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Classes of On-Chip Protocol

  1. Reciprocally-degrading: such as handshake protocols studied earlier: throughput is inversely proprotional to target latency in terms of clock cycles,
  2. Delay-tolerant: such as AMBA-3 (ARM's AXI) and OCP's BVCI (below): new commands may be issued while awaiting responses from earlier,
  3. Reorder-tolerant: responses can be returned in a different order from command issue: helpful for DRAM access and needed for advanced NoC architectures.
  4. Virtual-circuit flow controlled: (beyond scope of this course): each source has a credit counter controlling how many packets it can send and priority mechanisms ensure responses are returned without deadlock.

For those interested in more detail: »Comparing AMBA AHB to AXI Bus using System Modelling

7: (C) 2008-11, DJ Greaves, University of Cambridge, Computer Laboratory.