Answered! For the following two operations give the result and indicate the status of the C, V, N, and Z bits in the Condition Code register….

For the following two operations give the result and indicate the status of the C, V, N, and Z bits in the Condition Code register.

a) $2A + $52 = ?

b) $AC + $8A = ?

Expert Answer

So, here you are asked the condition code register bits status after performing the hexadecimal additions.

Let’s know first when each bit is set i.e. its value is set to 1.

I am writing this for addition operation only..

Z bit is set if addition returns 0 here. i.e. Z = 1 when the result is 0.

C bit is set if a carry is produced by the addition.

N bit is set if the result is negative i.e. when the MSB – most significant bit in result comes 1, the number is said negative so N bit is set.

V bit is set when an overflow is occurred. This in simple terms means when you add 2 positive numbers and it produces a negative number; it is said an overflow.

Similarly, when you add 2 negative numbers and the result is positive number, it is also an overflow.

So, in general, overflow means when you add 2 numbers of same sign and the result produced is having different sign.

Let’s make it simpler. If both the numbers have 1 in MSB, and the result has 0 in MSB; or vice-versa, it is said an overflow.

So now let’s add the numbers and find the result and status of these bits.

(a).

$2A
+ $52
——
$7C

Condition Code register status:

The result is not zero so Z flag remains 0.

No carry is produced here so C flag is also 0.

To see the MSB, write this number in binary.

7C = 0111 1100

So, here the MSB is not 1 so the number is positive. So N bit is also set to 0.

To find out the overflow…

Overflow is detected when the carry is generated while adding the respective 4 bits in binary. This means that you consider A+2 separate and if it is producing a carry, it will be added with 2+5. It is not generating a carry here, but if it was generated, it would be an overflow.

As here no overflow is there, V bit is also remaining 0.

(b).

$AC
+ $8A
——
$36 and carry = 1.

Let’s find the condition register status..

Here, the carry flag is set because in result we get, A+8+1 is exceeding F, the maximum value so C = 1

Z bit is 0 as the result is non-zero.

The number in binary is : 36 = 00110110 so the MSB is 0.

So N bit is also 0.

Let’s find for the overflow bit.

1100 (C in binary)
+1010 (A in binary)
——
0110

See, here both the numbers having MSB 1 and result containing MSB 0.

So this is an overflow. So, V bit is set to 1.