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Memory Vocabulary

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Title: Memory Vocabulary


1
Memory Vocabulary
  • 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 be
    erased electrically by the user.
  • Memory is not alterable under normal operation.

2
Memory Vocabulary
  • 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.
  • SRAM static RAM has the following
    characteristics
  • Read, Write operations take equal amounts of time
  • Access to any random location takes same
    amount of time.
  • Fastest access time of memory types.
  • Basic memory cell is a latch, takes 6 transistors
    per memory bit.

3
Static Random Access Memory (SRAM)
  • The serial EEPROM had low pin count because of
    serial I2C interface
  • Speed was sacrificed
  • Random access read took 5 bytes (3 bytes to set
    address, 2 bytes for read). Total of 45 bit times
    (5 9), _at_400 KHz this took 2.5 ?s 45 112.5
    ?s!
  • Write speed was slow because non-volatile memory
    (page write took about 6.5 ms for 64 bytes, or
    about 101 ? s/byte.
  • An SRAM (Static Random Access Memory) has
  • equal read/write times, measured in nanoseconds
  • volatile (contents not maintained when power
    removed)
  • equal cycle time, access time
  • typically has a parallel interface

4
Typical SRAM Control Lines
K x N
Addresslog2(K)-10
DataN-10
MEM
CS
Data bus is bidirectional
OE
W
Chip Select must be asserted before Memory will
respond to read or write operation. If negated,
data bus is high impedance. OE Asserted for
read operation, Memory will drive data lines. W
Asserted for a write operation (Memory inputs
data from data pins, processor writes to memory).
5
Internal Architecture
Bit Line Bias
Row Address
Cell
MemoryOrganization
Row Decode
Word lines(1 per row)
Sense Amps
Column Address
Bit lines(2 per column)
Column Decode
Data lines
6
Static Ram Cell
Wordline
B (bitline)
BB (bitline_bar)
7
Cypress 8K x 8 SRAM
Multiple Chip Enables more flexibility Data bus
is bi-directional (8 I/O lines). Note a more
precise name for this memory is Asynchronous
SRAM. It has no clock input, any change on
address inputs accesses a new location.
8
Read Cycle
Cycle Time how fast can I start another
operation? Access Time how fast is data
ready?For SRAMs, Cycle Time Access Time,
this is a feature of SRAMs.
9
Write Cycle
10
Write Timings
Notice that Write Cycle Read Cycle. All
volatile RAM types have this feature.
11
PIC To SRAM
means low true
8Kx8
PIC
A70
RA70
A128
RB40
IO70
RC70
Vdd
CE2
CE1
OE
RD0
WE
RD1
To read Address on RA, RB. RC port is all
inputs RD0 0, RD1 1. To write Address
on RA, RB. RC port is all outputs RD0 1, RD1
0.
12
Expanding Memory
8Kx8
RAM1
A70
RA70
A128
RB40
IO70
RC70
CE2
RB5
CE1
OE
RD0
PIC
WE
RD1
8Kx8
RAM0
A70
RAM1 accessed when RB5 1
A128
IO70
CE1
CE2
RAM0 accessed when RB5 0
OE
WE
13
More Memory
RAM0
RAM1
13
A120
RA70
RB40,
A120
8
IO70
RC70
IO70
OE
RD0
OE
WE
RD1
WE
CE1
PIC
CE1
RB65
RAM2
2-to-4 decoder
RAM3
RB6500
A120
A120
2
0
RB6501
IO70
IO70
1
RB6510
OE
OE
2
WE
WE
RB6511
3
CE1
CE1
chip select decoding
14
Memory Map
Total memory 4 8K 32 K ( 25 210), 15
address lines
Selects chip
Selects location within chip
RB40,RA70
RB65
1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 4 3 2 1 0 9 8
7 6 5 4 3 2 1 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 1 1 1 1 1 1
1 1 1 1 1 1 1
0x0000 low RAM00x1FFF high
0 1 0 0 0 0 0 0 0 0 0 0 0 0 00 1 1 1 1 1 1 1
1 1 1 1 1 1 1
0x2000 low RAM10x3FFF high
1 0 0 0 0 0 0 0 0 0 0 0 0 0 01 0 1 1 1 1 1 1
1 1 1 1 1 1 1
0x4000 low RAM20x5FFF high
1 1 0 0 0 0 0 0 0 0 0 0 0 0 01 1 1 1 1 1 1 1
1 1 1 1 1 1 1
0x6000 low RAM30x7FFF high
15
Other SRAM Types
  • SSRAM - Synchronous SRAM
  • Has a clock input
  • Address, data lines latched on clock edge
  • Can perform burst cycles
  • What is a burst cycle?
  • After first data value is output based upon
    address, data values in successive locations are
    output without needing to change address bus
  • Internal counter used for address value

