Question

Please include explanation.

For Problems 14 and 15, use the following code segment. mov AL, 1Ah mov BL, 2Bh xor AL, BL xor BL, AL xor AL, BL 14. What is in AL after the above code segment is executed. (a) 1Ah (b) 2Bh (c) 3Ch (d) none of the above

Answer 1

In the problem, we have been given two instructions mov and xor.

• The instruction mov copies the data referred by the second operand into the first operand. For example, after executing mov A, 0110 the value of A will become 0110.
• The instruction xor does the bitwise XOR of operand 1 and operand 2 and stores the result in operand 1 if the xor is written as XOR OPERAND1 OPERAND2

14.

Now, let us look at the numerical statement by statement

• mov AL, 1Ah

AL becomes 1Ah i.e. 00011010

• mov BL, 2Bh

BL becomes 2Bh i.e. 00101011

Now the following three statements will be done with the help of truth table of xor (bitwise XOR)

XOR Truth Table

A	B	A xor B
0	0	0
1	0	1
0	1	1
1	1	0

• xor AL, BL

AL = 00011010

BL = 00101011

AL xor BL(according to truth table) = 00110001

so AL = 00110001, BL = 00101011

• xor BL, AL

BL = 00101011

AL = 00110001

BL xor AL(according to truth table) = 00011010

so BL = 00011010, AL = 00110001

• xor AL, BL

AL = 00110001

BL = 00011010

AL xor BL(according to truth table) = 00101011

so AL = 00101011 i.e. 2Bh, BL = 00011010 i.e 1Ah

Hence the correct answer is (b) i.e. 2Bh