ITPEC FE Subject A April 2026 Question 11

128 MB/s — convert the clock to bus cycles, then multiply by the bus width in bytes.

• •Bus cycle rate: 64 MHz ÷ 4 = 16 million bus cycles per second
• •Bus width: 64 bits = 8 bytes per bus cycle
• •Transfer rate: 16 × 8 = 128 MB/s

Answer (b)

Why not others:
- (c) 512 — the result of using the raw 64 MHz clock as the bus cycle rate (64 × 8), ignoring that one bus cycle takes 4 clock cycles
- (a) 12.8 and (d) 1280 — a factor-of-ten slip either side of the correct 128

Key rule: Divide the input clock by the number of clock cycles per bus cycle before multiplying, and convert the bus width from bits to bytes — the two most common mistakes in this calculation.

AI-generated — may contain errors