Передача параметров в функции Опции

[Altair]

procedure       _Pos{ substr : ShortString; s : ShortString ) : Integer};
asm
{     ->EAX     Pointer to substr               }
{       EDX     Pointer to string               }
{     <-EAX     Position of substr in s or 0    }

        PUSH    EBX
        PUSH    ESI
        PUSH    EDI

        MOV     ESI,EAX { Point ESI to substr           }
        MOV     EDI,EDX { Point EDI to s                }

        XOR     ECX,ECX { ECX = Length(s)               }
        MOV     CL,[EDI]
        INC     EDI             { Point EDI to first char of s  }

        PUSH    EDI             { remember s position to calculate index        }

        XOR     EDX,EDX { EDX = Length(substr)          }
        MOV     DL,[ESI]
        INC     ESI             { Point ESI to first char of substr     }

        DEC     EDX             { EDX = Length(substr) - 1              }
        JS      @@fail  { < 0 ? return 0                        }
        MOV     AL,[ESI]        { AL = first char of substr             }
        INC     ESI             { Point ESI to 2'nd char of substr      }

        SUB     ECX,EDX { #positions in s to look at    }
                        { = Length(s) - Length(substr) + 1      }
        JLE     @@fail
@@loop:
        REPNE   SCASB
        JNE     @@fail
        MOV     EBX,ECX { save outer loop counter               }
        PUSH    ESI             { save outer loop substr pointer        }
        PUSH    EDI             { save outer loop s pointer             }

        MOV     ECX,EDX
        REPE    CMPSB
        POP     EDI             { restore outer loop s pointer  }
        POP     ESI             { restore outer loop substr pointer     }
        JE      @@found
        MOV     ECX,EBX { restore outer loop counter    }
        JMP     @@loop

@@fail:
        POP     EDX             { get rid of saved s pointer    }
        XOR     EAX,EAX
        JMP     @@exit

@@found:
        POP     EDX             { restore pointer to first char of s    }
        MOV     EAX,EDI { EDI points of char after match        }
        SUB     EAX,EDX { the difference is the correct index   }
@@exit:
        POP     EDI
        POP     ESI
        POP     EBX
end

Почему у нас получается фактически _pos(EAX,EDX):EAX ?