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
C
.
.
.
Bit Line
Sense
Amp
Where Is Memory in Chip?
Memory Layer
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
Implementation of Memory Cell
WL
GND
n+
n+
n+
n+
n+
p
p
p
p
p
n+
n+
n+
n+
n+
Random Access Memory
Column Decoder
In
…
Read/Write
Address strobe
…
Address lines
Row Decoder
Out
Memory Array
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
Floating Gates
(Control Gate)
(Storage Gate)
n+
n+
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
SRAM
Read
speed
Write
speed
+++ +++
Volatility Density
-
-
++
DRAM
+
+
--
PROM
+
-
+
Flash
Memory
+
+
+
Power
Rewrite
++
-
++
+
-
+
+
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
Arm Head
Actuator
Inner Outer
Sector
Track Track
Platter
• Platters: Information stored magnetically on both surfaces
• Bits are recorded in tracks, which in turn are divided into
sectors (e.g., 512 Bytes).
• Actuator moves the head over track, select surface, wait for
sector rotates under head, then read or write.
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