Question

Write an assembly program In MASM assembler to calculate the sum of odd numbers only stored in a 16-bit integers array.
The size of the array is provided in a separate variable. Use the following design:

Sum = 0
FOR i = 0 TO SizeOfArray - 1
    IF Arr[i] % 2 = 1
          Sum = Sum + Arr[i]
    END IF
END FOR

Answer 1

Assume that the array starts from the memory Location 500, size of array = 5 and the RESULT SUM IS stored at location 0.

So We will add the numbers at locations 501 and 503 (odd locations).

Find below a generic MASM Assembler code for adding numbers at odd places in an array!

400	MOV SI, 500	SI is the register storing the memory address!
403	MOV CL, [SI]	Register CL stores the value of Arr[i]
405	INC SI
406	MOV CH, 00	CH<-00
408	MOV AL, 00	Register AL stores the value of the variable SUM!
40A	MOV BL, [SI]	BL<-[SI]
40C	TEST SI, 01	SI AND 01 (THE MAIN CATCH) Explanation: In order to check if a number is odd we and it with the number 01. This works because if a number is odd it will have 1 as the LSB in its representation.(Eg. 1-> 0001 3->0011 5->0101 but 8->1000(Even)). Now, if you bitwise AND a number with 01 (0001): It will be zero at all places (since 0001 has zeroes at all places except LSB) except maybe at the LSB!   If the number is even => 0 at LSB => AND will produce the number 0000. If the number is odd => 1 at LSB => AND will produce the number 0001.
40F	JZ 413	Jump to 413 memory location if zero flag is set
411	ADD AL, BL	SUM +=Arr[i]
413	INC SI	SI<-SI+1
414	LOOP 40A	jump to 40A memory location if the content of CX is not equal to zero
416	MOV [600], AL	[600], AL FINAL STORAGE OF SUM
41A	HLT	end

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