Extended Message Box for Win32

; Message definition in .rdata
Errm01	dd	MB_YESNO | MB_ICONSTOP | MB_DEFBUTTON2 | MB_APPLMODAL
		db	'MESSAGE BOX DEMO', 0
		db	13, 10, 9, 'Do you want to continue', 0

	; Example of how you would call function
		xor	eax, eax		; hWnd, owners window handle
		mov	edx, Errm01		; Message definition
		call	MsgBox

; ============================================================================================ 
  ; 				      *** MSGBOX ***

  ;	ENTRY:	EAX = hWnd
  ;		EDX = Base pointer ot message parameters

  ;	LEAVE	EAX = Users response as defined in MessageBox

; .............................................................................................
  FMTM		EQU	FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM

  MsgBox	push	esi
		push	edi			; Save essential registers
		push	ebx

		enter	0x1c8, 0		; Frame for MessageBox parameters
		mov	ebx, esp		; Let EBX point to base of these params
		add	ebx, byte 24
		mov	esi, edx
		cld				; Assure auto increment of indices

	; Due to the two modes of operation of this procedure, default to simple message
	; box without extended information from GetLastError.

		mov	[ebx], eax		; hWnd, handle of owner window
		lodsd				; Get style
		mov	[ebx+12], eax		; uType, style of message box
		mov	[ebx+8], esi		; lpCaption, title of message box
		push	esi
		call	_lstrlenA@4		; Determine length of title text
		inc	eax			; Bump past strings terminating byte
		add	esi, eax
		mov	[ebx+4], esi		; lpText, text in message box

	; The single determining factor whether procedure operates in extended mode or not
	; is return value from GetLastError.  If it isn't null, then that means we want
	; to show error code, address of offending call, systems text associated with error
	; and message box text from parameter 2 of MessgeBox.

		call	_GetLastError@0		; Get error code if it exists.
		and	eax, eax
		jnz	.Ext_Mode

		; NOTE: I've choosen to use this method because Extended Mode code is
		; greater than 128 bytes which puts it past a near jump.

	; Display message box in either simple or extended mode, get response from
	; operator and let calling procedure determine appropriate action.

  .Done		lea	esp, [ebp - 0x1b0]	; Point to parameters intialized earlier.
		call	_MessageBoxA@16		
		leave

		pop	ebx
		pop	edi			; Restore essential registers
		pop	esi

		ret				; EAX = Return value of MessageBox call.

  ; -------------------------------------------------------------------------------------------
  ; Normally I would put this in .rdata, but sake of this demonstration I'll leave it in
  ; this segment

  .OutFmt	db	9, 'Error < %d > @ [ %X ] ', 13, 10, 13, 10, '%s%s', 0
  ; -------------------------------------------------------------------------------------------

	; To create the extended string contents for message box, establish the 6 params
	; that are required by wsprint being; OutputBuffer, FormatStr, ErrorCode,
	; CallAddress, ErrorCodeStr, MsgBoxText.
	
  .Ext_Mode	mov	edi, esp		; Establish pointer to wsprintf params
		push	eax			; Save error code from GetLastError
		mov	eax, esp
		add	eax, byte 0x3c		; Base of Destination Ascii buffer
		stosd				; 1st Param for wsprintf
		mov	eax, .OutFmt		; Formatting definition
		stosd				; 2nd Param for wsprintf
		pop	eax			; Restore Error code
		stosd				; 3rd Param for wsprintf

	; Procedure assumes that the first occurence of E8 before the call to MSGBOX was
	; the API function that generated this error.

		mov	esi, edi		; Save next pointer to wsprintf param
		mov	edi, [ebp+16]		; Get callers return
		sub	edi, byte 6		; Point to just before call to MSGBOX
		mov	 al, 0xE8		; Searching for call instruction
		xor	ecx, ecx
		dec	ecx			; Kind of large but no matter
		std				; We want to search backward
		repnz	scasb
		inc	edi			; Points to call to API
		cld				; Re-establish direction to default
		mov	eax, edi
		xchg	esi, edi
		stosd				; 4th Param for wsprintf (Offending call).

	; Use FormatMessage to determine text associated with this error from system

		xor	eax, eax
		push	eax			; Arguments = NULL (No insertion points)
		push	eax			; nSize, not required
		push	edi			; We'll use this location temporaritly
		push	dword SUBLANG_DEFAULT << 10 | LANG_NEUTRAL
		push	dword [edi - 8]		; Error code
		push	eax
		push	dword FMTM
		call	_FormatMessageA@28
		and	eax, eax
		jnz	$+4

		int	3			; In the unlikely event FormatMessage fails.

		mov	eax, [edi+12]
		add	edi, byte 4		; Jump over value just save by FormatMessage
		stosd				; Save address to string
		call	_wsprintfA		; Format entire message

		pop	eax			; Point to newly created string
		mov	[edi+4], eax		; and save for MessageBox API
		push	dword [edi-8]		; Recover memory allocated by FormatMessage
		call	_LocalFree@4
		jmp	.Done			; Complete display and operators response.