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Main Memory
Main memory (primary memory) is held on chips
Backing storage is not main memory
Backing storage is sometimes called secondary memory
Main MemoryMain
Memory
11110011 11110011
Address
00000000000000010000001000000011000001000000010100000110000001110000100000001001
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)
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
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.
Temporary storage outside RAMWhy?
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
Temporary storage outside RAMMain
MemoryAddress
0000000000000001000000100000001100000100000001010000011000000111000010000000100111100011
0011001100001110111100001111001111001010
11001100000000110001000101000000
Cache Memory
10000000100000011000001010000011
Address
Cache• small amount of memory built into the processor• physically closer to the processor than RAM• stores the next instructions to be read
Temporary storage outside RAMMain
MemoryAddress
0000000000000001000000100000001100000100000001010000011000000111000010000000100111100011
0011001100001110111100001111001111001010
11001100000000110001000101000000
Cache Memory
10000000100000011000001010000011
Address
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)
Temporary storage outside RAMRegisters
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
Temporary storage outside RAMVirtual 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
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
CreditsHigher 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)