1.
Explain what happen in executing the EXCH instruction and in cache coherence operations to
cause the processor to jump back to spin at lock it again?
Solution
The EXCH instruction exchanges the contents of the register R1 to register R2 and vice versa.It
returns 0 if successful.
In a shared memory multiprocessor with a separate cache memory for each processor , it is
possible to have many copies of any one instruction operand : one copy in the main memory and
one in each cache memory. When one copy of an operand is changed, the other copies of the
operand must be changed also. Cache coherence is the discipline that ensures that changes in the
values of shared operands are propagated throughout the system in a timely fashion.
There are three distinct levels of cache coherence:
The BNEZ instruction checks if EXCH returned 0 or not if it is not 0 then it goes to lockit.
the lockit function executes until the EXCH is successful.