Lecture on Electronic Memories

What Is Electronic Memory?

• Electronic device that stores digital information• Types

– Volatile v. non-volatile– Static v. dynamic– Embedded v. discrete– Read only v. Read & Write

• Magnetic memories are not made of electronic devices. Information is stored by permanent magnetization of magnetic material.

Memory Terminology• Volatile Memory: Memory maintaining content only with power on

• Non-volatile Memory: Memory maintaining content without power

• Random Access Memory (RAM): Volatile read-&-write memory

• Read Only Memory (ROM): Memory with fixed content

• Static RAM (SRAM): Fast volatile memory made of an array of flip flops

• Dynamic RAM (DRAM): Dense volatile memory made of a capacitor controlled by a transistor.

• Content Addressable Memory (CRAM): Volatile memory addressable by content, rather than address

• Cache Memory: SRAM attached to processor for fast access

Example of Single SRAM “Cell”

SRAM

• Made of fast charging and discharging transistors• Each cell is a pair of cross-connected inverters (flip

flop)• Cache memories are SRAMs because of speed need• No refresh needed as long as power is on• Need multiple transistors for one cell – maximum

several megabytes in a single chip (not too dense) • Can be incorporated into a larger chip (e.g., CPU) or

made into a separate chip.

SRAM

Example of Single DRAM “Cell”

Word Line

Bit Line

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Sense Amp

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Memory Layer

Where Is Memory in Chip?

DRAM

• Main external memory of PC

• Each cell is made of a single transistor.

• More dense, and a very large number of cells can be put into a chip. – Multiple giga bytes in a chip

• Typically made into separate memory chips.

• Needs to be refreshed periodically (8 ms, 1% time) even if the power is on. – Capacitive leakage

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WLGND

Implementation of Memory Cell

Random Access Memory

Column Decoder

Memory Array

…In

Out

Row

Dec

oder

…Address lines

Address strobe

Read/Write

Can be SRAM, DRAM, or any memory enabled with read & write.

Nonvolatile Memory

• In volatile SRAM and DRAM, when power is removed, the content is lost.

• Non-volatile memories physically alter cells depending on content (floating gate)

• Example: Programmable Read Only Memory (PROM) (old), flash memory (new)

• Non-volatile memories use floating gates that can be charged or discharged by electrons moving through the oxide layer.

Programmable Read Only Memory

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Floating Gates (Control Gate)(Storage Gate)

To store one bit, run a current from source to drain, and assert a large voltage on control gate, creating a strong electric field to push electrons into storage gate (hot-electron injection).

Flash Memory

• Can be read, written, and re-written.

• Can be read in nano seconds, and written in milli seconds.

• Block of memories can be erased in a single action or "flash." – Basis for the name “flash”

Memory Comparison

Type Read speed

Write speed

Volatility Density Power Rewrite

SRAM +++ +++ - - ++DRAM + + - - ++ - ++PROM + - + + -Flash Memory + + + + +

Mass Storage

• For storage of large amounts of information (i.e., 10’s giga bytes), magnetic film storage is needed.

• Information is stored in the magnetic domains on a ferromagnetic film, and can be written or read by movable heads.

• Today, practical limit of memory is 10’s of giga (109) bytes.

• Today, need for data storage is 10’s of tera (1012) bytes

Magnetic Domains

Each magnetic domain stores one bit of information.

Magnetism

• Ferromagnetic materials have a quantum interaction which makes adjacent atoms line up their magnetic fields in the same direction.

N N N N N N N N N N N N N S S S S S S S S S S S S S

Magnetic Interaction

• Two magnets line up in opposing directions, reducing total magnetic field.

• Magnetic domains can be changed by applying magnetic fields.

• Permanent magnet: Magnetic domains aligned in one direction

N S S N

Magnetic Disk

• Platters: Information stored magnetically on both surfaces

• Actuator moves the head over track, select surface, wait for sector rotates under head, then read or write.

• Bits are recorded in tracks, which in turn are divided into sectors (e.g., 512 Bytes).

Platter

OuterTrack

InnerTrackSector

Actuator

HeadArm

Writing to Magnetic Media (Disk)

• Magnetic storage material (platter) is made of a thin film of ferromagnetic material.

• A small electromagnet (head) is used to create domains oriented in a particular direction.

Reading from Magnetic Media

• Conventional read heads for magnetic media work like the secondary coil of a transformer.

• Produces a voltage across the read head (i.e., secondary coil)

Performance of Magnetic Disk

• Rotational Latency:– Typically rotate at 3,600 to 15,000 RPM

– Average latency: 8 ms at 3600 RPM, 2 ms at 15,000 RPM

• Transfer Time is a function of :– Transfer size (usually a sector): 1 kilo bytes per sector

– Rotation speed: 3,600 RPM to 15,000 RPM

– Recording density: bits per inch on a track

– Diameter typically between 2.5 and5.25 in

– Typical transfer time: 2 to 80 mega bytes per second

Magnetic Disk v. Memory

• DRAM ~10X slower than SRAM– Successive bytes 4x faster than first byte for

DRAM

• Disk ~100,000X slower than DRAM– First byte is ~100,000X slower than successive

bytes on disk