Support in ISA

• Recall: All instructions in a processor execute uninterrupted.
• Within a single processor, instructions are atomic.
• Pentium ISA provide xchg instruction. (Swap instruction)
  xchg(register r, memory_address a) {
  t = r; r = *a; *a = t;
  }
  • One Read and One write in memory and register each.

Implementing Locks Using Swap

• Acquire Lock:
  Register AX = TRUE;
  While (AX=TRUE) xchg(AX, &lock);
• Release Lock:
  • Lock = false;