Main memory consists of storage locations that can be uniquely addressed and hold data in the form of words. The two main types of main memory are RAM and ROM. RAM is volatile and used to store running programs and data, while ROM is non-volatile and used for firmware like the bootstrap loader. Cache memory and processor registers provide even faster temporary storage than main memory to improve system performance. Virtual memory uses the hard disk as an extension of main memory when RAM is insufficient. The closer the storage is to the processor, the faster the access speed.

2. Main Memory
Main memory (primary memory) is held on chips
Backing storage is not main memory
Backing storage is sometimes called
secondary memory

3. Main Memory
Main Memory
11110011 11110011
Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
00001001
Main memory consists of storage locations which are
identified by unique addresses
Each storage location holds one word
(the number of bits the CPU can process in a single
operation)
8 bit computer holds 8 bits in a storage location
16 bit computer holds 16 bits in a storage location
There are two types of main memory:
Random Access Memory (RAM)
Read Only Memory (ROM)

4. Random Access Memory
• holds data/application programs from input devices/backing storage
• can hold the operating system
• all data is lost when the computer is switched off
Static RAM (SRAM)
Contents of memory will be retained as long as power is applied to the chips
Dynamic RAM (DRAM)
A continuous signal is needed to refresh (re-write) its contents
Static RAM has faster access speeds than DRAM
RAM is mainly made up of DRAM because it needs less power and its circuitry is simpler

5. Read Only Memory
• has contents which are fixed when the chip is manufactured
• holds the bootstrap loader part of the operating system
• retains its data when the computer is switched off
Programmable ROM (PROM)-
Empty of data when the chip is manufactured, can
be programmed by the user. Once programmed the
data cannot be erased.
Erasable PROM (EPROM)-
Like PROM only the chip can be removed from the
computer and the program erased and another
stored in its place using ultraviolet light.
Electrically EPROM (EEPROM) -
Like EPROM but electricity is used to erase and
reprogram selected contents.

6. Temporary storage outside RAM
Why?
Reading / writing data to and from memory can slow down the system
performance
To combat this problem, the processor can store data in cache memory
or its internal registers instead of the RAM
Solution

7. Temporary storage outside RAM
Main Memory Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
0000100111100011
00110011
00001110
11110000
11110011
11001010
11001100
00000011
00010001
01000000
Cache Memory
10000000
10000001
10000010
10000011
Address
Cache
• small amount of memory built into the processor
• physically closer to the processor than RAM
• stores the next instructions to be read
Processor

8. Temporary storage outside RAM
Main Memory Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
0000100111100011
00110011
00001110
11110000
11110011
11001010
11001100
00000011
00010001
01000000
Cache Memory
10000000
10000001
10000010
10000011
Address
Processor
Cache
• write through cache - contents of the RAM are updated at the same time as the contents of cache
• write back cache - contents of the RAM are updated only when the contents of cache are cleared
• cache uses static memory (SRAM)

9. Temporary storage outside RAM
Registers
Storage locations inside the processor which hold
• Data being processed (Memory Data Register)
• Addresses of memory locations to be accessed (Memory Address Register)
• Instructions being executed (Instruction Register)
There are other many other registers
Register contents can be accessed faster than RAM or cache memory

10. Temporary storage outside RAM
Virtual Memory
Part of the backing storage (hard disk) is set aside for
virtual memory
This disk area is used as a temporary location for
programs and data
It is useful when the computer does not have enough
RAM to store the entire program and its data
It is slower to access data held in virtual memory than
data held in main memory

11. Memory Comparison
The speed at which data can be accessed depends on the distance
between it and the processor
Registers
Cache memory
RAM
Virtual memory
Faster access
Slower access

12. Credits
Higher Computing – Computer Structure – Computer Memory
Produced by P. Greene for the City of Edinburgh Council 2004
Adapted by M. Cunningham 2010
All images licenced under Creative Commons 3.0
• 1GB DDR3 Memory Module by William Warby (wwarby on Flickr)
• Hard disc drive assembled by Crispin Semmens (conskeptical on Flickr)
• Mac ED Logic Board by Jesus A Alvarez (maczydeco on Flickr)