Question

Assembly Language. Write a procedure that reverses order of a 1-D array using the stack. The array is a string array, and the address along with the number of elements are passed through the stack. Then write a complete program to test your procedure.

Answer 1

As per your question, First I will describe the steps involved in-order to reverse a 1-D string array. Next, I will write the procedure in 8086 assembly language(please refer to in-line comments for better understanding). Further, The complete 8086 assembly program for validating the procedure. So, please bear with me:-

The basic steps for reversal of any string array are followings:-

1. Define or create a valid string
2. Iterate or Traverse through each character of the string
3. Push each character of the string in the stack
4. Count the number of characters
5. Load the beginning address of the string
6. POP out every top character of the stack until the count is not equal to zero
7. Put the character and reduce the count and increase the address accordingly
8. Continue until the count is greater than zero
9. Load the effective address of the string in dx using the LEA command
10. Print the string by calling the interrupt with 9H in AH
11. The string must be terminated by the ‘$’ sign

Now, the procedure of string reversal as per your question:-

REVERSE PROC 
    ; load the offset of 
    ; the string  
    MOV SI, OFFSET STRING  
  
    ; count of characters of the;  
    ;string  
    MOV CX, 0H  
  
    LOOP1: 
    ; compare if this is;  
    ;the last character  
    MOV AX, [SI]  
    CMP AL, '$'
    JE LABEL1  
  
    ; else push it in the;  
    ;stack  
    PUSH [SI]  
  
    ; increment the pointer;  
    ;and count  
    INC SI  
    INC CX  
  
    JMP LOOP1  
  
    LABEL1: 
    ; again load the starting;  
    ;address of the string  
    MOV SI, OFFSET STRING  
  
        LOOP2:  
        ;if count not equal to zero  
        CMP CX,0  
        JE EXIT  
  
        ; pop the top of stack  
        POP DX  
  
        ; make dh, 0  
        XOR DH, DH  
  
        ; put the character of the;  
        ;reversed string  
        MOV [SI], DX  
  
        ; increment si and;  
        ;decrement count  
        INC SI  
        DEC CX  
  
        JMP LOOP2  
  
                  
    EXIT: 
    ; add $ to the end of string  
    MOV [SI],'$ '
    RET  
          
REVERSE ENDP

=> The entire program in order to test the above procedure:-

.MODEL SMALL 
.STACK 100H 
.DATA 

; The string to be printed 
STRING DB 'God Bless You', '$'

.CODE 
MAIN PROC FAR 
MOV AX,@DATA 
MOV DS,AX 

; call reverse function 
CALL REVERSE 

; load address of the string 
LEA DX,STRING 

; output the string 
; loaded in dx 
MOV AH, 09H 
INT 21H 

; interrupt to exit
MOV AH, 4CH 
INT 21H 

MAIN ENDP 
REVERSE PROC 
        ; load the offset of 
        ; the string 
        MOV SI, OFFSET STRING 

        ; count of characters of the; 
        ;string 
        MOV CX, 0H 

        LOOP1: 
        ; compare if this is; 
        ;the last character 
        MOV AX, [SI] 
        CMP AL, '$'
        JE LABEL1 

        ; else push it in the; 
        ;stack 
        PUSH [SI] 

        ; increment the pointer; 
        ;and count 
        INC SI 
        INC CX 

        JMP LOOP1 

        LABEL1: 
        ; again load the starting; 
        ;address of the string 
        MOV SI, OFFSET STRING 

                LOOP2: 
                ;if count not equal to zero 
                CMP CX,0 
                JE EXIT 

                ; pop the top of stack 
                POP DX 

                ; make dh, 0 
                XOR DH, DH 

                ; put the character of the; 
                ;reversed string 
                MOV [SI], DX 

                ; increment si and; 
                ;decrement count 
                INC SI 
                DEC CX 

                JMP LOOP2 

                                
        EXIT: 
        ; add $ to the end of string 
        MOV [SI],'$ '
        RET 
                
REVERSE ENDP 
END MAIN 

=> The Run output:-

uoY sselB doG

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Thank You...