Cache coherence

• deals with ordering of writes to a single memory location
• only needed for systems with caches

Memory consistency

• deals with ordering of reads/writes to all memory locations
• needed in systems with or without caches

P0:
S1: datum = 5;
S2: datumIsReady = 1;

P1:
S3: while (!datumIsReady);
S4: … = datum 

Program-order expectation: Programmers expect memory accesses in a thread to be executed in the same order in which they occur in the source code. Atomicity expectation: A read or write happens instantaneously with respect to all processors.