Memory

Question 1

What is SRAM?

Answer 1

Static Random Access Memory - Retains data while power is supplied, but will lose it once power is lost. More expensive, but faster

Question 2

What is DRAM?

Answer 2

Dynamic Random Access Memory - Loses data if we don’t constantly refresh it

Question 3

Why is SRAM more expensive than DRAM?

Answer 3

It requires more than one transistor per bit of memory

Question 4

What are SRAM and DRAM used for?

Answer 4

SRAM for caches and DRAM for MM

Question 5

Describe a magnetic disk

Answer 5

• Circular platters with 2 surfaces that can store data

- platters are stacked on top of each other around a spindle with an assembly head attached to each one

Question 6

What is a track?

Answer 6

Each platter is divided into tracks - one rotation around the platter at a given distance

Question 7

What is a cylinder?

Answer 7

All of the tracks from the same distance from the spindle

Question 8

What is a sector?

Answer 8

Each track is divided into sectors - has header and checksum bits

Question 9

What are the things that can slow down magnetic disk access?

Answer 9

Seek time (move head to correct cylinder), rotational latency, data read, controller time (check checksum), I/O bus time (send data back to mm), queuing delay

Question 10

What are the slowest components of magnetic disk access?

Answer 10

Disk access time is dominated by the seek time and rotation speed

Question 11

What is the main difference between optical disks and magnetic disks?

Answer 11

Use laser instead of assembly head ==> allows for optical disks to be portable

Question 12

What is a con of optical disks?

Answer 12

They must be standardized because they are portable => slows rate of improvement

Question 13

What is magnetic tape used for?

Answer 13

Backup storage

Question 14

What are the pros/cons of magnetic tapes?

Answer 14

(+)Large capacity and easily replaceable
(-) sequential access
(-) expensive because no one is buying them

Question 15

What is SSD?

Answer 15

Solid-state disk. Attempt to use benefits of DRAM for hard drive storage

Question 16

What are the implementation options for SSD?

Answer 16

1. DRAM + Battery
   - Fast, reliable, but very expensive and not good for archiving (remember DRAM is not persistent)
2. Flash
   - Low power, faster than disks (but slower than DRAM)
   - But smaller capacity than disks (GBs vs TBs)
   - Keep data without power

Question 17

What is Hybrid Magnetic-Flash Storage?

Answer 17

tries to combine best of both worlds - use flash for cache for the disk, use magnetic disk for storage. Most frequent data is put in the cache. Both mediums are persistent, so power loss is not an issue

Question 18

Pros/cons of magnetic disks

Answer 18

(+) low cost / GB
(+) huge capacity
(-) eats power
(-) slow (must move mechanical head)
(-) sensitive to impacts while spinning (head can scratch disk and permanently damage it)

Question 19

Pros/cons of flash

Answer 19

(+) fast
(+) power efficient
(+) no moving parts