Post on 26-Dec-2015
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
C
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