DYNASM RevStr(BYVAL s AS STRING) AS STRING
   // get length of string
   // divide by two
   // setup pointers to head and tail
   // iterate from 1 to (length \ 2)
   //   swap head with tail
   //   increment head pointer
   //   decrement tail pointer

   ENTER 0, 0 // = PUSH EBP: MOV EBP, ESP
   PUSH EBX // by Windows convention EBX, EDI, ESI must be saved before modification

   MOV EAX, s // get string pointer
   MOV ECX, EAX // duplicate it

   .WHILE BYTE PTR [ECX] <> 0

	INC ECX // propagate to tail

   .WEND

   MOV EDX, ECX // duplicate tail pointer
   DEC EDX // set it to last byte before trailing zero

   SUB ECX, EAX // get length in ECX in 1 CPU cycle
   SHR ECX, 1 // get length \ 2 in 1 CPU cycle; that's the beauty of power-of-two division

   .WHILE ECX > 0

      MOV BL, [EDX] // no need to XOR; just overwrite BL and BH contents
      MOV BH, [EAX] // DynAsm deduces data size from destination register sizes

      MOV [EDX], BH // ditto, source register sizes
      MOV [EAX], BL

      INC EAX // propagate pointers
      DEC EDX

      DEC ECX // decrement counter

   .WEND

   // point to start of string again
   MOV EAX, s // MOV = 1 CPU cycle, PUSH + POP = 2 CPU cycles

   POP EBX // by Windows convention ESI, EDI, EBX must be restored if modified
   LEAVE // = POP EBP
   RET
END DYNASM