Architecture 07

Question 1

What is memory hierarchy?

Answer 1

The levels of computer storage, ranging from small, fast and expensive storage (i.e registers) to large, slow and cheap storage (i.e disk)

Question 2

How is memory hierarchy utilised?

Answer 2

• Processing happens near top
• Bring data up levels where needed
• Bring new data down to non-volatile levels for safety/sharing

Question 3

Why are fast processors small?

Answer 3

The larger the distance the electric signals need to travel the greater the delay in transmission

Question 4

Why is there a memory hierarchy?

Answer 4

• Processors need to be small and fast, but you can store large amounts of data here

Question 5

What are the properties of storage mechanisms?

Answer 5

• Access paradigm (serial vs random)
• Volatility (keeping data without power)
• Write-ability (write-once vs re-writable)
• Latency vs bandwidth
• Density (space requirements)
• Energy usage (idle as well as in operation)
• Dependability and error correction

Question 6

What is the guiding principle of managing different memory technologies?

Answer 6

• Keep currently/frequently used data in faster, more expensive memory
• Keep other data in larger, slower memory

Question 7

What is a Virtual Machine?

Answer 7

A VM is a mechanism that hides the details of the underlying physical memory to provide a more convenient memory evnironment

Question 8

What is a virtual address space?

Answer 8

These are addresses that a processor uses, where the MMU translates each address into an underlying physical memory.

Question 9

Past Paper Question: What is a memory cache and why are caches so important in modern computer architecture?

Answer 9

• A cache is placed on the path between a mechanism that makes requests and a mechanism that answers requests, and the cache is configured to intercept and handle all requests
• It acts as high speed, temporary storage: it keeps a local copy of selected data, and answers requests from the local copy whenever possible

Question 10

Past Paper Question: What do the terms spatial locality and temporal locality mean in reference to memory accesses, and why are they important?

Answer 10

• Spacial locality is where memory being accessed is often near memory accessed recently
• Temporal locality is when accesses to the same area of memory tend to be grouped in time
• These are important when a decent proportion of data we want is in fast memory already

Question 11

What is meant by Direct Mapped?

Answer 11

This is a cache optimisation technique, where each chunk of data has just one place where it can be
• Cache is smaller than slower memory, so multiple chunks can be in that place

Question 12

What is meant by Fully Associative?

Answer 12

This is a cache optimisation technique, where any chunk of data can be anywhere
• More complex data structure/hardware to look up whether a chunk is in cache