Consider an 8-bit ALU with a 4-bit status register. The four status bits are symbolized by C, S, Z, and V which stands for carry, sign, zero, and overflow, respectively. Let A= 1111 0000 and B = 0001 0100 and ALU performs A - B. Which of the following represents the state of 4-bit status register?

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AhmedNazir · Answer 1 · Sep 04, 2022 04:29:57

Option 2 : 1100

The correct answer is option 2.

Concept:

To subtract B from A.

Write the expression (A-B) as:

(A - B) = A + (-B) Now (-B) can be written as (2’s complement of B).

The Given data:

An ALU has 8-bit with a 4-bit status register.

C=carry,

S=sign,

Z=zero,

V=overflow.

A= 1111 0000

B = 0001 0100

Explanation:

A-B= A + (2’s complement of B)

2’s complement of B=1’s complement +1

B = 0001 0100

1’s complement of B= 1110 1011

2’s complement of B= 1110 1100

A + (2’s complement of B)=

A= 1111 0000

+ B= 1110 1100

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A-B= 1101 1100

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The last four bits are 4-bit status register= 1100

Hence the correct answer is 1100.

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