You are building a computer system with in-order execution that runs at 1 GHz an

Question

You are building a computer system with in-order execution that runs at 1 GHz and has a CPI of 1, with no memory accesses. The memory system is a split L1 cache. Both the I-cache and the D-cache are direct mapped and hold 32 KB each, with a block size of 64 bytes.

The memory system is split L1 cache. Both the I-cache and the D-cache are direct mapped and hold 32KB each, with a block size of 64 bytes.

The I-cache has a 2% miss rate, and the D-cache is a write-through with 5% miss rate.

The hit cycles for both the I-cache and the D-cache take 1 cycle (1 cycle takes 1 ns).

The L2 cache is a unified write-back with a total size of 512 KB and a block size of 64 bytes.

The hit cycle of the L2 cache is 15 cycles. The local hit rate of the L2 cache is 80%.

An L2 data write miss takes extra 15 ns.

Given: Hit time=Hit Rate*Hit Cyles

L2 miss penalty = 100nsec

Compute the AMAT for both instruction and data memories

Explanation / Answer

Solution: Assume a L2-miss will cause the memory transfers to both of L1 and L2.
Cycle time to transfer one bus word (64b) = 1/100MHz = 10ns
Time to transfer memory block to cache way (64B): 64B/64b * 10ns = 80ns
For Instruction:
AMAT = 98%*1ns (L1 instruction hit)
+2%*( (L1 instruction miss)
80%*15ns (L2 instruction hit)
+20%*(20ns+80ns)) (L2 instruction miss: access latency + transfer)
= 1.62 ns
For data:
AMAT = 95%*1ns (L1 data hit)
+ 5%*( (L1 data miss)
80%*15ns (L2 data hit)
+20%*(15ns+20ns+80ns)) (L2 data miss: write miss + latency +
transfer)
= 2.7ns
Overall AMAT = 1.62 + 2.7 = 4.32ns

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