Memory Access

• Concurrent Read (CR) -- multiple processors read from same memory location at the same time.
• Concurrent Write (CW) -- multiple processors write to same memory location at the same time.
• Exclusive Read (ER) -- no two processors read from same memory location at the same time.
• Exclusive Write (EW) -- no two processors write to same memory location at the same time.
• EREW, CREW, ERCW, and CRCW are all PRAM models
• EREW and CRCW are most prevalent
• EREW is easy to implement (constraint maintained by programmer)
• In the CRCW model, need to determine what to do when two or more processors write to the same location
  • Arbitrary -- select actual written value arbitrarily.
  • Priority -- write value from processor with lowest index.
  • Combining -- write some combination of values (sum, maximum).
  • Common -- processors must all write same value.