Memory Hierarchy

Question 1

define locality of reference

Answer 1

a processor’s memory access tent to cluster - i.e tend to access the same set of memory locations repeatedly over a short period of times

Question 2

what are the 4 reasons that make locality make sense

Answer 2

1. instructions other than fetch & jump are fetched sequentially
2. iterations repeat a small number of contiguous instructions
3. the nesting of function calls is usually not very deep
4. data structures are usually accessed sequentially

Question 3

what are the 2 dimensions of locality

Answer 3

1. temporal
2. spatial

Question 4

what is temporal locality

Answer 4

a programme tends to reference recently accessed memory locations again in the near future

Question 5

draw a probability distribution of temporal locality

Question 6

what is spatial locality

Answer 6

a programme tends to reference memory locations that are near one another

Question 7

draw a probability distribution of spatial locality

Question 8

what are the 4 types of locality

Answer 8

1. instruction spatial
2. instruction temporal
3. data spatial
4. data termporal

Question 9

what are the 3 main memory trade offs

Answer 9

1. ^ memory speed = ^ cost per bit
2. ^ memory size = v speed
3. ^ memory size = v cost per bit

Question 10

write out the memory hierachy

Answer 10

1. CPU
2. cache
3. main-memory
4. solid-state memory
5. virtual memory & file/database memory
6. office bulk memory

Question 11

what changes as you travel down the memory hierarchy

Answer 11

• cheaper
• slower
• larger

Question 12

what are the 5 measures of performance modelling

Answer 12

1. expected access time
2. average cost per bit
3. average cost per bit/relative memory size
4. access efficiency
5. average access time

Question 13

expected access time formula

Answer 13

ts = h.t + (1 - h)(t1 + t2)
= t1 + (1 - h).t2
where:
- h = M1 hit ratio
- t1 = access time of M1
- t2 = access time of M2

Question 14

average cost per bit formula

Answer 14

Cs = (C1.S1 + C2.S2)/(S1 + S2)
where:
- Cs = average cost per bit
- C1 average cost per bit of M1
- C2 = average cost per bit of M2
- S1 = size of M1
- S2 = size of M2

Question 15

average cost per bit/relative memory size formula

Answer 15

Cs/C2 = (C1/C2 + S2/S1)/(1 + S2/S1)

Question 16

access efficiency formula

Answer 16

T1/Ts = T1/(T1 + (1 - H).T2/T1
= 1/(1 + (1 - H).T2/T1)
where:
- T1 = access time of M1
- T2 = access time of M2

Question 17

average access time formula

Answer 17

Ts = sum of (harmonic mean(1 - hj)(tihi)