What 'E'?

Understanding cache coherence protocols and the MESI extension

This is an easy computer architecture question testing your grasp of snoopy-cache coherence and how protocols evolve to address real-world performance constraints. It appears frequently in systems interviews at firms that care about low-level performance tuning.

The question asks you to compare two related coherence protocols and identify what specific problem the additional state solves. To answer well, you need to understand the state transitions and communication patterns in a simpler protocol, then recognize what inefficiency the extra state eliminates. The focus is on why the design choice matters, not just what the states are.

• Cache-line states and their semantics
• Bus traffic and coherence messages
• Trade-offs between protocol complexity and performance