Transcript Memory Hierarchy

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…
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)
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)
Pentium Memory System – 4G capacity
64 bit data bus, 32 bit Address Bus
Can see use of CS, W and OE signals.
Main Memory Characteristics
• Semiconductor Chips
 Housed in DIP Packages
 DIP Packages Mounted on SIMM, DIMM Circuit Boards
• Characteristics
 Access Times
(read,write,erase)
 Faster is Better (varies from minutes to a few ns)
 Volatility
 Ability to Retain Data After Power is Removed
 Power Consumption
 Less is Better (mW to nW typical)
 Density
 Larger is Better (bits/sq. micron or transistors/bit)
 Cost
 Less is Better
Three 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.
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 parallel
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
Video RAM
• VRAM – dual ported, allowing one port to continuously
refresh the display while the other port writes data to the
display. Results in lower eye strain than possible with
DRAM.
• WRAM windows RAM – optimized for graphics
displays using Windows. Can refresh screen more
quickly than VRAM
• SGRAM – synchronous graphics RAM – single
ported RAM used on accelerator cards. Two memory
pages can be open at the same time. Able to clear
memory very quickly. 3D graphics.
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 Eraseable PROM – contents be
erased electrically by the user.
– Memory is not alterable under ‘normal’ operation.
4x5 PROM Organization
1-Bit Storage Cell
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
Semiconductor Memory Device Architecture
Storage Cell Array
A1
A0
24
Decoder
Buffers
D0
D1 D2
D3
D4
DECODER REVIEW
•
n×2n Device
–
n encoded inputs
–
2n decoded outputs
A1
D3
D2
D1
D0
24
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)