated, 0 if not
        DX = Handle to allocated block.

    ERRS:    BL = 80h if the function is not implemented.
        BL = 81h if a VDISK device is detected.
        BL = A0h if all available extended memory is allocated.
        BL = A1h if all available extended memory handles are in use.

This function is similar to the existing Allocate Extended Memory,
except that it uses a 32-bit instead of a 16-bit value to specify the
amount of memory requested.  It allocates from the same memory and
handle pool as the current function.  Since it requires a 32-bit
register, this function can be supported only on 80386 and higher
processors, and XMS drivers on 80286 machines should return error code
80h.

Get Extended EMB Handle Information (Function 8Eh)

    ARGS:    AH = 8Eh
        DX = Extended memory block handle.

    RETS:    AX = 1 if the block's information is found, 0 if not
        BH = Block lock count
        CX = Number of free EMB handles in the system
        EDX = Block's length in Kb.

    ERRS:    BL = 80h if the function is not implemented.
        BL = 81h if a VDISK device is detected.
        BL = A2h if the handle is invalid.

This function is similar to the Get EMB Handle Information function.
Since it uses a 32-bit register to report the block size, it can be used
to get information on blocks larger than 64 Mb.  It also uses a 16-bit
instead of 8-bit register to report the number of free handles, allowing
the handle pool to be extended beyond 256 entries.

Because of its reliance on a 32-bit register, this function is available
on 80386 and higher processors.  XMS drivers on 80286 machines should
return error code 80h if this function is called.


Reallocate Any Extended Memory (Function 8Fh)

    ARGS:    AH = 8Fh
        EBX = New size for extended memory block, in Kb.
        DX = Unlocked handle for memory block to be resized.

    RETS:    AX = 1 if the block is reallocated, 0 if not

    ERRS:    BL = 80h if the function is not implemented.
        BL = 81h if a VDISK device is detected.
        BL = A0h if all available extended memory is allocated.
        BL = A1h if all available extended memory handles are in use.
        BL = A2h if the handle is invalid.
        BL = ABh if the block is locked.

This function is similar to the existing Reallocate Extended Memory,
except that it uses a 32-bit instead of a 16-bit value to specify the
amount of memory requested.  It allocates from the same memory and
handle pool as the current function.  Since it requires a 32-bit
register, this function can be supported only on 80386 and higher
processors, and XMS drivers on 80286 machines should return error code
80h.


PRIORITIZING HMA USAGE:

For DOS users to receive the maximum benefit from the High Memory Area,
programs which use the HMA must store as much of their resident code in
it as is possible. It is very important that developers realize that the
HMA is allocated as a single unit.

For example, a TSR program which grabs the HMA and puts 10K of code into
it may prevent a later TSR from putting 62K into the HMA.  Obviously,
regular DOS programs would have more memory available to them below the
640K line if the 62K TSR was moved into the HMA instead of the 10K one.

The first method for dealing with conflicts such as this is to require
programs which use the HMA to provide a command line option for
disabling this feature.  It is crucial that TSRs which do not make full
use of the HMA provide such a switch on their own command line
(suggested name "/NOHMA").

The second method for optimizing HMA usage is through the
/HMAMIN=parameter on the XMS device driver line.  The number after the
parameter is defined to be the minimum amount of HMA space (in K-bytes)
used by any driver or TSR. For example, if "DEVICE=HIMEM.SYS /HMAMIN=48"
is in a user's CONFIG.SYS file, only programs which request at least 48K
would be allowed to allocate the HMA.  This number can be adjusted
either by installation programs or by the user himself.  If this
parameter is not specified, the default value of 0 is used causing the
HMA to be allocated on a first come, first served basis.

Note that this problem does not impact application programs.  If the HMA
is available when an application program starts, the application is free
to use as much or as little of the HMA as it wants.  For this reason,
applications should pass FFFFh in DX when calling Function 01h.


HIGH MEMORY AREA RESTRICTIONS:

    Far pointers to data located in the HMA cannot be passed to DOS. DOS
    normalizes any pointer which is passed into it. This will cause data
    addresses in the HMA to be invalidated.

    Disk I/O directly into the HMA (via DOS, INT 13h, or otherwise) is
    not recommended.

    Programs, especially drivers and TSRs, which use the HMA *MUST* use
    as much of it as possible. If a driver or TSR is unable to use at
    least 90% of the available HMA (typically ~58K), they must provide a
    command line switch for overriding HMA usage. This will allow the
    user to configure his machine for optimum use of the HMA.

    Device drivers and TSRs cannot leave the A20 line permanently turned
    on. Several applications rely on 1MB memory wrap and will overwrite
    the HMA if the A20 line is left enabled potentially causing a system
    crash.

    Interrupt vectors must not point into the HMA.  This is a result of
    the previous restriction. Note that interrupt vectors can point into
    any allocated upper memory blocks however.

