HW # 8. Theme: Integer Arithmetic Solution

    1. In the following code sequence, show the value of AL after each shift or rotate instruction has executed. This question is to be done by hand, not by running a program.
        mov cl, 1
        mov al, 12h            ; al= 00010010b
        rol al, cl             ; al= 00100100b = 24h
        mov al, 34h            ; al= 00110100b = 34h
        mov cl, 2              
        ror al, Cl             ; al= 00001101b = 0Dh
        stc                
        mov al, 56h            ; al= 01010110b = 56h
        mov cl, 3             
rcl al, cl            ; al= 10110101b = B5h
        stc                
        mov al, 78h            ; al= 01111000b = 78h
        mov cl, 1            
        rcr al, cl            ; al= 10111100 = BCh

    2. (a) Write a program which calculates EAX*2810 using binary multiplication. (Only typewritten code required)

(b) Consider the following unsigned value: 1234ABCD.  Let this value be stored in register EAX.  Write a program that will extract the decimal digits from this value using shifts and logical instructions.   Place the first two decimal numeric digits in DH and the other two into DL.  Submit a screenshot of the console output of the program and the asm/lst file. 


    3. (a) What will be the contents of AX and DX after the following operation?  What may happen if you do not set dx to 0 in the beginning?  You must work this problem by hand, not by a program run.  
    mov dx, 0
    mov ax, 1111h
    mov cx, 2222h
    mul cx            ; AX= 8642, DX = 0246
Explanation: When multiplying a 16 bit, the higher order gets stored in DX. If DX was not set to 0 before the operation occurred, then the value that gets stored into DX could be incorrect.

    (b) When does an IDIV instruction cause an overflow?  Provide an example.

Answer: An overflow is caused by a quotient produced by the instruction that is too large (or too small if working with signed integers) for the destination operand. When an overflow happens, the program abruptly stops and an error code is provided.
Example: mov ax, 700
                 mov bl, 2
                 idiv bl           ; 350 cannot be stored in AL – overflow error

    (c) What will be the values of DX:AX after the following instructions execute?  What might be the use of such a sequence of instructions in a 16-bit computer?
    mov ax, 0h
    mov dx, 0h
    sub ax, 2h    ; AX = FFFEh
    sbb dx, 0    ; DX = FFFFh

Explanation: These instructions, along with the use of DX and AX, can emulate 32-bit integers and perform 32-bit operations, as 16-bit computers cannot naturally use any 32-bit operations/registers/integers.


    4. Write a program which will have the register ‘AX’ as its input.  It would display the contents of AX in binary on the screen.  You should use shifts to achieve the function. Demonstrate by using several values of AX.  



  
    5. Write a program that performs C = A + B using extended addition.  See textbook pg. 270-271.  
Use the following:
Apple        QWORD     1111222233334444h 
Berry        QWORD     13572468ABCD0000h
Cherry    QWORD     ?
You may only use16-bit registers to perform the addition, e.g. AX, BX etc. 
Submit the asm/list file and a screenshot of your code printing the contents of all the arrays after the run.