[HaBo]

.data
    TargetName        db            "Patchtest.exe", 0
    BackupName        db            "Patchtest.bak", 0
    PatchOffset        dd            103Bh
    Old                db            75h, 39h, 6Ah, 64h
    New                db            0EBh, 39h, 6Ah, 64h
...

.data?
    hInstance            HINSTANCE    ?
    hTarget            HINSTANCE    ?
    
    RBuffer            dd            ?
    BytesRead        db            ?
    BytesWritten        db            ?

.code
...
Patch proc hWnd:HWND
    invoke GetFileAttributes, ADDR TargetName
    .if    eax != FILE_ATTRIBUTE_NORMAL
        invoke SetFileAttributes, ADDR TargetName, FILE_ATTRIBUTE_NORMAL
    .endif
    
    invoke CreateFile, ADDR TargetName, GENERIC_READ + GENERIC_WRITE, FILE_SHARE_READ + FILE_SHARE_WRITE, NULL, OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL, NULL
    .if eax != INVALID_HANDLE_VALUE
        mov hTarget, eax
        
        invoke SendDlgItemMessage, hWnd, CBBackup, BM_GETCHECK, NULL, NULL
        .if eax == BST_CHECKED
            invoke CopyFile, ADDR TargetName, ADDR BackupName, TRUE
            invoke List, hWnd, ADDR s_Backup
        .endif
        
        invoke SetFilePointer, hTarget, PatchOffset, NULL, FILE_BEGIN
        invoke ReadFile, hTarget, ADDR RBuffer, 4, ADDR BytesRead, NULL
        .if BytesRead == 4
            mov eax, dword ptr [RBuffer]
            .if eax == dword ptr [Old]
                invoke SetFilePointer, hTarget, PatchOffset, NULL, FILE_BEGIN
                invoke WriteFile, hTarget, ADDR New, 1, ADDR BytesWritten, NULL
                .if BytesWritten == 1
                    invoke List, hWnd, ADDR Success
                .else
                    invoke List, hWnd, ADDR f_Write
                .endif
            .elseif eax == dword ptr [New]
                invoke List, hWnd, ADDR f_Already
            .else
                invoke List, hWnd, ADDR f_Version
                invoke MessageBox, hWnd, ADDR RBuffer, ADDR BytesWritten, MB_OK
            .endif
        .else
            invoke List, hWnd, ADDR f_Read
        .endif
    .else
        invoke List, hWnd, ADDR f_Open
    .endif
    invoke CloseHandle, hTarget
    Ret
Patch endp

virtual address - imagebase - code base + pointer to rawdata for .text section or .code section
0x40103B - 0x400000 - 1000 + 400 = 0x43B