I/O Interfaces & Buses
In AXI, what is the advantage of issuing the next read request without waiting for the previous one's response (having multiple outstanding requests)?
- ARequests keep flowing during memory latency, hiding it and raising bandwidth
- BFewer requests make the circuit smaller
- CResponses always come back in reverse order
Show answer and explanation
Answer: A. Requests keep flowing during memory latency, hiding it and raising bandwidth
If each request waits for its response before the next is issued, with 100 cycles of memory latency the bus sits idle most of the time. AXI tags requests with IDs and allows several to be outstanding, so data keeps returning during the latency and bandwidth is used well. Requests with different IDs may complete out of order, so whichever finishes first can be returned first.
- A. Correct. IDs are used to tell them apart.
- B. The number of requests does not decrease.
- C. Ordering follows per-ID rules, not always reversed.