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Memory Hierarchy - Electrical and Computer Engineering

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Memory Hierarchy
Access/Speed
Cost/Bit
Registers
Cache
Main Memory
Fixed Disk
(virtual memory)
Tape
Floppy
Zip
CD-ROM
CD-RWR
Capacity
Connection of memory to the
Processor
K-bit
address bus
Memory
N-bit data bus
Up to 2k addressable
locations
MAR
MDR
Word length = n bits
Control lines
R/W’, IRQ,IE, etc…
Control Connections
Address[log2(K)-1:0]
CS
OE
WE
M
E
M
Data[N-1:0]
Chip Select – must be asserted before Memory will respond to
read or write operation. If negated, data bus is high impedance.
May have more than one – if so, all must be asserted.
OE – Asserted for read operation, Memory will drive data lines.
WE – Asserted for a write operation (Memory inputs data from
data pins, processor writes to memory).
There may only be one control line (R/W)
Main Memory Characteristics
• Semiconductor Chips
пѓ� Housed in DIP Packages
пѓ� DIP Packages Mounted on SIMM, DIMM Circuit Boards
• Characteristics
пѓ� Access Times
(read,write,erase)
пЃ¶ Faster is Better (varies from minutes to a few ns)
пѓ� Volatility
пЃ¶ Ability to Retain Data After Power is Removed
пѓ� Power Consumption
пЃ¶ Less is Better (mW to nW typical)
пѓ� Density
пЃ¶ Larger is Better (bits/sq. micron or transistors/bit)
пѓ� Cost
пЃ¶ Less is Better
Timing Characteristics
• Memory Access Time –
– The time from a valid address being placed
on the address bus until valid data appears on
the data bus.
• Memory Write Time –
– The time from when a valid address is placed
on the address bus until the value on the data
bus is captured by memory.
• Faster is Better!!!!
Address/Data Connections
KxN
Address[log2(K)-1:0]
M
E
M
Data[N-1:0]
K locations, N bits per location
Address bus has log2(K) address lines, data bus has N data
lines.
Address pins labeled An-1 – A 0 where A0 is least significant
Data pins labeled Dn-1 – D0 where D0 is least significant
Memory Chips
• Most devices are 8-bits wide (Byteaddressable); some are 16-bits, others 1
bit wide.
• Listing refer to memory locations x
bits/location
– 1Kx8, 16Kx8
• Often classified by total bit capacity
– 1Kx8 (8K device)
– 64Kx4 (256K device)
Pentium Memory System – 4G capacity
64 bit data bus, 32 bit Address Bus
Can see use of CS, W and OE signals.
Semiconductor Memory Device Architecture
Storage Cell Array
A1
A0
2п‚ґ4
Decoder
Buffers
D0
D1 D 2
D3
D4
DECODER REVIEW
•
nГ—2n Device
–
n encoded inputs
–
2n decoded outputs
A1
D3
D2
D1
D0
2п‚ґ4
Decoder
A0
A1 A0
D3 D2 D1 D0
0
0
1
1
0
0
0
1
0
1
0
1
0
0
1
0
0
1
0
0
1
0
0
0
Buffers
•
Single-Ended Amplifier
–
Gain: Av
–
1 input voltage, 1 output voltage referenced to common ground
Vin
•
Av
Vout= Av Vin
Differential Amplifier
–
Gain: Av
–
2 input voltages, 1 output voltage referenced to common ground
V1
+
V2
-
Av
Vout= Av (V1-V2)
ROM
• ROM – Read Only Memory - a type of memory that cannot
be written, can only be read. Contents determined a
manufacture time.
– ROM is non-volatile – contents remain even when
power is off.
• PROM – Programmable ROM – a type of memory whose
contents can be programmed by the user
– OTP – One Time Programmable, a PROM is OTP if
contents can be programmed only once.
• EEPROM – Electrically Erasable PROM – contents can be
erased electrically by the user.
– Memory is not alterable under �normal’ operation.
4x5 PROM Organization
1-Bit Storage Cell
Basic Types of RAM
• RAM – Random Access Memory
– memory that can be both read and written during normal operation.
– Contents are non-volatile, will be lost on power off.
Static RAM
Fast access time (used for off-processor cache)
Does not have to be refreshed
Dynamic RAM
Slower access time
Must be refreshed
much more dense
Static RAM
• Fastest access time of memory types. Typically the type
of RAM used primarily in Level -2 cache.
• Read, Write operations take equal amounts of time.
• Access to any �random’ location takes same amount of
time.
• Basic memory cell is a latch, takes 6 transistors per
memory bit.
 SRAM –static - high speed memory that does not require a refresh
operation. Much faster than dynamic RAM, with speeds between 8-12
nsec.
• PBSRAM – pipeline burst - static RAM that has been enhanced by the
use of burst technology. Multiple requests can be collected together
and sent as a single pipelined request. Bus speeds of 75MHz or higher.
Static RAM Cell
Dynamic RAM
•Must be refreshed within less than a millisecond
•Most main memory is dynamic RAM (least expensive)
–FPO Fast Page Mode – Can only match speed of 30MHz data
bus
–EDO Extended Data Out – 66MHz motherboards or less
–BEDO – burst enhanced data-out –
–SDRAM – Synchronous dynamic – operates synchronously with
system clock and data bus. Can handle 100MHz or more
–DDR – Double Data Rate – can transmit data on both edges of
the clock
–RD – Rambus – operates in a serial fashion rather than
SIMMS and DIMMS
•
Mount Memory Device Packages on Circuit Boards to
Conserve Space
•
30-Pin SIMM – First – Single Byte Access
•
•
72-Pin SIMM – Four Byte (32 bit) Access
•
•
Used in “Pairs” Since in x86 1 Word=16 bits
Need Pairs for Pentium+ Since 64 bit Data Bus
168-pin DIMM – Eight Byte (64 bit) Access
SIMM – Single In-Line Memory Module
DIMM – Dual In-Line Memory Module
8M x 32 (32MB) SIMM
Flash Memory
• Hybrid of RAM/ROM
• Have basically replaced EEPROMs
• Memory parts can be electrically erased and
reprogrammed without removing the chip.
• The entire chip (or block) must be erased at one time.
Individual byte erasure is not possible.
• Silicon Hard Disks – PCMCIA (Personal Computer
Memory Card International Association) – credit card
size
• ROM BIOS, Font cards for printers, automotive industry
diagnostic codes, modems, Ethernet cards
Flash Memory Examples
•
•
•
•
•
Computer BIOS Memory
Compact Flash (In Digital Cameras)
Smart Media (Digital Cameras)
Memory Stick (Digital Cameras)
PCMCIA Type I and Type II (solid state
disks in laptops)
• Memory Cards for video game consoles
Main Memory Technologies
Mask
Programmable
ROM
OTP
RO
M
UVEPR
OM
EEPRO
M
FLASH
MEMOR
Y
CACH
E
SRAM
SSRA
M
x
Volatile
DRA
M
SDRA
MM
x
x
x
x
x
X
Nonvolatile and byte erasable
Capacitor Storage Cell
x
Fastest Access Times
x
x
x
Highest Density
Fusible Links
x
Core Memory SIMMS and DIMMS
x
x
Quartz-glass window
x
Support burst transfers using a clock
Requires periodic refreshing
Cannot be programmed by user
Multiple programs and erases without be
removed
x
x
x
x
X
X
x
X
X
X
X
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