Physical Memory

ByGregory Marshall

MEMORY HIERARCHY

Fast and expensive

Slow and Inexpensive

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

AccessTime

Cost/MB Typical Amount Used

Typical Cost

Registers 0.5 ns High 2 KB -

Cache 5-20 ns $80 2 MB $160

Main Memory 40-80 ns $0.40 512 MB $205

Disk Memory 5 ms $0.005 40 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

RANDOM ACCESS MEMORY(RAM)

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

MEMORY CHIP ORGANIZATION

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.

READ-ONLY MEMORY(ROM)

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.

FLASH MEMORY

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.