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......

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

n+ p p p p p WL GND Implementation of Memory Cell

Random Access Memory Column Decoder Memory Array … In Out Row Decoder … 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 n+ 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 TypeRead speed Write speed VolatilityDensityPowerRewrite 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 (10 9 ) bytes. Today, need for data storage is 10’s of tera (10 12 ) 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 SS 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 Outer Track Inner Track Sector 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