1. Hardware
Main memory
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2. Learning Objectives
Explain the differences between RAM and ROM memory.
Show understanding of the need for primary storage.
Explain the differences between Static RAM (SRAM) and Dynamic RAM (DRAM).

3. Primary / Main Memory / RAM
Processor’s memory.
Data or instructions that are being held in locations other than in main memory.
e.g. surface of a disk drive,
Cannot be used by the processor until it is moved into the processor’s memory.
As it is so important it is called Main / Primary memory.
The primary memory of the computer is the memory that is advertised as being “128 Mbytes of RAM”.
This simply describes the capacity of the memory.

4. RAM (Random Access Memory)
RAM stores the programs that are being used by the computer (including the operating system) and the data that is being used with those programs.
When the computer is switched off all this data is lost.

5. Memory Unit / Primary / Main Memory
There are two other parts of the memory unit/primary/main memory besides the main large-capacity RAM:
Processor registers and processor cache memory which are both located inside the processor.
Processor registers are dealt with in “Processor Fundamentals” later.
So conceptually the term main memory includes "stuff" in the processor and this is why “main memory” can be considered to located in the processor
(or at least part of it is – the cache or processor’s memory & registers).

6. ROM (Read Only Memory) ROM is memory that cannot be altered.
This means that even switching the computer off will not affect the contents of the ROM.
There is very little that needs to be stored in ROM so it tends to be very small.
In the past the whole operating system was stored on ROM, but that means that if you want to use your computer in a different way or you want to install the latest edition of an operating system to replace the one in use, you can’t because the computer can’t delete the old one.
When the computer is turned on it would be there.

7. Bootstrap / Booting
So the operating system is stored on the hard drive of a computer along with all the other programs that may be used.
A problem arises because when the computer is switched on it needs the operating system to be able to do anything useful, so a small program is stored on ROM whose job it is to go and get the operating system from the hard drive so that the computer can work.
This little program is called the bootstrap, and the process of retrieving the operating system is known as booting.

8. Basic Input Output System (BIOS)
Includes the bootstrap program
This part is held in the ROM as it is unchangeable and needs to be there when the computer is switched on.
See previous slide.
The rest of the BIOS is held on secondary storage e.g. the hard drive as it is changeable.
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9. ROM
Note that in the past the whole operating system was stored on ROM, but that means that if you want to use your computer in a different way or you want to install the latest edition of an operating system to replace the one in use, you can’t because the computer can’t delete the old one.
When the computer is turned on it would be there.

10. RAM & ROM - Similarities
Both have random access (even though ROM stands for Read Only Memory it still has random access).
Otherwise access data, programs and files would be too slow.
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11. Difference between main memory and auxiliary (secondary) storage
Transitory.
Secondary storage:
(Semi-) Permanent.
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12. Why use auxiliary (secondary) storage?
To store the operating system, application software and files:
When the power to the processor is switched off (as RAM would lose them).
To allow them to be alterable and upgradeable (as ROM is Read Only Memory).
To store files which are too large to be stored in the processor itself, until they are needed (as RAM and ROM have much smaller storage capacities).
To allow for the transfer of data from one machine to another (as RAM, ROM and the internal hard disk are installed internally).
A full operating system requires a large amount of storage space that is better utilised in the computer memory for applications.
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13. Difference between static RAM (SRAM) and dynamic RAM (DRAM)?
Both:
Divided into memory cells which each hold 1 bit of information.
DRAM:
Each memory has only two parts: a transistor and a capacitor.
Each capacitor holds a bit of information - a 0 or a 1- but “leaks” so needs recharging regularly.
SRAM:
Each memory cell based on the logic circuit known as flip-flop which takes 4 or 6 transistors along with some wiring, but never has to be refreshed.
Therefore significantly faster than DRAM but takes a lot more space on a chip than a dynamic memory cell, so less memory per chip, and more expensive than DRAM.

14. DVD-RAM / Flash Memory DVD-RAM: Flash Memory:
Data is stored using lasers.
Uses phase changing recording, in which varying laser intensities cause targeted areas in the phase change recording layer to alternate between an amorphous and a crystalline state.
Uses a rotating disk with concentric tracks.
Allows read and write operations to occur simultaneously.
Flash Memory:
Most are NAND-based.
There are no moving parts.
Uses a grid of columns and rows that has two transistors at each intersection. It is not possible to over-write existing data so it is necessary to first erase the old data and then write the new data in the same location.
One transistor is called a floating gate and the second transistor is called the control gate.
Memory cells store voltages which can represent either a 0 or a 1 and essentially the movement of electrons is controlled to read/write

15. Exam Question a) State two ways that RAM and ROM memory differ. (2)
b) Explain what types of data would be stored in each of RAM and ROM memory, giving reasons for your answers.
(4)

16. Answer a)
RAM is volatile (will lose its contents when the power is switched off), while ROM is not.
The contents of RAM can be altered, whereas the data stored in ROM cannot be altered.
ROM tends to be much smaller than RAM.

17. Answer b)
RAM would contain user files and software that is in current use.
RAM is used for these files because they have to be alterable, and the user will want to use different software from time to time, implying that the original software will need to be erased so that it can be replaced. Losing these files when the machine is switched off is not a problem because they will have been saved on secondary storage.

18. Answer b) ROM is used to store the bootstrap program.
ROM is used because when the computer is initially switched on there must be a program present that can be used to load up the rest of the operating system from secondary storage. This program must not be altered because without it the computer cannot start to work.

19. Note Notice the way that the answers have been phrased in part (a).
The question asked for a comparison of ROM and RAM so both need to be mentioned when you are giving an answer.
To say that RAM is volatile is true, but does not answer the question until you say that ROM is not.
Part (b) is asking for reasons to be given.
Notice that the reasons given related to the answers given in part (a).