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.
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.
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
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 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:
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
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.
The following XMS API functions are available:
Each is described below.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 pair 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.
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.
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-bit pointers into the block become invalid and should no longer be used.
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).
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.
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.
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.
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.
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.
Entry:
AH = 88h
Exit:
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.
Errors:
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.
Entry:
AH = 89h EDX = Amount of extended memory requested, in Kb.
Exit:
AX = 1 if the block is allocated, 0 if not DX = Handle to allocated block.
Errors:
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.
Entry:
AH = 8Eh DX = Extended memory block handle.
Exit:
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.
Errors:
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.
Entry:
AH = 8Fh EBX = New size for extended memory block, in Kb. DX = Unlocked handle for memory block to be resized.
Exit:
AX = 1 if the block is reallocated, 0 if not
Errors:
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.
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.
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
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:
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:
NOTES:
;
; 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
....
Copyright (c) 1988, Microsoft Corporation