Questions for Cache Hierarchy

Q1: Where can a block be placed in the upper level?

• (Block placement)
  Q2: How is a block found if it is in the upper level?
• (Block identification)
  Q3: Which block should be replaced on a miss?
• (Block replacement)
  Q4: What happens on a write?
• (Write strategy)

Q1: Where can a block be placed in the upper level?

Set = # of cacheline / way
2 - way set associative = 2 cache line (way) per set

Cache Block = chunk of memory data
Cache line = actual stroage location
Way = position name of that line

                 Way 0              Way 1
               ↓                    ↓
Set 0      [ Cache Line ]      [ Cache Line ]
Set 1      [ Cache Line ]      [ Cache Line ]
Set 2      [ Cache Line ]      [ Cache Line ]
Set 3      [ Cache Line ]      [ Cache Line ]

As you can see here,

• Number of ways = 2 (position name)
• Number of cache lines = 8 (actual stroage location)

1. Fully associative

Anywhere in the cache

2. Direct mapped

At a fixed place in the cache
Index=Block_Address mod Cache_Size

3. Set associative

Anywhere within a restricted set of places
Set_Index=Block_Address mod Number_Of_Sets

2 way means you have two cache line,
and 12 mod 4 = 0 so, you can put data at set 0

Q2: How is a block found if it is in the upper level?

Data address in main memory is divided into following fields:

• Block offset – where within a block do we start accessing the data we need
• Block Address
  

Each cache record stores

• Tag
• Valid bit – whether this record contains valid information
• Data – the actual block is stored here