Q. Use of array in assembly language?

Let's write a program to add two 5-byte numbers stored in an array. For illustration two numbers in hex can be:

Let's also presume that numbers are represented as the lowest significant byte first and put in memory in two arrays. The result is stored in third array SUM. SUM also includes the carry out information therefore would be 1 byte longer than number arrays.

; Program: Add two five-byte numbers using arrays

; ALGORITHM:

;           Make count = LEN

;           Clear the carry flag

;           Load address of NUM1

;           REPEAT

;                       Put byte from NUM1 in accumulator

;                       Add byte from NUM2 to accumulator + carry

;                       Store result in SUM

;                       Decrement count

;                       Increment to next address

;           UNTIL count = 0

;           Rotate carry into LSB of accumulator

;           Mask all but LSB of accumulator

;           Store carry result, address pointer in correct position.

; PORTS   : None used

; PROCEDURES: None used

; REGISTERS: Uses CS, DS, AX, CX, BX, DX

DATA SEGMENT

                        NUM1 DB 0FFh, 10h ,01h ,11h ,20h

                        NUM2 DB 10h, 20h, 30h, 40h ,0FFh

                        SUM DB 6DUP (0)

DATA ENDS

                        LEN EQU 05h; constant for length of the array

CODE SEGMENT

                        ASSUME CS: CODE, DS: DATA

START:           MOV AX, DATA; initialise data segment

                        MOV DS, AX; using AX register

                        MOV SI, 00; load displacement of 1^stnumber.

                                                ; SI is being used as index register

                        MOV CX, 0000; clear counter

                        MOV CL, LEN; set up count to designed length

                        CLC   ; clear carry. Ready for addition

AGAIN:          MOV AL, NUM1 [SI]; get a byte from NUM1

ADC AL, NUM2 [SI]; add to byte from NUM2 with carry

MOV SUM [SI], AL; store in SUM array

INC SI

LOOP AGAIN; continue until no more bytes

RCL AL, 01h; move carry into bit 0 of AL

AND AL, 01h; mask all but the 0^th bit of AL

MOV SUM [SI], AL; put carry into 6^th byte

FINISH:          MOV AX, 4C00h

INT 21h

CODE             ENDS

END START