ITPEC FE Subject A April 2025 Question 1

Topic: Basic Theory

For an 8-bit binary integer, which of the following is the operation that extracts the most significant 4-bits while making the other bits zero?

a)Bitwise AND operation with 11110000.b)Bitwise NAND operation with 11110000.c)Bitwise OR operation with 00001111.d)Bitwise XOR operation with 00001111.

Explanation

AND with 11110000 — a mask keeps the bits where it has 1 and clears the bits where it has 0.

AND gives 1 only when both bits are 1, so ANDing a bit with 1 passes it through unchanged and ANDing it with 0 forces it to 0. The mask 11110000 has 1s over the upper four bits and 0s over the lower four.

Example: 10110101 AND 11110000 = 10110000. The upper half survives and the lower half becomes zero.

Answer (a)

Why not others:

•(b) NAND with 11110000 — NAND inverts the AND result: the upper four bits come out inverted and the lower four become 1, not 0
•(c) OR with 00001111 — OR with 1 forces a bit to 1, so the lower four bits become 1111 instead of 0000
•(d) XOR with 00001111 — XOR with 1 flips a bit, so the lower four bits are inverted, not cleared

Key rule: AND with a mask clears the bits where the mask has 0, OR sets the bits where it has 1, and XOR inverts the bits where it has 1.

Basic Theory · AI-written, may contain errors

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