Thursday, December 24, 2009

Assembly Language Part 6

Logic Instructions

The ability to manipulate the individual bits of a Data is one of the key advantages of assembly language. In order to do so we use Logic instructions. Let us first remind the basic ligic instructions for now,


AND 
 0 0 -> 0
 1 0 -> 0
 0 1 -> 0
 1 1 -> 1

OR 
 0 0 -> 0
 1 0 -> 1
 0 1 -> 1
           1 1 -> 1            



XOR 
 0 0 -> 0
 1 0 -> 1
 0 1 -> 1
 1 1 -> 0


NOT

 0 -> 1
 1 -> 0
 

Now lets see where we use these functions in Assembly -

AND   ->  To clear
OR     ->  To set
XOR   ->  To compliment desired bit positions
NOT   ->  To compliment all bits

AND (Used to Clear )

The form is
AND DESTINATION, SOURCE

Suppose we want to clear the sign bit of AL then the code will be

AND AL,7Fh                            ;

we know that the sign bit is the MSB so if we AND it with the value which has all 1s except of the MSB as 0 then

AL   =   10101100
Msk =   01111111

-----------------------------
rslt  =   00101100


OR (used to set)

The form is
OR DESTINATION, SOURCE

Suppose we want to set the sign bit of AL then the code will be

OR AL,80h                            ;

we know that the sign bit is the MSB so if we OR it with the value which has all 0s except of the MSB as 1 then

AL   =   00101100
Msk =   10000000
-----------------------------
rslt  =   10101100

XOR (Used to compliment selected bits )

The form is
XOR DESTINATION, SOURCE

Suppose we want to change the sign bit of AL then the code will be

XOR AL,80h                            ;

we know that the sign bit is the MSB so if we XOR it with the value which has all 0s except of the MSB which is 1 then


AL   =   10101100
Msk =   10000000
-----------------------------
rslt  =   00101100

Here you may have noticed that destination bit is complimented if that bit position of the Mask is 1 and unchanged if it is 0. (XOR x , 0 - x) (XOR  x,1)= x'



NOT (Used to compliment all bits )

The form is
NOT DESTINATION

Suppose we want to invert all the bits of AL then the code will be

NOT AL                            ;

You can see that NOT takes only one argument.

Suppose AL = 10110010 , after executing the NOT instruction it will be 01001101


Problem (Convert ASCII value to number)
When we enter a character in Assembly the equivalent ASCII value is stored in the memory. Say we pressed 3 then the ASCII value 33h will be saved in AL. we have to convert it to 3...

If we AND a value with 0000  1111 then the relust will be 30 less than the original value

AL   =   0011 0011   =33
Msk =   0000 1111
------------------------------
             0000 1111   =3

so the code will be

AND AL, 0Fh

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