MESI & MOESI

MESI: Adding the Exclusive State

MESI adds a fourth state to MSI:

State	Meaning	Dirty?	Shared?
M (Modified)	Only copy, modified	Yes	No
E (Exclusive)	Only copy, clean	No	No
S (Shared)	Clean, others may have copies	No	Yes
I (Invalid)	Not valid	—	—

The Key Optimization

When Core 0 reads a line and no other cache has it:

• MSI: goes to S → later write requires BusUpgr (bus transaction)
• MESI: goes to E → later write just transitions E→M silently (no bus transaction!)

This silent upgrade is a major performance win because many programs read data before writing it, and often only one core accesses a given line.

How Does the Cache Know?

On a BusRd, the bus has a SHARED signal. If any other cache asserts SHARED (they also have the line), the requesting cache goes to S. If nobody asserts SHARED, it goes to E.

Interactive: Cache State Transitions

Click Read/Write on each core to see MSI and MESI state transitions in action. Watch how the states change and how they coordinate:

Reset

Core 0

I

Invalid

ReadWrite

Core 1

I

Invalid

ReadWrite

Memory[A] = 42

States: M=Modified, S=Shared, I=Invalid

System initialized. Memory[A] = 42

Try these scenarios

1. Read sharing: Core 0 Read → Core 1 Read (both go to S)
2. Write invalidation: Core 0 Read → Core 1 Read → Core 0 Write (Core 1 gets invalidated)
3. MESI silent upgrade: Switch to MESI → Core 0 Read (goes to E) → Core 0 Write (E→M, no bus traffic)
4. Write-back on eviction: Core 0 Write → Core 1 Read (Core 0 must flush M data)

MOESI: Adding the Owned State

MOESI adds a fifth state:

State	Meaning
O (Owned)	This cache has the authoritative copy, but others have S copies. Memory is stale.

Why Owned?

In MESI, when Core 0 has a line in M and Core 1 reads it:

1. Core 0 flushes the dirty data to memory (expensive write)
2. Both caches go to S

In MOESI:

1. Core 0 supplies the data directly to Core 1 (cache-to-cache transfer)
2. Core 0 goes to O (responsible for the data), Core 1 goes to S
3. Memory is not updated — saved a memory write!

The O state means “I’m the owner; if anyone needs this data, I’ll supply it. Memory is stale.”

MOESI Summary

State	Only copy?	Dirty?	Responsible for writeback?
M	Yes	Yes	Yes (on eviction)
O	No (others in S)	Yes	Yes (on eviction)
E	Yes	No	No
S	No	No	No
I	—	—	—

Real-World Usage

Protocol	Used by
MESI	Intel (with extensions), many embedded
MOESI	AMD (with extensions)
MESIF	Intel server (F = Forward, similar role to O)

All real implementations add vendor-specific extensions, but the core state machine is recognizable from these standard protocols.