1. Lesson Objectives Aims You should be able to:
Describe the difference between RAM and ROM
Explain the need for ROM in a computer system
Describe the purpose of RAM in a computer system
Explain how the amount of RAM affects system performance
Virtual Memory

2. RAM We should know by now:
The computer consists of a few main components (CPU, Memory, Storage, I/O)
All computers need software to do something
This is just a list of instructions
The CPU carries these instructions out (which is what makes it all “work”)

3. Definition
Quite simply, RAM is:
A temporary area of storage for programs and data that are currently being used

4. This includes the Operating System!
Properties of RAM
When you run a program:
It is copied from storage in to RAM
The program is used
Data is stored in RAM whilst in use
Once the program is closed, any changes/stored data is copied back to storage
The memory is erased (marked as free)
This includes the Operating System!

5. Why use RAM?
Storage, whilst big, is very slow. Look at the relative speeds of computer components:
Operation
Access Time
Relative time in seconds
1 CPU cycle
0.3 ns
1 s
Level 1 cache access
0.9 ns
3 s
Level 2 cache access
2.8 ns
9 s
Level 3 cache access
12.9 ns
43 s
Main memory access
120 ns
6 min
Solid-state disk I/O μs
2-6 days
Rotational disk I/O
1-10 ms
1-12 months
Internet: SF to NYC
40 ms
4 years
Internet: SF to UK
81 ms
8 years
Internet: SF to Australia
183 ms
19 years
OS virtualization reboot
4 s
423 years
SCSI command time-out
30 s
3000 years
Hardware virtualization reboot
40 s
4000 years
Physical system reboot
5 m
32 millenia

6. RAM
We could operate a computer with no RAM but it would be sloooooooow.
RAM is a happy medium between storage and CPU – running at about half CPU speed or more.
RAM provides fast access to programs and data and is stable enough to be reliable in use

7. How it affects performance - Size
The more RAM a system has, the more it can do quickly at any one time
More RAM = More space for more OPEN programs and data
This means the system SHOULD be faster – why?
It does not need to rely on virtual memory (which is slow)
It has more space for OPEN programs and data
It would NOT give an increase in performance if you weren’t using all your RAM in the first place!

8. Types of Memory Cache Special type of memory found INSIDE the CPU
Very small quantities (1-8mb typically)
Runs at (or almost) CPU Speed
Enables the CPU to run at full speed rather than idling when waiting for instructions from RAM
Stores frequently/recently/about to be executed data/instructions
Provides a massive performance increase as data does not have to be fetched from RAM

9. Flash – called memory but technically a storage technology
Uses non-volatile memory chips
Very fast permanent storage
Limited read/write life
Not as fast as RAM so doesn’t replace it in computer systems.

10. Memory and storage performance is a trade off between:
Speed
Size
Volatility
This is the reason why we don’t just get rid of memory and have fast storage instead…!

11. Performance Compared Cache RAM/ROM Flash Memory/SSD HDD
Magnetic Backup Tape
Faster, Smaller, More Volatile
Slower, Bigger, Less Volatile

12. Virtual Memory
Ask yourself a question:
Why do we need storage if we have RAM?
Or vice-versa – why do we need RAM if we have storage?

13. Virtual Memory
If a computer runs out of RAM, it shouldn’t just crash/stop
Instead we can use a file on the hard drive/secondary storage which pretends to be RAM
This is called virtual memory
Allows us to remove idle programs out of RAM to free up space
Programs can be swapped in and out of physical memory as necessary
Gives a solution to lack of RAM
Will see a significant slow down in system performance when used

15. ROM The final type of memory is ROM Read Only Memory
This cannot be changed, only read
Holds code which is executed at power on (called firmware or BIOS or UEFI)
Used to boot the computer – without it, the computer would do nothing at power on
Usually the code will look to start an operating system of some sort
Also used in games cartridges