Обьясните assume fs:nothing - Assembler

.386
   .model flat,stdcall
   option casemap:none
   include \masm32\include\windows.inc
   include \masm32\include\kernel32.inc
   include \masm32\include\comdlg32.inc
   include \masm32\include\user32.inc
   includelib \masm32\lib\user32.lib
   includelib \masm32\lib\kernel32.lib
   includelib \masm32\lib\comdlg32.lib
 
   SEH struct
   PrevLink dd ?    ; the address of the previous seh structure
   CurrentHandler dd ?    ; the address of the exception handler
   SafeOffset dd ?    ; The offset where it's safe to continue execution
   PrevEsp dd ?      ; the old value in esp
   PrevEbp dd ?     ; The old value in ebp
   SEH ends
 
   .data
   AppName db "PE tutorial no.2",0
   ofn OPENFILENAME <>
   FilterString db "Executable Files (*.exe, *.dll)",0,"*.exe;*.dll",0
                    db "All Files",0,"*.*",0,0
   FileOpenError db "Cannot open the file for reading",0
   FileOpenMappingError db "Cannot open the file for memory mapping",0
   FileMappingError db "Cannot map the file into memory",0
   FileValidPE db "This file is a valid PE",0
   FileInValidPE db "This file is not a valid PE",0
 
   .data?
   buffer db 512 dup(?)
   hFile dd ?
   hMapping dd ?
   pMapping dd ?
   ValidPE dd ?
 
   .code
   start proc
   LOCAL seh:SEH
   mov ofn.lStructSize,SIZEOF ofn
   mov ofn.lpstrFilter, OFFSET FilterString
   mov ofn.lpstrFile, OFFSET buffer
   mov ofn.nMaxFile,512
   mov ofn.Flags, OFN_FILEMUSTEXIST or OFN_PATHMUSTEXIST 
     or OFN_LONGNAMES or OFN_EXPLORER or OFN_HIDEREADONLY
   invoke GetOpenFileName, ADDR ofn
   .if eax==TRUE
       invoke CreateFile, addr buffer, GENERIC_READ, 
       FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL
       .if eax!=INVALID_HANDLE_VALUE
          mov hFile, eax
          invoke CreateFileMapping, hFile, NULL, PAGE_READONLY,0,0,0
          .if eax!=NULL
             mov hMapping, eax
             invoke MapViewOfFile,hMapping,FILE_MAP_READ,0,0,0
             .if eax!=NULL
                mov pMapping,eax
                assume fs:nothing
                push fs:[0]
                pop seh.PrevLink
                mov seh.CurrentHandler,offset SEHHandler
                mov seh.SafeOffset,offset FinalExit
                lea eax,seh
                mov fs:[0], eax
                mov seh.PrevEsp,esp
                mov seh.PrevEbp,ebp
                mov edi, pMapping
                assume edi:ptr IMAGE_DOS_HEADER
                .if [edi].e_magic==IMAGE_DOS_SIGNATURE
                   add edi, [edi].e_lfanew
                   assume edi:ptr IMAGE_NT_HEADERS
                   .if [edi].Signature==IMAGE_NT_SIGNATURE
                      mov ValidPE, TRUE
                   .else
                      mov ValidPE, FALSE
                   .endif
                .else
                    mov ValidPE,FALSE
                .endif
   FinalExit:
                .if ValidPE==TRUE
                    invoke MessageBox, 0, addr FileValidPE, addr AppName, MB_OK+MB_ICONINFORMATION
                .else
                   invoke MessageBox, 0, addr FileInValidPE, addr AppName, MB_OK+MB_ICONINFORMATION
                .endif
                push seh.PrevLink
                pop fs:[0]
                invoke UnmapViewOfFile, pMapping
             .else
                invoke MessageBox, 0, addr FileMappingError, addr AppName, MB_OK+MB_ICONERROR
             .endif
             invoke CloseHandle,hMapping
          .else
             invoke MessageBox, 0, addr FileOpenMappingError, addr AppName, MB_OK+MB_ICONERROR
          .endif
          invoke CloseHandle, hFile
       .else
          invoke MessageBox, 0, addr FileOpenError, addr AppName,
   MB_OK+MB_ICONERROR
       .endif
   .endif
   invoke ExitProcess, 0
   start endp
 
   SEHHandler proc uses edx pExcept:DWORD, pFrame:DWORD, pContext:DWORD, pDispatch:DWORD
       mov edx,pFrame
       assume edx:ptr SEH
       mov eax,pContext
       assume eax:ptr CONTEXT
       push [edx].SafeOffset
       pop [eax].regEip
       push [edx].PrevEsp
       pop [eax].regEsp
       push [edx].PrevEbp
       pop [eax].regEbp
       mov ValidPE, FALSE
       mov eax,ExceptionContinueExecution
       ret
   SEHHandler endp
   end start