ERROR CODE INDEX:

If AX=0000h when a function returns and the high bit of BL is set,

BL=    80h if the function is not implemented
    81h if a VDISK device is detected
    82h if an A20 error occurs
    8Eh if a general driver error occurs
    8Fh if an unrecoverable driver error occurs
    90h if the HMA does not exist
    91h if the HMA is already in use
    92h if DX is less than the /HMAMIN= parameter
    93h if the HMA is not allocated
    94h if the A20 line is still enabled
    A0h if all extended memory is allocated
    A1h if all available extended memory handles are in use
    A2h if the handle is invalid
    A3h if the SourceHandle is invalid
    A4h if the SourceOffset is invalid
        A5h if the DestHandle is invalid
       A6h if the DestOffset is invalid
    A7h if the Length is invalid
    A8h if the move has an invalid overlap
    A9h if a parity error occurs
    AAh if the block is not locked
    ABh if the block is locked
    ACh if the block's lock count overflows
    ADh if the lock fails
    B0h if a smaller UMB is available
    B1h if no UMBs are available
    B2h if the UMB segment number is invalid


IMPLEMENTATION NOTES FOR DOS XMS DRIVERS:

    A DOS XMS driver's control function must begin with code similar to
    the following:

XMMControl  proc    far

        jmp     short XCControlEntry    ; For "hookability"
        nop                   ; NOTE: The jump must be a short
        nop                 ;  jump to indicate the end of
        nop                 ;  any hook chainThe nop's
                        ;  allow a far jump to be
                        ;  patched in.
XCControlEntry:


    XMS drivers must preserve all registers except those containing
    returned values across any function call.

    XMS drivers are required to hook INT 15h and watch for calls to
    functions 87h (Block Move) and 88h (Extended Memory Available). The
    INT 15h Block Move function must be hooked so that the state of the
    A20 line is preserved across the call. The INT 15h Extended Memory
    Available function must be hooked to return 0h to protect the HMA.

    In order to maintain compatibility with existing device drivers, DOS
    XMS drivers must not hook INT 15h until the first non-Version Number
    call to the control function is made.

    XMS drivers are required to check for the presence of drivers which
    use the IBM VDISK allocation scheme.  Note that it is not sufficient
    to check for VDISK users at installation time but at the time when
    the HMA is first allocated.  If a VDISK user is detected, the HMA
    must not be allocated. Microsoft will publish a standard method for
    detecting drivers which use the VDISK allocation scheme.

    XMS drivers which have a fixed number of extended memory handles
    (most do) should implement a command line parameter for adjusting
    that number (suggested name "/NUMHANDLES=")

    XMS drivers should make sure that the major DOS version number is
    greater than or equal to 3 before installing themselves.

    UMBs cannot occupy memory addresses that can be banked by EMS
    4.0.EMS 4.0 takes precedence over UMBs for physically addressable
    memory.

    All driver functions must be re-entrant.  Care should be taken to
    not leave interrupts disabled for long periods of time.


    Allocation of a zero length extended memory buffer is allowed.
    Programs which hook XMS drivers may need to reserve a handle for
    private use via this method.  Programs which hook an XMS driver
    should pass all requests for zero length EMBs to the next driver in
    the chain.

    Drivers should control the A20 line via an "enable count." Local
    Enable only enables the A20 line if the count is zero.  It then
    increments the count. Local Disable only disables A20 if the count
    is one.  It then decrements the count. Global Enable/Disable keeps a
    flag which indicates the state of A20.  They use Local
    Enable/Disable to actually change the state.

    Drivers should always check the physical A20 state in the local
    Enable-Disable calls, to see that the physical state matches the
    internal count.  If the physical state does not match, it should be
    modified so that it matches the internal count.  This avoids
    problems with applications that modify A20 directly.


IMPLEMENTATION OF CODE FOR HOOKING THE XMS DRIVER:

In order to support the hooking of the XMS driver by multiple pieces of
code, the following code sample should be followed. Use of other methods
for hooking the XMS driver will not work in many cases. This method is
the official supported one.

The basic strategy is:

    Find the XMS driver header which has the "near jump" dispatch.

    Patch the near jump to a FAR jump which jumps to my HOOK XMS driver
    header.

NOTES:

    This architecture allows the most recent HOOKer to undo his XMS
    driver hook at any time without having to worry about damaging a
    "hook chain".

    This architecture allows the complete XMS hook chain to be
    enumerated at any time. There are no "hidden hooks".

    This architecture allows the HOOKer to not have to worry about
    installing an "INT 2F hook" to hook the AH=43h INT 2Fs handled by
    the XMS driver. The base XMS driver continues to be the only one
    installed on INT 2Fh AH=43h.

    This avoids all of the problems of undoing a software interrupt
    hook.

  ;
  ; When I wish to CHAIN to the previous XMS driver, I execute a FAR JMP
  ; to the address stored in this DWORD.
  ;
  PrevXMSControlAddr    dd      ?

