L8 - Memory

Question 1

What is the name of the memory internal to the processor?

Answer 1

Registers

Question 2

How is external memory accessible by the processor?

Answer 2

Through IO modules

Question 3

Cache is a form of ______ memory.

a) internal
b) external

Answer 3

a) internal

Question 4

In what unit is memory capacity usually expressed

Answer 4

Bytes

Question 5

What are the four methods of accessing units of data?

Answer 5

Sequential access

Direct access

Random access

Associative access

Question 6

Define “associative access”

Answer 6

Word is retrieved based on a portion of its own contents rather than its address

Each location has its own address

Constant retrieval time

Cache memory may use associative

Information can be searched by type

***SPECIAL CASE OF RANDOM ACCESS***

Question 7

Define “random access”

Answer 7

Each location has unique, wired in address

Any location can be selected and accessed

Access time is constant - same for any location

Order unimportant

Example: Main memory, cache

Question 8

Define “direct access”

Answer 8

Shared read-write mechanism

Individual blocks have unique physical address

Access time depends on physical position of data

Example: Harddrive (R/W Mechanism is the magnetic head)

Question 9

Define “sequential access”

Answer 9

Memory organized into units of data calld records

Access is made linearly, i.e. one record after another

Access time depends on position of required data

Example: Tape reel

Question 10

What are the three performance parameters use with regard to memory?

Answer 10

Access time (latency)

Memory cycle time

Transfer rate

Question 11

What is the access time (latency) of a memory system?

Answer 11

For random-access - time taken for a read/write operation

For non-random-access memory - time taken to position the read/write mechanism at desired location

Question 12

Define “memory cycle time”

Answer 12

Access time plus any additional time before another access can commence

Example: time for R/W Head to move back to default position or time for SSD to calculate the address/difference etc.

Concerned with system bus, not processor

Question 13

Define “transfer rate” as it applies to memory systems.

Answer 13

Rate at which data can be transferred into or out of a memory unit

For random access memory, 1/(cycle time)

Question 14

What is memory hierarchy?

Answer 14

Balance of cost, speed and capacity

In general, increased speed = increased cost & decreased capacity

increased capacity = decreased speed & increased price

etc.

So for a good balance can use many different memory components

Question 15

Describe the process of reading from memory

Answer 15

1) Source address put in Memory Address Register
2) Address bus set to value of MAR
3) Control line set to “read”
4) Data bus set to value at address in MAR
5) Memory buffer register set to value of Data bus

Question 16

Describe the process of writing to memory

Answer 16

1) Destination address put in Memory address register
2) Data to be written put in Memory buffer register
3) Address bus set to value of MAR
4) Control line set to indicate write operation
5) Data bus set to contents of MBR
6) Data at address in Address bus set to value of MBR

Question 17

Cache memory is a very common modification to what?

Answer 17

Von Neumann Machine

Question 18

Why is cache memory used?

Answer 18

Faster memory access because reads/writes are cached and cache memory is very fast

Question 19

Describe the cache read operation

Answer 19

Receive Read Address from CPU

If block containing address present in cache, fetch and return it from the cache

Otherwise, fetch from the external memory and then store in the cache, using a replacement algorithm if not enough space

Question 20

What are the three most common replacement algorithms

Answer 20

Least recently used - most effective; replace the block that was least recently used; most popular option

First-in-first-out - replace block that has been cached the longest

Least frequently used - replace block that has experienced fewest writes

Question 21

What are the two write cycle schemes using the cache?

Answer 21

Write through - all write operations also write to the cache; bit pointless to have a cache in this case

Write back - minimises memory writes, updates only made in cache, then at some point all the blocks from the cache are written to the main memory

Question 22

How do hard disks store data?

Answer 22

Magnetic substrate which can store a magnetic state indefinitely

Question 23

What is the read/write head in a HDD made of?

Answer 23

Crystal.

Question 24

How and why does sectore size change between tracks in a HDD?

Answer 24

Outermost tracks have larger sectors

Question 25

What is Rotational Delay in HDDs?

Answer 25

Delay while the head finds the right sector

Question 26

What is seek time in HDDs?

Answer 26

Time taken to position head at tracks

Mean seek time is 1/3*time from outermost to innermost track

Question 27

What is the access time of a HDD?

Answer 27

Sum of seek time and rotational delay

Time it takes to get into position to read or write

Question 28

How do you calculate the average rotational latency of a HDD?

Answer 28

Time taken for disk to rotate 180 degrees

Question 29

Why is rotational speed an issue with cds?

Answer 29

Data density same everywhere so must spin slower at edge of disc to maintain same data rate