Question

Question1. (lesson 2)

In the following code we determine a number if its even or odd. Explain what logic is used for that?   

section .text

   global _start            ;must be declared for using gcc

      

_start:                     ;tell linker entry point

   mov   ax,   8h           ;getting 8 in the ax

   and   ax, 1              ;and ax with 1

   jz    evnn

   mov   eax, 4             ;system call number (sys_write)

   mov   ebx, 1             ;file descriptor (stdout)

   mov   ecx, odd_msg       ;message to write

   mov   edx, len2          ;length of message

   int   0x80               ;call kernel

   jmp   outprog

evnn:  

  

   mov   ah, 09h

   mov   eax, 4             ;system call number (sys_write)

   mov   ebx, 1             ;file descriptor (stdout)

   mov   ecx, even_msg      ;message to write

   mov   edx, len1          ;length of message

   int   0x80               ;call kernel

outprog:

   mov   eax,1              ;system call number (sys_exit)

   int   0x80               ;call kernel

section   .data

even_msg db 'Even Number!' ;message showing even number

len1 equ $ - even_msg

   

odd_msg db 'Odd Number!'    ;message showing odd number

len2 equ $ - odd_msg

                    

Question 2. (lesson 1 and lesson 3)

                    Write a program that does the following(modification from assignment 3)

            1. Asks the user to enter two numbers (each is a byte) A and B

            2. Multiply the numbers and stores the result in variable C.

                  3. so that if the product  exceeds 10, the program would print “result is greater than 10”.

                      Otherwise (if result < 10), it prints the result as usual.

Answer 1

First question

To find whether a number a number is even or odd, we check its least significant bit in binary representaion. If the Least Significant Bit is 1, the number is odd, otherwise it is even. Here, AND operation is performed between the variable ax and 1. In the line jz evnn, it is being checked whether after performing the AND operation, the value in z is zero(0) or not. If the value of z is zero, the number is even. Otherwise it is odd.

In AND operation, the answer is one only when both the input bits are one, otherwise the answer is zero. So, the last bit of the number when combined with one through AND operation, gives one when it is one(indicating number is odd), or gives zero, when it is zero(indicating number is even).

Second Question

global_start

A DB ?
B DB ?
MSG1 DB 10,13,”ENTER FIRST NUMBER : $”
MSG2 DB 10,13,”ENTER SECOND NUMBER : $”
MSG3 DB 10,13,”Result is greater than 10 : $”

ENDS

CODE SEGMENT
ASSUME DS:DATA CS:CODE
START:
MOV AX,DATA
MOV DS,AX
LEA DX,MSG1
MOV AH,9
INT 21H
MOV AH,1
INT 21H
SUB AL,30H
MOV A,AL

LEA DX,MSG2
MOV AH,9
INT 21H
MOV AH,1
INT 21H
SUB AL,30H
MOV B,AL
MUL A
MOV C,AL
AAM
ADD AH,30H
ADD AL,30H
MOV BX,AX
CMP BX, #10
GT BX,#10 
LEA DX,MSG3
MOV AH,9
INT 21H
MOV AH,2
MOV DL,BH
INT 21H
MOV AH,2
MOV DL,BL
INT 21H
MOV AH,4CH
INT 21H
ENDS
END START