A Level Computer Science OCR Practice Exam

What does the 8th bit in ASCII usually serve for?

1. Representing an additional character

2. Error checking

3. Indicating the end of a string

4. Logical operations

The correct answer is: Error checking

In ASCII, the 8th bit is often used for error checking. ASCII originally defines a 7-bit encoding system, which allows for 128 unique character representations. However, in situations where an 8th bit is added, it can be used to accommodate additional information beyond standard character representation. One common use of this 8th bit is for parity checking, which is a simple error detection scheme. Parity bits are used to ensure that the number of bits set to 1 in a given set of bits is either even or odd. If the received data has an incorrect parity, it indicates that an error may have occurred during transmission, prompting further investigation, such as retransmission of data. The other options do not utilize the 8th bit in the context of standard ASCII. Representing an additional character would typically rely on different encodings, not just the 8th bit. Indicating the end of a string is commonly done with specific terminators (like null characters in C), rather than a dedicated bit in ASCII. Logical operations pertain more to operations in programming and processor instructions, rather than character encoding. Hence, the primary function of the 8th bit in ASCII systems is indeed related to error checking.