ITPEC FE Morning October 2020 Question 4

Bitwise AND Masking — extract specific bits from a byte using a bitmask.

To extract the lower 7 bits from an 8-bit code (ignoring the MSB parity bit), perform AND with 7F:

• •7F = 0111 1111 in binary
• •AND with 1 preserves the bit; AND with 0 clears it
• •Result: bit 7 (parity) is cleared, bits 0–6 remain unchanged

Example: 1010 0110 AND 0111 1111 = 0010 0110

Why not others:
- (a) AND with 0F — 0000 1111 clears bits 4–6 too, losing data
- (c) OR with 0F — OR sets bits to 1, doesn't extract; corrupts lower nibble
- (d) XOR with FF — inverts all 8 bits (bitwise NOT), doesn't extract

Key rule: To isolate specific bits, use AND with a mask where target bits = 1 and unwanted bits = 0.

AI-generated — may contain errors