In character encoding using ASCII (American Standard Code for Information Interchange), adding a 0 to the leftmost (most significant) bit to form an 8-bit byte from a 7-bit character is called ________.
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Azero padding
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Bsign extension
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Cparity insertion
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DMSB masking
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Ebit stuffing
Answer
Correct Answer: zero padding
Explanation
Introduction / Context:ASCII is historically a 7-bit code. In many digital systems, bytes are 8 bits wide, so a 7-bit ASCII character is commonly stored in an 8-bit location by adding an extra bit on the left. This question asks for the proper term used when a 0 is added to that most significant position to fill the byte neatly without changing the character value.
Given Data / Assumptions:
- ASCII characters are 7 bits in their original definition.
- Storage and transmission often use 8-bit bytes.
- The added bit is placed in the most significant position (leftmost bit).
- The added bit is 0, not 1.
Concept / Approach:When extending a binary value to a wider width without changing its magnitude, adding zeros to the left (high-order side) is called “zero padding” or “zero extension.” This preserves the numeric/encoded meaning and is distinct from adding a parity bit or performing sign extension (used with signed integers).
Step-by-Step Solution:
Start with a 7-bit ASCII code, for example 1000001 (letter ‘A’).Form an 8-bit byte by adding a zero on the left: 01000001.Note that the character meaning remains unchanged; only width increased.Recognize the operation name as zero padding (zero extension).Verification / Alternative check:Compare numerical values before and after: 1000001 (binary 65) becomes 01000001 (binary 65). The value is identical, confirming a padding (not a reinterpretation).
Why Other Options Are Wrong:
- Sign extension: Copies the sign bit (1 for negative in signed integers), not applicable to ASCII characters.
- Parity insertion: Adds a parity bit for error detection, typically separate from the stored character byte.
- MSB masking: Refers to clearing or ignoring the MSB using a mask, not adding a new bit.
- Bit stuffing: Protocol-level addition of bits in streams to avoid reserved patterns, not fixed-width padding.
Common Pitfalls:Confusing parity with padding; assuming sign rules apply to character encodings; thinking the added bit changes the code point. It does not; it merely fits the code into an 8-bit container.
Final Answer:zero padding