I. About the readers and article's files overview II. MASM vs NASM : Syntax overview
III. A skeleton VxD
IV. More VxD examples
V. FAQs
VI. About the writer

I. About the readers and article's files overview

As for the files in this article:

NASMVXD.TXT     -   This article.
VXDN.INC        -   Contains VxD related definitions and macros for NASM.
WINDDK.INC      -   This is used by VXDN.INC and should'nt be directly included
by you. It contains VxD related EQU's and it also has VxD
services covering VMM,Shell,Debug,...

II. Overview about MASM & NASM

Okay, now the syntax differences between MASM & NASM.

Processor Mode:

To enable the use of 386+ protected mode instructions you used to put a '.386p' in MASM, no need for that in NASM, however you have to explicitly set the default bitness to 32 via the 'BITS 32' directive (and to 16 in the real mode initialization segment).

MASM: .386p
NASM: BITS 32

Segments specification:

MASM has lot of segments declaration macros unlike NASM in which you have to name the segment as you stated it in the .DEF file.

The 5 basic segment definition macros are:

    MASM:                    NASM:            Description
-----                    -----            -----------
VxD_CODE_SEG/ENDS        segment LTEXT   Protected mode code seg.
VxDDATA_SEG/ENDS        segment LDATA   Protected mode data seg.
VxDICODE_SEG/ENDS       segment ITEXT   Protected mode initialization
code segment. (usually optional)
VxDIDATA_SEG/ENDS       segment IDATA   Protected mode initialization
data segment. (usually optional)
VxDREAL_INIT_SEG/ENDS   segment _RTEXT   Real mode initialization
segment. (optional too)

Notice that NASM does not need a segment closing macro unlike MASM.

To start a new segment just declare it like 'segment _LTEXT' and everything after that line will go to that segment.

Please do not use the intrinsic form of the segment macro (e.g. [segment _LTEXT]) as certain VxD macros rely on saving/restoring the current segment and they would fail should you use the intrinsic form.

Check the FAQ for a brief segment overview or NASMDOC.TXT for full overview.

Virtual Device Desciptor Block (DDB) Declaration:

MASM

---

Declare_Virtual_Device Name, MajorVer, MinorVer, CtrlProc, DeviceNum,

InitOrder, V86Proc, PMProc, RefData

NASM

---

Due to the fact that NASM does not support string concatenation in macros yet (there exist patched versions which do), the declaration is a bit

different

Declare_Virtual_Device Name, 'Name', MajorVer, MinorVer, CtrlProc,

DeviceNum, InitOrder, V86Proc, PMProc, RefData

Params 5 to 9 are optional, since most of the time they are generic (not used).

The extra parameter is 'Name' which will become the DDB_Name field in the DDB (this is the name by which the VxD will be known to the VMM), Name itself determines the name for the Control Procedure and the Service Table (if used).

The DDB must be declared inside the _LDATA segment.

Example

segment LDATA
DeclareVirtual_Device SAMPVXD1, 'SAMPVXD1', 1, 0, SAMPVXD1_Control

Control Procedure Definition:

MASM

---

Begin_Control_Dispatch NAME

Control_Dispatch Message,Proc End_Control_Dispatch

NASM

---

This will be a little new for you since you have to do it by hand and not by similar macros:

segment _LTEXT

VXDNAME Control

cmp eax,VM_INIT je OnVmInit

        cmp  eax,W32_DEVICEIOCONTROL
je   OnDIOC
cmp  eax,<system message>
je   <Desired Event handler proc>
clc  ; At any time during initialization, a virtual device can set the
; carry flag and return to the VMM to prevent the virtual device
; from loading. This means that the carry flag must be cleared to
; allow loading.
retn

OnVmInit

; Do some code ret

OnDIOC: ; OnDeviceIoControl

        ; ESI points to a DIOCParams struct
cmp   word [esi+DIOCParams.dwIoControlCode],MY_DIOC_CODE
je    domycode
retn   ; Don't forget to put a return as you're used to put a
; "EndProc procname"

Any Other procedure Definition

Using NASM's normal procedure definition you can define a new proc as usual: "procname :".
As for calling conventions you have to access the stack yourself or use some other NASM macros.

Using VxdCall and VMMCall

In NASM you can call: VMMCall Service,param1,{param2},[ [{]param3[}] ],....

III. A skeleton VxD

In NVXDSKEL.DEF you can specify if it will be a DYNAMIC or a STATIC VxD like:

VXD MYVXD DYNAMIC ; dynamic vxd
VXD MYVXD ; static vxd

NVXDSKEL.DEF

VXD NVXDSKEL DYNAMIC

SEGMENTS

  _LTEXT      CLASS 'LCODE'   PRELOAD NONDISCARDABLE
_LDATA      CLASS 'LCODE'   PRELOAD NONDISCARDABLE
_TEXT       CLASS 'LCODE'   PRELOAD NONDISCARDABLE
_DATA       CLASS 'LCODE'   PRELOAD NONDISCARDABLE
CONST       CLASS 'LCODE'   PRELOAD NONDISCARDABLE
_ITEXT      CLASS 'ICODE'   DISCARDABLE
_IDATA      CLASS 'ICODE'   DISCARDABLE
_PTEXT      CLASS 'PCODE'   NONDISCARDABLE
_PDATA      CLASS 'PDATA'   NONDISCARDABLE SHARED
_STEXT      CLASS 'SCODE'   RESIDENT
SDATA      CLASS 'SCODE'   RESIDENT

