-------------------------------===START===---------------------------
; set a couple of options for the assembler
.386P
Locals
jumps

.Model Flat ,StdCall
mb_ok equ 0
hWnd equ 0
FILE_ATTRIBUTE_NORMAL equ 080h
OPEN_EXISTING equ 3
GENERIC_READ equ 80000000h
GENERIC_WRITE equ 40000000h


--==declaration of all used API-functions==--

extrn ExitProcess : PROC ;procedure to end the program
extrn MessageBoxA : PROC ;procedure to show a MessageBox
extrn CreateFileA : PROC ; " ... to open a file
extrn ReadFile : PROC ;read a block of a file
extrn WriteFile : PROC ;write a block into a file
extrn CloseHandle : PROC ;close file
extrn GetFileSize : PROC ;get the filesize
extrn GlobalAlloc : PROC ;allocate memory
extrn GlobalFree : PROC ;set (free) memory
extrn SetFilePointer : PROC ;set the filepointer

--==here begins our Data==--

.Data

caption db "_masta_'s essay on Win32-ASM-Coding, part 1",0

 ;captionstring, 0-terminated

text db "Hi, nice to CU again",13,10
db "This tut will describe you how to make",13,10
db "Win32-ASM Search and Destroy patchers",0

 ;introtext, 0-terminated

err_cap db "ERROR",0  ;caption for errormessage

openerr db "Error on opening File",0  ;errortext opening file
memerr db "Error on allocating memory",0  ;errortext alloc. memory
byterr db "File is here, but i can't find the original bytes",0

 ;error while bytesearch

readycap db "Ready",0 ;caption for 'done'

readytxt db "Ok, file is patched",0 ;text for 'done'

file db "make.old",0  ;what file we want to patch?
org_val db "Xmas'97" ;original values
new_val db "_masta_" ;new values
len equ $-new_val ;how many values (length)
;org_val and new_val must be equal

fhandle dd  ? ;variable for the filehandle
fsize dd  ? ;variable for the filesize
memptr dd  ? ;pointer to allocated memory
bread dd  ? ;number of read bytes
bwrite dd  ? ;number of written bytes

--==and here we start with our code==--

.Code
Main:
push mb_ok  ;PUSH value for 'uType'
push offset caption  ;PUSH pointer to caption
push offset text  ;PUSH pointer to Text
push hWnd  ;PUSH Masterhandle
call MessageBoxA  ;CALL MessageBoxA

push 0 ;for Win95 always 0
push FILE_ATTRIBUTE_NORMAL ;standard Fileattributes
push OPEN_EXISTING ;open existing file
push 0 ;no Security-attributes
push 0 ;disable Share-Mode
push GENERIC_READ + GENERIC_WRITE ;read- and writeaccess
push offset file ;offset of the filename
Call CreateFileA ;open file
mov fhandle,eax ;save filehandle
cmp eax,0FFFFFFFFh ;if eax=FFFFFFFF then
; error
jnz file_is_here

push mb_ok
push offset err_cap
push offset openerr
push hWnd
call MessageBoxA  ; showerrormessage
jmp end_ ; jump to end

file_is_here: ;file is there, so go on

push 0 ;can be 0, if the filesize is less
 ; then 4,3 GB :)
push fhandle ;PUSH filehandle
Call GetFileSize ;get the filesize
mov fsize,eax ;save the filesize

push fsize ;PUSH filesize=size of the buffer
push 0 ;0=GMEM_FIXED -> fixed memory-area
Call GlobalAlloc ;allocate as much as memory as filesize
mov memptr,eax ;save pointer to memory-area

cmp eax,0 ;if eax=0, then there were errors
jnz mem_ok

push mb_ok
push offset err_cap
push offset memerr
push hWnd
call MessageBoxA
jmp end_kill_handle ;end program, close file b4

mem_ok: ;memory is allocated -> next step

push 0 ;set to 0 in most cases
push offset bread ;pointer to number of read bytes
push fsize ;read how many bytes?,
; fsize=whole file
push memptr ;save where? ->allocated memory
push fhandle ;filehandle
Call ReadFile ;read file!

read_ok:

mov edi,memptr ;set EDI to memory-area
mov ecx,fsize ;set ECX (for repnz) to filesize
mov esi,offset org_val ;set ESI to the string to find
mov al, byte ptr [esi] ;load AL with the first byte

loop_:
repnz scasb ;repeat until ECX=0 or AL equals
;the value of the byte [EDI], EDI is
;incremented by 1 every run
cmp ecx,0 ;If ECX=0, nothing is found
jz not_found


here_is_something: ;found matching byte

push ecx ;save register
push edi
push esi
dec edi ;EDI-1, cos REPNZ SCASB is one step too far
mov ecx,len ;ECX=length of the patch
repz cmpsb ;repeat until the values in the memory of
;[EDI] and [ESI] are different,
;or ecx=0
cmp ecx,0 ;If ecx=0, then the org_val is in memory
jz patch ;->jump to patcher

not_that: ;it is not yet here

pop esi ;POP ESI
pop edi
pop ecx
jmp loop_ ;search next byte

patch: ;start of the patcher
pop esi ;POP registers
pop edi
pop ecx
dec edi ;EDI-1
inc ecx ;ECX+1
mov eax,fsize
sub eax,ecx ;compute Offset
push 0 ;offset from the beginning of the file
push 0 ;is 0, if file < 4,3GB
push eax ;offset
push fhandle ;filehandle
call SetFilePointer ;set FilePointer

push 0 ;normally 0
push offset bwrite ;how many bytes where written?
push len ;length of the bytes to write
push offset new_val ;offset to new values
push fhandle ;filehandle
Call WriteFile ;write block to file

push mb_ok
push offset readycap
push offset readytxt
push hwnd
call MessageBoxA  ;OK, patch is done!

jmp end_kill_all ;END! Cleanup!

not_found:

push mb_ok
push offset err_cap
push offset byterr
push hWnd
call MessageBoxA  ;the bytes where not in the file

end_kill_all:

push memptr ;pointer to Memoryarea
call GlobalFree ;enable (free) memory

end_kill_handle:

push fhandle ;PUSH filehandle
call CloseHandle ;CloseHandle


end_:

CALL ExitProcess  ;Quit program
End Main  ;end of code, JUMP-spot (main)
-----------------------==END OF SOURCE==----------------------------

-------------START-------------make.bat
tasm32 /mx /m3 /z /q tut
tlink32 -x /Tpe /aa /c tut,tut,, import32.lib
--------------END--------------make.bat