W13D2_WeiJie

W13D2/W13D2_Wei_Jie.md

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+#### Hardware Support for Memory Reference Speculation
+
+We have just one LR register, so we can do ll/sc in ll/sc which called nested ll/sc.
+
+When $M_{x}\not= M_{y}$, 
+
+1. store $M_{y}$
+2. load $M_{x}$
+
+We can first load $M_{x}$, then store $M_{y}$. It is an optimization.
+
+When $M_{x}= M_{y}$, we must obey the order----Hazard: error occurs when Mx = My. 
+
+#### Spinlock
+
+A locking mechanism is called a spinlock (non-blocking locker).
+
+In spinlock, the task cannot sleep while waiting for the lock, whereas in mutex, the task can sleep while waiting for the lock. 
+
+Load link & Store conditional: 
+
+eg1:
+
+1. mov R3, R4
+2. ll  R2, 0(R1)
+3. sc  R3, 0(R1)
+4. beqz R3
+5. mov R4, R2
+
+eg2:
+
+1. ll 	R2, 0(R1)
+2. addi R2, R2 #1 
+3. sc	 R2, 0(R1)
+4. beqz R2
+
+
+
+Compare and swap is different from ll/sc because it just compares the value but not pays attention to whether it has been modified.