Show how the Flag Register is affected by addition of F5 H and 0B H.

#Flag_Register
#8051Flag_Register
#Flag_Register_Status

T
, we'll go through the addition step by step, highlighting how each flag is set or cleared based on the result. The common flags in a typical 8-bit processor's flag register include the Carry flag (C), Zero flag (Z), Sign flag (S), Parity flag (P), Auxiliary Carry flag (AC), and Overflow flag (O/V).

1111 0101 (F5 H)
+ 0000 1011 (0B H)
-----------
1 0000 0000 (100 H)

Result: The sum is
10
0
16
100
16
​
, but since we are dealing with an 8-bit register, the result is
0
0
16
00
16
​
with a carry out.

Affect on Flags:
Carry Flag (C):

Since the result produced a 9th bit (carry out), the Carry flag is set (C = 1).
Zero Flag (Z):

The result is
0
0
16
00
16
​
, which is zero. Thus, the Zero flag is set (Z = 1).
Sign Flag (S):

The Sign flag is based on the most significant bit of the result. The result is
0
0
16
00
16
​
(
0000
 
000
0
2
0000 0000
2
​
), so the Sign flag is cleared (S = 0).
Parity Flag (P):

Parity flag is set if the number of set bits in the result is even.
0000
 
000
0
2
0000 0000
2
​
has zero set bits (which is even), so the Parity flag is set (P = 1).
Auxiliary Carry Flag (AC):

Auxiliary carry is set if there is a carry out from the lower nibble (4 bits) to the upper nibble. Here:
Lower nibble of
𝐹
5
16
F5
16
​
is
010
1
2
0101
2
​
.
Lower nibble of
0
𝐵
16
0B
16
​
is
101
1
2
1011
2
​
.
Adding these:
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Copy code
0101
+ 1011
------
1 0100 (carry from the lower nibble)
So, there is a carry out from the lower nibble to the upper nibble. Hence, the Auxiliary Carry flag is set (AC = 1).
Overflow Flag (O/V):

Overflow occurs when the sign of the result does not match the expected sign based on the operands. In this case:
Adding two positive numbers
𝐹
5
16
F5
16
​
(which is actually negative in two's complement) and
0
𝐵
16
0B
16
​
does not produce an overflow in the signed context.
As the final carry is outside the 8-bit range, the overflow flag should be cleared (O/V = 0).
Final Flag Register States:
Carry Flag (C): 1
Zero Flag (Z): 1
Sign Flag (S): 0
Parity Flag (P): 1
Auxiliary Carry Flag (AC): 1
Overflow Flag (O/V): 0
So, after the addition of
𝐹
5
16
F5
16
​
and
0
𝐵
16
0B
16
​
, the flags in the Flag Register are set as follows:

C = 1
Z = 1
S = 0
P = 1
AC = 1
O/V = 0