Q. Write a program for comparing two strings?

Let's write a program for comparing two strings. Determine the following piece of code that has been written in C to compare two strings. Let's presume that 'str1' and 'str2' are two strings which are initialised by some values and 'ind' is index for these character strings:

for (ind = 0; ( (ind <9) and (str1[ind] = = str2[ind]) ), ind + +)

Intermediate code in assembly language produced by a non-optimising compiler for the above piece may appear like:

MOV IND, 00; ind: = 0

L3:                               CMP IND, 08; Ind < 9

                                    JG L1   ; not so; skip

                                    LEA AX, STR1; offset of str1 in AX register

                                    MOV BX, IND; it uses a register for indexing into 

                                                                        ; The array

LEA CX, STR2; str2 in CX

                                    MOV DL, BYTE PTR CX [BX]

                                    CMP DL, BYTE PTR AX [BX]; str1 [Ind] = str2 [Ind]

                                    JNE L1    ; no, skip

                                    MOV IND, BX 

                                    ADD IND, 01

L2:                               JMP L3    ; loop back

L1:

What we find in above code: a large code which could have been improved further if 8086 string instructions would have been used.

; Program: Matching two strings of similar length stored in memory locations.

; REGISTERS: Uses CS, DS, ES, AX, DX, CX, SI, DI

DATA SEGMENT

            PASSWORD              DB                  'FAILSAFE'              ; source string

            DESTSTR                   DB                  'FEELSAFE'              ; destination string

            MESSAGE                 DB                  'String are equal $'

DATA ENDS

CODE SEGMENT

                        ASSUME CS: CODE, DS: DATA, ES: DATA

                                    MOV AX, DATA

                                    MOV DS, AX   ; Initialise data segment register

                                    MOV ES, AX   ; Initialise extra segment register

; As destination string is considered to be in extra segment. Please note that ES is also 

; initialised to the same segment as of DS.

LEA SI, PASSWORD; Load source pointer

                                    LEA DI, DESTSTR; Load destination pointer

                                    MOV CX, 08   ; Load counter with string length

                                    CLD   ; Clear direction flag so that comparison is

                                                ; done in forward direction.

EPE CMPSB; Compare the two string byte by byte

                                    JNE NOTEQUAL; If not equal, jump to NOTEQUAL

                                    MOV AH, 09; else display message 

                                    MOV DX, OFFSET MESSAGE;

                                    INT 21h; display the message

NOTEQUAL:             MOV AX, 4C00h; interrupt function to halt

                                    INT 21h

CODE ENDS

END