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CPI Execution: Average Memory Access Time (AMAT)

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CPI Execution: Average Memory Access Time (AMAT)

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Questions and Answers

What is the formula for Memory Stall Cycles per Average Instruction (CPIStall), and how does it relate to MemAccess/Instruction and MemStall?

CPIStall = (MemAccess/Instruction)(MemStall) = (1 + %load + %store)(1 – hi)(Mi)

What is the Ideal CPI, and how does it relate to CPIPerfect and CPIStall?

CPI = CPIPerfect + CPIStall, and in the ideal case, CPIStall = 0, so CPI = CPIPerfect

What is the calculation for AMAT, and how does it relate to MemStall?

AMAT = 1 + MemStall, where MemStall = (Miss Rate)(Miss Penalty)

What is the Speedup of Ideal CPI, and how does it relate to CPI and CPIPerfect?

Speedup = CPI/CPIPerfect, where CPI = CPIPerfect in the ideal case Signup and view all the answers

In a system with no cache, what is the miss rate, and how does it affect MemStall?

The miss rate is 1, resulting in MemStall = (1)(Miss Penalty) Signup and view all the answers

In the example given, what is the calculation for CPI, and how does it relate to CPIPerfect and CPIStall?

CPI = CPIPerfect + CPIStall = 1.1 + 140 = 141.1 Signup and view all the answers

What does the Average Memory Access Time (AMAT) represent in the context of CPU execution?

The number of cycles required to complete an average memory access request by the CPU. Signup and view all the answers

Derive the formula for AMAT in terms of hit rate, access time, miss rate, and miss penalty, given that the hit access time t1 is usually 1 cycle and the miss penalty is T2.

AMAT = ti + (mi)(T2) or AMAT = 1 + (mi)(T2) since ti = 1. Signup and view all the answers

What is the significance of the term MemStall in the context of CPU execution, and how is it related to AMAT?

MemStall represents the memory stall cycles per memory access, and it is equal to (m1)(T2) or (Miss Rate)(Miss Penalty). AMAT is directly affected by MemStall, with the formula AMAT = 1 + MemStall. Signup and view all the answers

What is the relationship between the hit rate (hi) and miss rate (mi) in the context of CPU memory access?

The miss rate (mi) is equal to 1 – hi, indicating that the sum of the hit rate and miss rate is always equal to 1. Signup and view all the answers

What is the implication of a higher miss rate on the Average Memory Access Time (AMAT)?

A higher miss rate leads to a higher AMAT, as it increases the number of stall cycles added to the CPU execution cycles for one memory access. Signup and view all the answers