16
Synchronous SRAM (SSRAM)
Note clock lines to SRAM. A DDR (Double Data
Rate) SSRAM means that data is clocked on both
clock edges.
17
SSRAM Normal Cycle vs Burst Cycle
Address ready, 1st data ready in 2 clocks,
successive locations ready in successive clocks.
This burst cycle is a 2-1-1-1 cycle. Diagram
shows burst suspended for 1 clock between 3rd
4th clock. Normal 2-1-1-1 burst takes 5 clocks.
Address ready,Data ready in clocks.
18
Why Burst Cycles?
  • Burst Cycles more efficient at block memory
    transfers (memory location is successive
    locations)
  • Normal cycle would take 8 clocks to transfer data
    from 4 locations.
  • 2-1-1-1 burst takes only 5 clocks.
  • Why do we want to transfer blocks of memory
    efficiently?
  • For cache fill operations.
  • So what is a cache?

19
A Cache System
1st level cache
CPU
Single chip(die)
Memory (SRAM)
2nd level cache
Processor ModuleCasing
Memory (SSRAM)
3rd level cache
Memory (SSRAM)
Motherboard
Main Memory (DRAM)
20
What is a Cache?
  • The closer a memory is to a CPU, the faster the
    data transfer between CPU and Memory.
  • Cant get any closer than on the same chip as the
    CPU!
  • For high performance microprocessors, need LOTS
    of memory
  • Dont have enough room to put all memory on same
    chip as CPU
  • Put some memory on same die as CPU, will not be
    able to hold all needed data/programs but will
    hold most frequently used data/programs
  • Will need to swap out some data if we dont find
    what we need in the cache (a miss!).

21
Memory Hierarchy
Access/Speed
Cost/Bit
The further away, the slower the access time, the
higher the capacity (density).
Registers
The further away, the cheaper per bit.
Cache
Main Memory
Fixed Disk
Tape
Floppy Zip
CD-ROM CD-RWR
Capacity
22
Intel 3rd Generation IA-64 (executes new 64-bit
instructions) 0.13?
L1 instr. cache
IA-32 CPU (pentium, for compatibility)
L2 cache
cache and memory account for about 70 of the
die.
L3 cache
23
Main Memory and DRAM
  • For main memory, we need a memory whose primary
    characteristics are
  • Dense (lots o bits!!!)
  • Cheap (cause we dont want to spend too much for
    our lots o bits!)
  • Good at block transfers, can be rotten at random
    accesses.
  • How do we achieve density? (lots o bits?)
  • Less transistors per bit!
  • Dynamic RAM (DRAM) - one transistor capacitor
    per bit!!!!!

24
DRAM Memory Cell
Word line
Turn this transistor on to access data
Cs
Memory value stored on capacitor ( a very small
capacitor...)
Bit line
Data value appears on bit line
25
DRAM Characteristics
  • Very dense (high capacity). Cheap per bit.
  • Slow for Random Access (access to any location)
  • Cycle time gtgt access time, Read Cycle time
    Write cycle time.
  • Has special access modes to speed block transfers
  • Important since transfers to DRAM in modern
    computer system is always block-oriented for
    cache fills.
  • Only has half the address pins that you would
    expect
  • 1M x 8 DRAM has only 10 address pins instead of
    20
  • Reduces package size, can pack more DRAM chips
    per unit area. Address values multiplexed
    between row/column addresses

26
A 1M x 16 DRAM (Micron Tech.)
A0-A9 - 10 address pins DQ1-DQ16 16 data
pins RAS - row address strobe asserted when
address pins contain row address. CASL, CASH -
column address strobe asserted when address
pins contain row address. Both need to be
asserted for 16 bit transfers, only one for
either high or low byte transfers.
27
Address Muxing
Assume a 16-bit data wide bus (D0-D15), and 20
address lines A20-A1 (no A0 pin since 16 bit-wide
data bus). How are addresses split between Row,
Column?
A20-A11
A10-A1
Varies the fastest.
Row Address Column Address
A9-A0
CPU
A20-A1
DRAMcontroller
DRAM
RAS
Chip select
CASL
Decode
CASH
28
DRAM Controllers
  • External logic called a DRAM controller needed to
    interface to DRAMs.
  • DRAM interfacing more complex than SRAM
  • Provides muxing of Address lines
  • Assertion of RAS, CAS lines
  • Also keeps DRAM contents refreshed
  • Capacitors tend to leak. Memory contents needs to
    continually accessed in order to keep contents
    valid.
  • Special refresh cycles are supported by DRAMs
    to support refreshing all of bits in a row with
    one cycle
  • DRAM controller responsible for running refresh
    cycles.