  ;
  ; The next two data items are needed ONLY if I desire to be able to undo
  ; my XMS hook.
  ; PrevXMSControlJmpVal stores the previos XMS dispatch near jump offset
  ; value that is used to unhook my XMS hook
  ; PrevXMSControlBase stores the address of the XMS header that I hooked
  ;
  PrevXMSControlBase    dd      ?
  PrevXMSControlJmpVal  db      ?

  ;
  ; This is MY XMS control header.
  ;  MyXMSControlFunc proc FAR
        jmp     short XMSControlEntry
        nop
        nop
        nop
  XMSControlEntry:

  ......

  Chain:
        jmp     cs:[PrevXMSControlAddr]

  MyXMSControlFunc endp


  .......
  ;
  ; This is the code which installs my hook into the XMS driver.
  ;
  ;
  ; See if there is an XMS driver to hook
  ;
        mov     ax,4300h
        int     2Fh
        cmp     al,80h
        jne     NoXMSDrvrToHookError
  ;
  ; Get the current XMS driver Control address
  ;
        mov     ax,4310h
        int     2Fh
  NextXMSHeader:
        mov     word ptr [PrevXMSControlAddr+2],es
        mov     word ptr [PrevXMSControlBase+2],es
        mov     word ptr [PrevXMSControlBase],bx
        mov     cx,word ptr es:[bx]
        cmp     cl,0EBh                         ; Near JUMP
        je      ComputeNearJmp
        cmp     cl,0EAh                         ; Far JUMP
        jne     XMSDrvrChainMessedUpError
  ComputeFarJmp:
        mov     si,word ptr es:[bx+1]           ; Offset of jump
        mov     es,word ptr es:[bx+1+2]         ; Seg of jump
        mov     bx,si
        jmp     short NextXMSHeader

  ComputeNearJmp:
        cmp     word ptr es:[bx+2],9090h        ; Two NOPs?
        jne     XMSDrvrChainMessedUpError       ; No
        cmp     byte ptr es:[bx+4],90h          ; Total of 3 NOPs?
        jne     XMSDrvrChainMessedUpError       ; No
        mov     di,bx                           ; Save pointer to header
        xor     ax,ax
        mov     al,ch                           ; jmp addr of near jump
        mov     [PrevXMSControlJmpVal],al
        add     ax,2                            ; NEAR JMP is 2 byte instruction
        add     bx,ax                           ; Target of jump
        mov     word ptr [PrevXMSControlAddr],bx
  ;
  ; Now INSTALL my XMS HOOK
  ;
        cli                             ; Disable INTs in case someone calls
                                        ;       XMS at interrupt time
        mov     byte ptr es:[di],0EAh   ; Far Immed. JUMP instruction
        mov     word ptr es:[di+1],offset MyXMSControlFunc
        mov     word ptr es:[di+3],cs
        sti
    .....

  ;  ; Deinstall my XMS hook. This can be done IF AND ONLY IF my XMS header
  ;   still contains the near jump dispatch
  ;
        cmp     byte ptr [MyXMSControlFunc],0EBh
        jne     CantDeinstallError
        mov     al,0EBh
        mov     ah,[PrevXMSControlJmpVal]
        les     bx,[PrevXMSControlBase]
        cli                             ; Disable INTs in case someone calls
                                        ;       XMS at interrupt time
        mov     word ptr es:[bx],ax
        mov     word ptr es:[bx+2],9090h
        mov     byte ptr es:[bx+4],90h
        sti
    ....


IMPLEMENTATION NOTES FOR HIMEM.SYS:

    HIMEM.SYS currently supports true AT-compatibles, 386 AT machines,
    IBM PS/2s, AT&T 6300 Plus systems and Hewlett Packard Vectras.

    If HIMEM finds that it cannot properly control the A20 line or if
    there is no extended memory available when HIMEM.SYS is invoked, the
    driver does not install itself. HIMEM.SYS displays the message "High
    Memory Area Unavailable" when this situation occurs.

    If HIMEM finds that the A20 line is already enabled when it is
    invoked, it will NOT change the A20 line's state.  The assumption is
    that whoever enabled it knew what they were doing.  HIMEM.SYS
    displays the message "A20 Line Permanently Enabled" when this
    situation occurs.

    HIMEM.SYS is incompatible with IBM's VDISK.SYS driver and other
    drivers which use the VDISK scheme for allocating extended memory.
    However,  HIMEM does attempt to detect these drivers and will not
    allocate the HMA if one is found.

    HIMEM.SYS supports the optional "/HMAMIN=" parameter.  The valid
    values are decimal numbers between 0 and 63.

    By default, HIMEM.SYS has 32 extended memory handles available for
    use. This number may be adjusted with the "/NUMHANDLES=" parameter.
    The maximum value for this parameter is 128 and the minimum is 0.
    Each handle currently requires 6 bytes of resident space.

Extended Memory Specification (XMS) version 3.0


Copyright (c) 1988,1992 Microsoft Corporation