Module 6: memory

Question 1

Describe the function of primary and secondary storage and the main differences between them (give 1 example of each)

Answer 1

primary: directly accessible by the processor
e.g. ram and rom

secondary: devices that can store data more permenently
e.g. hdd, ssd, CD/DVD

Question 2

List the sequence of steps for a primary memory read. Explain how delays in these
steps contribute to the access time

Answer 2

access time is the time taken to retrieve data from storage.

1. address voltages settle
2. decode the address
3. select the correct memory location
4. transfer data to data buffer that is connected to data bus
5. enable data onto the bus
6. data voltages settle

(this part is calebs wording) if there is a delay, the time taken to retrieve data will take longer.

Question 3

Describe the similarities and differences between ROM and RAM.

Answer 3

ROM:
- Non-volatile memory
- cant be written to normal operation
- normmally used for startup code and low-level functions

RAM (read write memory) :
- Volatile (data disappears when no power)
- can be read and written
- main working memory

Question 4

What is firmware? Describe at least 2 applications for firmware

Answer 4

firmware is software embedded in hardware (ROM)

applications for firmware:
BIOS/ UEFI, hard disk drives & SSDs, Graphics cards

Question 5

How does an SSD work? What are the advantages of an SSD over HDD?

Answer 5

Flash ROM technology that works similar to RAM, but transfers blocks of data rather than single bytes

advantages:
fast access (no seek time)
- access time = 35 - 100 microseconds
- 100x faster than HDD
Quiet:
- no spinning disks
Reliable:
- no moving parts
Fast startup
- no delays waiting for disks to get up to speed

Question 5

What are the differences between static RAM and dynamic RAM? Based on their

relative strengths and weaknesses, describe suitable applications for each.

Answer 5

Static: stores data in logic circuits
pro:
- fast and simple implementation

Con:
- space and power inefficient

Applications:
- CPU cache

Dynamic: Data stored as charge on small MOS capacitors
pro:
- higehr density
- power requirements are lower

Con:
- period charging due to leakage of capacitors
- more complex - needs refresh circuits

Applications:
- main memory (RAM)

Question 6

Describe the principles of operation of a hard disk drive. Explain the factors that contribute to the access time for a hard disk

Answer 6

Data is stored on metallic platters (‘hard disks’)
- Platters are coated on both sides with magnetic oxide
- The magnetic ‘orientation’ of material changed to represent 1s or 0s

• Heads read the 1’s and 0’s from the platter Or write them to the platter – Heads for top and bottom of each platter
• Platters spin at high speed
• Actuator arm moves the heads together radially allows any spot to be accessed

Access time is relatively long
- 4 to 20 ms (milliseconds), depending on drive
- Seek time in addition to normal access delays
- Seek time = time to get head positioned over data location
- Time to move head to track + time for disk to spin required sector under head

Question 6

How do tape drives store data and what are they primarily used for? Give reasons why they are used for that application

Answer 6

store data magnetically with read/ write headers

archival purposes because tape can safely store data for decades as long as temperature. humidity conditions are maintained.

Question 7

What is cache memory and why is it needed?

Describe the difference between L1 and L2 cache memory. Illustrate this using a diagram

Answer 7

Small amount of faster memory that sits ‘between’ CPU and main memory

Stores copies of frequently used data and instructions to improve performance of primary storage

Cache closest to processor (core) is called L1 Cache (on cache)
L2 (off cache)
increments the further it is from the processor

Question 8

How data is organised in a HDD?

Explain how data is distributed to minimise access time.

Answer 8

Data on hard drives is organised in tracks,

Tracks are subdivided into blocks called sectors,

So data is normally spread across the same track on the different surfaces of the platters

This combination of tracks is called a cylinder