29
RAS asserted first
CAS asserted next
30
Block transfer. Access different bits on same
row, change column address.
31
Timing Values
  • Random access Trc 104ns, Trac 50 ns
    (access time from row address strobe assertion)
  • Block Mode
  • Access time to data on row 50 ns
  • To column locations on same row 25 ns
  • Block transfer much more efficient than random
    access.

32
DRAM Chip Generations
  • Each time a new DRAM Chip generation is released,
    capacity goes up by 4x
  • 16K x 1 (late 70s), 64K x 1, 256K x 1, 1M x 1,
    4M x 1, 16M x1, 64M x 1, 256M x 1 (current), 1G
    x 1 (samples available), 4G x 1 (in labs)
  • Why? because of muxed addresses, 1 address pin is
    actually two address bits. Two address bits give
    4X more locations.

33
DRAM Modules SIMMs and DIMMs
  • Mount Memory Device Packages on Circuit Boards
    to Conserve Space
  • 30-Pin SIMM First Single Byte Access
  • Used in Pairs Since in x86 1 Word16 bits
  • 72-Pin SIMM Four Byte Access
  • Need Pairs for Pentium Since 64 bit Data Bus
  • 168-pin DIMM Eight Byte Access
  • SIMM Single In-Line Memory Module
  • DIMM Dual In-Line Memory Module

34
DRAM Modules
SIMM Single In-line Memory Module -- 72 pins
Capacity varies Picture shows 8M x 32 (32 MByte)
SIMMs/DIMMs invented to get denser packaging for
DRAMs on motherboards.
35
Schematic View of 72-Pin SIMM
36
Memory Device Specification
  • Dimension of Storage Cell Array
  • 8 Mb Refers to Eight Mega-bits (not
    Mega-Bytes!)
  • 8 Mb (lower case b !!)
  • (8)(1024)(1024)bits
  • (1)(1024)(1024)Bytes
  • 1MB (upper case B!!)
  • Single 16Mb Device can be Arranged as
  • 16M?1 bits
  • 4M ?4 nybbles
  • 2M ?8 bytes
  • 1M ?16 (words in the case of x86)
  • Many Devices are Typically byte-wide Devices
  • N?8

37
DRAM Variations
  • SDRAM Synchronous DRAM has clock, supports
    burst transfers
  • DDR-SDRAM - Double Data Rate SDRAM
  • Data transferred on EACH clock edge
  • Double the data rate of SDRAM
  • RDRAM Rambus DRAM
  • Will discuss this in more detail later
  • High speed signaling interface to support very
    fast data transfers
  • Very high latency (long access time to first
    location), but very fast at transferring
    successive locations claim is better at block
    mode transfers than SDRAM/DDR-SDRAM.

38
Dual Port Memories
Address
Address
CPU
Dual Port Memory(SRAM, SSRAM)
CPU
Data
Data
Control
Control
Support simulatenous access via two ports
(left/right sides). Support simultaneous read
accesses to different locations or same
locations.Suports simultaneous write accesses to
different locations Simultaneous write access to
same location is not supported.One obvious
application is multiple-processor systems, but
useful in many other situations.
39
Flash RAM
  • SRAM-like interface, density of SRAM
  • Non-volatile (retains contents when power is off)
  • Read Cycle time same as SRAM (10s of
    nanoseconds)
  • Write Cycle gtgt Read Cycle
  • write times in microseconds
  • Can write individual locations or blocks of
    locations
  • Applications include smart cards (credit cards,
    medical history cards, etc)
  • Intel is market leader, has a Flash RAM cell that
    stores 2-bits per cell (can sense 4 different
    volage levels from cell).

40
What do you have to know?
  • Differences between SRAM, DRAM in terms of
    density, speed, functionality
  • Pin functions of a SRAM
  • Memory decoding for SRAM via PIC example
  • Definitions of access time, cycle time for SRAM
  • Difference between Synchronous SRAM, Asynchronous
    SRAM
  • Dual Port, Flash Memory definitions
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