; Copy a piece of a string to another place

; INPUT Parameters:
;        PUSHD source string (offset)
;        PUSHD the bytes to skip from start (bytes)
;        PUSHD the bytes to copy (bytes)
;        PUSHD destination string (offset)

proc MidString source,start,count,destination
     mov ecx,[count]
     jecxz .exit ; if zero then exit
     mov esi,[source]
     add esi,[start]
     mov edi,[destination]
     cld
     rep movsb
     xor al,al ; to terminate the second string
     stosb
     .exit: ret
endp    

;; @name str.sub
;; @desc
;;    Extracts substring from another string.
;; @arg dest
;;    Pointer to destination string buffer.
;; @arg destlen
;;    Size of destination string buffer.
;; @arg src
;;    Pointer to source string.
;; @arg srclen
;;    Size of source string buffer.
;;    You can set this to -1 (FFFFFFFFh) if string is quaranteed to be properly zero-terminated.
;; @arg index
;;    Position in source string from where we start extracting.
;;    Position 0 corresponds to first character, included.
;; @arg len
;;    Length of string we want to extract.
;; @ret
;;    CF set on error, otherwise
;;    EAX = dest, and
;;    OF set if string overlapped and less than len characters were copied
;; @err ERR_OUT_OF_RANGE
;;    Position is outside of destination string, pos >= strlen(dest).
;;    You are trying to extract string from behind end of source string.
;; @err ERR_BUFFER_OVERFLOW
;;    Destination buffer is not big enough.
;;    Only first destlen bytes of extracted string were written.
;;    String in destination buffer is NOT zero terminated.
;; @err ERR_ZERO_SIZE
;;    destlen=0 or srclen=0.
;; @note If source string doesn't contain enough characters from given index,
;;       string shorter than len is copied and procedure returns with OF set.
;; @note If len=0, destination buffer is filled with empty string.
proc str.sub dest, destlen, src, srclen, index, len
 push    ecx esi
     pushf
       btc     dword [esp], 11 ;clear OF
   cld

     ;error if destlen=0
 cmp     [destlen], 0
        je      .error_destlen_zero

     ;error if srclen=0
  cmp     [srclen], 0
 je      .error_srclen_zero

      ;if len is zero, we just fill dest with empty string
        cmp     [len], 0
    je      .len_is_zero

    ;ECX = length of source string
      libcall str.len, [src], [srclen]
    jc      .rc
 mov     ecx, eax

        ;ERR_OUT_OF_RANGE if index >= length of source string
    cmp     [index], ecx
        jae     .error_index_out_of_range

       ;ECX = required length for the new string = min(index+len, str.len(string)) - index
 mov     eax, [index]
        add     eax, [len]
  cmp     eax, ecx
    jbe     @f
  mov     eax, ecx
    bts     dword [esp], 11 ;set OF
@@:
  sub     eax, [index]
        mov     ecx, eax

        ;ECX = length in dest buffer = min(ECX, destlen)
    mov     eax, [destlen]
      cmp     ecx, eax
    cmova   ecx, eax

        ;get position in source string from where to copy
   mov     esi, [src]
  add     esi, [index]

    ;copy actual part of string that should be copied
   libcall mem.copy, [dest], esi, ecx
  jc      .rc

     ;error if there is no place for ending zero, eg. if ECX = destlen
   cmp     ecx, [destlen]
      je      .error_dest_buffer_full

 ;otherwise add ending zero
  add     ecx, [dest]
 mov     byte [ecx], 0

   ;return dest
.done:      mov     eax, [dest]

.rnc:    btc     dword [esp], 1 ;clear CF
.r:     popf
        pop     esi ecx
     ret
.rc: bts     dword [esp], 1 ;set CF
      jmp     .r

      ;length of desired string is zero, just fill dest with empty string
.len_is_zero:
    mov     eax, [dest]
 mov     byte [eax], 0
       jmp     .done

.error_dest_buffer_full:
   mov     eax, ERR_BUFFER_FULL
        jmp     .rc

.error_destlen_zero:
 mov     eax, ERR_ZERO_SIZE
  jmp     .rc

.error_srclen_zero:
  mov     eax, ERR_ZERO_SIZE
  jmp     .rc

.error_index_out_of_range:
   mov     eax, ERR_OUT_OF_RANGE
       jmp     .rc

endp    

This package contains:
- complete sources
- full documentation (CHM, HTML)
- tutorial (FASM)
- binaries (LIB, ELF, MS-COFF, DLL)
- includes for FASM, MASM, NASM/YASM
- examples for FASM, MASM, NASM/YASM

;;Equivalent to MyString = Mid(MyString, Start, Count)
proc Mid source,start,count
   MOV eax,[source]
   ADD eax,[start]
   MOV ecx,[count]
   MOV byte[eax+ecx],0
endp