DBOSTART CLASS 'DBOCODE' PRELOAD NONDISCARDABLE CONFORMING DBOCODE CLASS 'DBOCODE' PRELOAD NONDISCARDABLE CONFORMING DBODATA CLASS 'DBOCODE' PRELOAD NONDISCARDABLE CONFORMING _RCODE CLASS 'RCODE'

EXPORTS
NVXDSKEL_DDB @1

NVXDSKEL.ASM

bits 32

%include "vxdn.inc"

segment _LDATA

Declare_Virtual_Device NVXDSKEL,'NVXDSKEL',1,0,NVXDSKEL_Control

segment _LTEXT

NVXDSKEL Control

clc retn

Assembling and linking:

• To assemble you must have NASM v0.98+ NASM NVXDSKEL.ASM -f win32 LINK NVXDSKEL.OBJ /VXD /DEF:NVXDSKEL.DEF

That's it!

IV. More VxD examples

VXDSAMP1.DEF

VXD VXDSAMP1 DYNAMIC

SEGMENTS

  _LTEXT      CLASS 'LCODE'   PRELOAD NONDISCARDABLE
_LDATA      CLASS 'LCODE'   PRELOAD NONDISCARDABLE
_TEXT       CLASS 'LCODE'   PRELOAD NONDISCARDABLE
_DATA       CLASS 'LCODE'   PRELOAD NONDISCARDABLE
CONST       CLASS 'LCODE'   PRELOAD NONDISCARDABLE
_ITEXT      CLASS 'ICODE'   DISCARDABLE
_IDATA      CLASS 'ICODE'   DISCARDABLE
_PTEXT      CLASS 'PCODE'   NONDISCARDABLE
_PDATA      CLASS 'PDATA'   NONDISCARDABLE SHARED
_STEXT      CLASS 'SCODE'   RESIDENT
SDATA      CLASS 'SCODE'   RESIDENT

DBOSTART CLASS 'DBOCODE' PRELOAD NONDISCARDABLE CONFORMING DBOCODE CLASS 'DBOCODE' PRELOAD NONDISCARDABLE CONFORMING DBODATA CLASS 'DBOCODE' PRELOAD NONDISCARDABLE CONFORMING _RCODE CLASS 'RCODE'

EXPORTS
VXDSAMP1_DDB @1

VXDSAMP1.ASM

bits 32

%include "vxdn.inc"

segment _LDATA

Declare_Virtual_Device VXDSAMP1,'VXDSAMP1',1,0,VXDSAMP1_Control

segment _LTEXT

VXDSAMP1 Control

cmp eax,W32_DEVICEIOCONTROL je OnDIOC

        clc
retn

OnDIOC

cmp dword [esi+DIOCParams.dwIoControlCode],1 je .1

        xor  eax,eax
jmp  .ret
.1:
VMMCall Get_Sys_VM_Handle
xor   esi,esi ; no callback
xor   edx,edx ; no ref data for callback
mov   eax,0
mov   ecx,Msg
mov   edi,Title
VxDCall SHELL_Message
.ret:
retn

segment _LDATA
Msg db 'Hello world!',0
Title db 'Title!',0

<EOF>

And another example that calls Int21/Ah=02,dl=7 to beep.

VXDSAMP2.ASM

bits 32

%include "vxdn.inc"

segment _LDATA

Declare_Virtual_Device VXDSAMP2,'VXDSAMP2',1,0,VXDSAMP2_Control

segment _LTEXT

VXDSAMP2 Control

        cmp  eax,W32_DEVICEIOCONTROL
je   OnDIOC
clc
retn

OnDIOC

cmp dword [esi+DIOCParams.dwIoControlCode],1 je .1

        xor  eax,eax
jmp  .ret
.1:
VxDCall Begin_Nest_V86_Exec
mov word [ebp+CRS.EAX],0x0200
mov word [ebp+CRS.EDX],0x0007
mov eax,0x21
VxDCall Exec_Int
VxDCall End_Nest_Exec
.ret:
retn

<EOF>

Use .DEF like previous example but change name to the new VxD name.

To test the last two examples, just open the VxD with CreateFileA() and then issue a DeviceIoControl() with code 1.

V. FAQs

{http://www.web-sites.co.uk/nasm/} As for LINK.EXE you can get it from the DDK or just download the MASM Pack from {http://win32asm.cjb.net}

Q) How can i add new services and use them with NASM? A) You can start by defining:

MyDevice_DeviceID equ 0x1234 ; must be word

and then define a service table like:

Begin_Service_Table MyDevice

      VMM_Service MyService0                 ; 0x0000 ord
VMM_Service MyService1                 ; 0x0001 ord
VMM_Service MyServiceN                 ; ord N

End_Service_Table MyDevice

VI. About the writers

fOSSiL
&
The Owl - For creating VXDN.INC and

           for showing how to write VxDs in NASM in the first place
by demonstrating it in IceDump (visit: {http://icedump.tsx.org}).
And for reviewing/editing this document.

Iczelion - For his awesome win32asm resource site and for his

good VxD tutorials. (visit: {http://win32asm.cjb.net})

UKC Team - For their support.

[The VXDN.INC and WINDDK.INC files can be obtained from

{http://asmjournal.freeservers.com/files/nasmvxd.zip}

where they have been archived along with the text of the article.]