1. Memory Reference

Memory Reference는 CPU가 메모리 계층(Cache / Memory)에 한 번 접근하는 것을 의미한다. 주로 두 종류가 있다.
A memory reference means one access to the memory hierarchy/cache.

In these cache problems, there are usually two kinds:
$\boxed{\text{Instruction fetch}}$
and
$\boxed{\text{Data access from load/store}}$

Every instruction needs to be fetched, so every instruction causes at least:

$\boxed{1\text{ memory reference}}$

If the instruction is a load or store, it also accesses data memory, causing another memory reference. So if 40% of instructions are loads/stores:

${Memory references/instruction}= 1+0.40$

Why do we need it? Because cache miss rate is often given per memory reference, but CPU performance is usually measured per instruction.

Stall CPI

Stall CPI = Memory reference / Instruction x MissRate x Misspenalty