RAM/ROM

Baa-RAM-ewe!

Learning Goals

By the end of the lesson I will be able to:

  • Describe the function of RAM in a computer system
  • Identify the benefits and drawbacks of RAM
  • Describe the function of ROM chips

What IS RAM?

RAM stands for Random Access Memory, which is a very fast but temporary form of data storage.

To continue our analogy of the human body, RAM would be "short-term" or "working" memory.

In the human body, short-term and working memory are used to store information you are currently using, such as this lesson.

How is it used?

The computer uses RAM to store programs, files and data that are currently being used.

The reason for this is time. Storage drives, such as hard disk drives, are extremely slow compared to RAM.

There are some tradeoffs for being so fast, but we will discuss that later.

Random Access?

The main reason we use the term random access to describe this type of memory is because, in theory it takes the same amount of time to access ANY memory address in RAM.

RAM with addresses

00

01

02

03

04

05

06

07

08

09

0A

0B

0C

0D

0E

0F

10

11

12

13

14

15

16

17

18

19

1A

1B

1C

1D

1E

1F

20

21

22

23

24

25

26

27

28

29

2A

2B

2C

2D

2E

2F

30

31

32

33

34

35

36

37

38

39

3A

3B

3C

3D

3E

3F

Random Access?

That random access means we may want to access the data in address 0x0A, then immediately look for the data in 0x32.

In RAM this would be very fast.

RAM with addresses

00

01

02

03

04

05

06

07

08

09

0A

0B

0C

0D

0E

0F

10

11

12

13

14

15

16

17

18

19

1A

1B

1C

1D

1E

1F

20

21

22

23

24

25

26

27

28

29

2A

2B

2C

2D

2E

2F

30

31

32

33

34

35

36

37

38

39

3A

3B

3C

3D

3E

3F

Short Term

RAM is also known as short term memory. This is because when the power is shut off, the RAM forgets everything it had stored.

This forgetfulness is known as volatility. So RAM is a type of volatile memory.

This is one of the tradeoffs for the memory being as fast as it is.

Scale and Cost

The other tradeoff of RAM is its cost. To make memory that is this fast is not cheap! As you can see HERE the price of RAM in the 1950’s and 60’s is MUCH higher than it is now.

Year Price per MB Year Price per MB

1957 $411,041,792 2000 $1.56

1980 $6,480 2010 $0.019

1990 $106 2017 $0.0049

Scale and Cost (Continued)

The cost was very prohibitive, and in some ways, still is. Hard Drive space is much cheaper than the $0.0049/MB of RAM.

With that said, the size of memory sticks, now known as DIMM’s (Dual Inline Memory Module), has grown incredibly over the last 20 years.

What about ROM?

ROM stands for Read Only Memory. Unfortunately, this is a somewhat unfortunate name because on today’s computers this type of memory is rarely TRULY read only.

ROM chips are used, usually, to store important, rarely changing things such as the BIOS.

What else about ROM?

Remember how RAM was considered volatile because when it loses power it forgets everything? ROM chips don’t do that.

A ROM chip is considered nonvolatile because it remembers everything even when there is no power.

4 - RAM/ROM - Google Slides