During the previous week I found out some tricks and tips in assembly, and thought of sharing them in this brief post.  I hope you will find these useful. I used NASM compiler to compile these codes.

Clearing a register:

Suppose you want to clear the ax register. XOR’ing it with itself will set all the bits in the register to zero.

xor ax, ax

Printing a single character on screen:

The interrupt 0x10 will print a given character on screen. The following code can be used to print a single character on screen.

mov al, '@'     ; character or the ASCII code of the character to be printed
mov ah, 0x0e    ; video – teletype output
int 0x10

Extracting some bits from a register:
Suppose you want to extract bits 3 and 4 of the AX register, and then print the decimal value on screen. The following code can be used to achieve this.
‘AND’ the register with a binary number which contains 1’s in the bit positions you want to extract. For our example it would be 0b0000000000011000 or 0x0018 (in hexadecimal). Then shift it right until our bits are the right-most bits. Then add 4b to it to get the ASCII number (ASCII value of ‘0’ is 48)

and ax, 0x0018    ; mask out bits 3 – 4
shr ax, 3         ; shift it right 3 times
add ax, 48        ; add 48 to get ASCII character
mov ah, 0x0e      ; print it
int 0x10

Remember the above code will work only if you are going to print values less than 9. If you want to convert a value more than 9 to decimal this will not work (I hope you understand why it doesn’t work 😉 ).

Printing out a register in binary:
For most of our debugging work, we will need to print the contents of a register in binary format. The following code can be used to print the contents of the CX register.

;print the CX reg
push dx              ; save registers dx,ax,bx
push ax
push bx
mov dx, cx
mov cx, 16           ; loop for 16 times
    mov ax, 32768    ; set  ax = ob1000....
    and ax, dx       ; mask out the MSB of dx and store it in ax
    shr ax, 15       ; shift it to right 15 times
    add al, 48       ; add 48 to get the ASCII (would be either 0 or 1
    mov ah, 0x0e
    int 0x10         ; print it out
    shl dx, 1        ; shift left dx by 1
loop print_loop

pop bx;              ; restore the registers
pop ax
pop dx

Hope these few tips will be helpful. That’s all for now :)

5 thoughts on “Assembly Tips and Tricks

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