JNTU MCA 2018 COMPUTER ORGANIZATION Previous Question Paper (January - 2018 - R17)

To design a 4-to-16 line decoder using 3-to-8 decoders, we require two 3-to-8 decoders and one NOT gate. A 3-to-8 decoder uses 3 input lines (A2, A1, A0) to produce 8 outputs (Y0 to Y7) and typically includes an enable input (E).

The design logic is as follows:

1. Connect the three least significant bits (A2, A1, A0) to the input lines of both 3-to-8 decoders.
2. Use the most significant bit (A3) as the control signal for the enable inputs.
3. Connect A3 directly to the enable input of the second decoder (active high).
4. Connect A3 through a NOT gate to the enable input of the first decoder (active low).
5. When A3 is 0, the first decoder is enabled, producing outputs Y0 through Y7. When A3 is 1, the second decoder is enabled, producing outputs Y8 through Y15.

This configuration effectively expands the address space from 3 bits to 4 bits, allowing the selection of one out of 16 output lines based on the input combination.

Locality of reference is a phenomenon observed in computer programs where the CPU tends to access a relatively small portion of its address space at any given time. This principle is the fundamental basis for the effectiveness of memory hierarchies, such as cache memory, as it allows the system to predict future memory accesses and pre-fetch data from slower main memory into faster cache.

It is categorized into two primary types:

• Temporal Locality: If a particular memory location is referenced, it is highly likely that the same location will be referenced again in the near future. This is commonly seen in loops, where instructions and variables are accessed repeatedly.
• Spatial Locality: If a particular memory location is referenced, it is highly likely that nearby memory locations will be referenced soon. This occurs due to the sequential nature of code execution and the storage of data structures like arrays in contiguous memory blocks.

By exploiting these properties, the computer system reduces the average memory access time, significantly improving overall processor performance.