Registers Top of the hierarchy Match the CPU speed Generally are large and consume large amounts of power Usually a small number of registers in a computer
Secondary/Offline Storage Bottom of the hierarchy Example: Magnetic tapes Cost per stored bit of data is generally small in terms of money and power. Access time is much longer than top-tier hierarchy storage.
Properties of memory Hierarchy Memory Type Access Time Cost/MBTypical Amount Used Typical Cost Registers0.5 nsHigh2 KB- Cache5-20 ns$802 MB$160 Main Memory40-80 ns$0.40512 MB$205 Disk Memory5 ms$0.00540 GB$200 The further you got down the hierarchy: – You get more storage for your money – You tend to use more of the cheap storage – The access time goes way up
What is RAM? It means that regardless of location in the physical storage data can be accessed in the same amount of time. There is some location dependence when compared with block access, but it is vast improvement over old days of drums and acoustic delays.
Static RAM (SRAM) Static RAM chips are based on flip-flops Fast Consume lots of power and are large Retains data as long as power is supplied Typically used for applications that are small but require speed Functional behavior of Static RAM
Dynamic RAM (DRAM) Stores data via a capacitor Data must be refreshed frequently through special circuitry. Smaller and slower than SRAM Stores much more info in smaller spaces than SRAM Typically used for main memory due to low cost, low power consumption, and high Density. Functional behavior of Dynamic RAM
2-1/2D organization Most prevalent organization for RAM integrated circuits. During Read Operation – Entire row is selected first – The row is then fed to Column MUX and it then selects the correct bit to read. During Write Operation – The DEMUX chooses the correct column – The row decoder then selects the proper row to write to.
What is it, and what is it good for? Generally speaking, ROM is used to store programs that are never changed. – Video games, Calculators, Microwaves, etc. – On computer boot BIOS is booted from ROM – Can be used for control units and ALU’s Data stored is non-volatile (data not lost when power turned off) ROM is simple, and all that is needed is a decoder and logic gates (no flip-flops).
ROM cont’d For high volume applications ROM is often factory programmed. There is programmable ROM (PROM) – Allowed delay of program, but still only writable once Early erasable PROM (EPROM) could be rewritten multiple times after being erased with UV light. Electrically erasable PROM (EEPROM) allow the contents to be rewritten electrically.
Example of ROM that stores four four-bit words. Each address input corresponds with a different word.
What is Flash Memory? Flash memory is also known as “flash RAM”, secure digital (SD) disks, or jump drives. Flash memory is a form of EEPROM. It is non-volatile memory A section of memory cells can be erased in a single step, or “flash”, thus we get the name Flash Memory. The key to flash memory is the dielectric material which is great support for the electrostatic field that contains the data.
Flash Memory cont’d Read and write speeds are on the order of 13 MB/s and 19 MB/s respectively – Places Flash close to hard disk drives in terms of function Disadvantage of flash mem. Is that it’s life time is only about 100,000 program/erase cycles – Limitation if we want to replace HDD with flash drives because virtual memory locations get hit with lots of writes/rewrites Apple iPods, Cameras, PDAs, and videogame consoles are some examples of where flash memory is used.