William Stallings

Computer Organization

and Architecture

8th Edition

Chapter 5

Internal Memory

Semiconductor Memory Types

Memory Type Category Erasure Write Mechanism Volatility

Random-access

memory (RAM)Read-write memory Electrically, byte-level Electrically Volatile

Read-only

memory (ROM)

Read-only memory Not possible

Masks

Nonvolatile

Programmable

ROM (PROM)

Electrically

Erasable PROM

(EPROM)

Read-mostly memory

UV light, chip-level

Electrically Erasable

PROM (EEPROM)Electrically, byte-level

Flash memory Electrically, block-level

Semiconductor Memory

• RAM

—Misnamed as all semiconductor memory is random access

—Read/Write

—Volatile

—Temporary storage

—Static or dynamic

Memory Cell Operation

Dynamic RAM

• Bits stored as charge in capacitors

• Charges leak

• Need refreshing even when powered

• Simpler construction

• Smaller per bit

• Less expensive

• Need refresh circuits

• Slower

• Main memory

• Essentially analogue

—Level of charge determines value

DRAM Cell Structure

DRAM Operation

• Address line active when bit read or written

—Transistor switch closed (current flows)

• Write

—Voltage to bit line

– High for 1 low for 0

—Then signal address line

– Transfers charge to capacitor

• Read

—Address line selected

– transistor turns on

—Charge from capacitor fed via bit line to sense amplifier

– Compares with reference value to determine 0 or 1

—Capacitor charge must be restored

Static RAM

• Bits stored as on/off switches

• No charges to leak

• No refreshing needed when powered

• More complex construction

• Larger per bit

• More expensive

• Does not need refresh circuits

• Faster

• Cache

• Digital

—Uses flip-flops

SRAM Cell Structure

Static RAM Operation

• Transistor arrangement gives stable logic state

• State 1—C1 high, C2 low

—T1 T4 off, T2 T3 on

• State 0—C2 high, C1 low

—T2 T3 off, T1 T4 on

• Address line transistors T5 T6 is switch

• Write – apply value to B & compliment to B

• Read – value is on line B

SRAM v DRAM

• Both volatile

—Power needed to preserve data

• Dynamic cell

—Simpler to build, smaller

—More dense

—Less expensive

—Needs refresh

—Larger memory units

• Static

—Faster

—Cache

Read Only Memory (ROM)

• Permanent storage

—Nonvolatile

• Used to store:

—Microprogramming (see later)

—Library subroutines

—Systems programs (e.g. BIOS)

—Function tables

Types of ROM

• Written during manufacture, a.k.a. ROM

—Very expensive for small runs

• Programmable (once)

—PROM

—Needs special equipment to program

• Read ―mostly‖

—Erasable Programmable (EPROM)

– Erased by UV

– One transistor/bit high density

—Electrically Erasable (EEPROM)

– Takes much longer to write (100 s) than read (100 ns)

– Can read and write individual Bytes

– 3 transistors/bit + ―charge pump‖ low density

Types of ROM

—Flash memory: in between EPROM and EEPROM

– Erase whole memory or one block electrically

– Cannot erase individual Bytes

– One transistor/bit high density

Source and explanations: http://www.techquark.com/2009/12/inside-flash-memory-and-drives.html

Although it’s

FGMOS, not the

usual MOS

Memory Organisation vs. Architecture

Example architecture:

• The programmer wants a total of 16 Mbit (= 2 MB) of memory, with a word size of 16 bit

This could be organized in two extreme ways:

• The word-per-chip system: One chip, containing 1 Million (220) 16-bit words

• The bit-per-chip system: 16 chips, each containing 1 Million 1-bit words

—Accessing a word means accessing (in parallel!) its 16 bits, one from each chip

Memory Organisation vs. Architecture

―Compromise‖ organisation:

16Mbit chip organised as a 2048 x 2048 x 4bit array

Important ―trick‖: reduce number of address pins by multiplexing

—Multiplex row address and column address

—11 pins to address (211=2048)

—What happens if a twelfth address pin is added? → memory chip capacities generally grow

by factors of 4

Simplified DRAM Read Timing (p.181 of our text)

DRAM Read timing diagram

Typical 16 Mb DRAM (4M x 4)

Refreshing

Modern chips are ―self-refresh‖, i.e. all refresh circuits are included on chip.

• Older chips depend (partially) on memory controller

There are several refresh methods. Simplest is RAS-only:

• Disable chip

• Count through rows and activate

• Read & Write back (entire row at once!)

• In any case, refresh takes time → Slows down the external accesses

RAS

Packaging

Explain how to connect

CE for 2 and 4 chips

How is the function of WE

and OE performed on the

EPROM chip?

Putting the

memory chips

together in a

module

Example:

256 kByte Module

made of 256 k x 1

chips

One level higher: 1MByte super-module

Each column is a

module from prev. slide

Interleaved Memory

A collection of DRAM chips can be:

• A memory module (as on prev. slides)

• A memory bank

• In a bank, consecutive words are stored on different chips

• K banks can service k requests (of consecutive words!) simultaneously—As in the case of caches, locality of reference

makes requests of consecutive words worthwhile

—What is being read may well be a block, going in a line of cache!

This concludes section 5.1

This is the material required for the midterm

To solve in notebook for next time:

End of chapter problems 5.2 and 5.3.

Next time:

• Review in class

• Exam in the lab

• Lost one lecture due to Dr. appt.

5.2 Error Correction

• Hard Failure

—Permanent physical defect

—Bit is ―stuck‖ at 0 or 1, or switches randomly

• Soft Error

—Random, non-destructive

—No permanent damage to memory

• Two types of codes:

—Only error detection

– E.g. even or odd parity bit

– E.g. ―Excess nine‖

—Error detection and correction

– E.g. Hamming code

Story from the early days

of the Univac …

Easy to implement using

XOR gates!

Error Detection and Correction

Redundant bits

Syndrome word

XOR gate

Draw the symbol and the truth

table for a 3-input XOR

Generates an even-parity bit!

4-bit example, using Venn diagram

Source: http://www.apl.ucl.ac.uk/lectures/3c42/3c42-13.htmlGo to the link for more examples and a nice discussion about possible

sources of errors!

Original 4 bits Adding K=3

parity bits

(even parity)

An error occurs.

How can we detect

and correct it?

8-bit example

• Problem: Venn diagrams with 4 or more bits become progressively harder to visualize

• Idea: Insert the parity bits in the word itself, in those positions which are powers of two

—Unlike normal bit numbering, start with position 1

Bit Position 12 11 10 9 8 7 6 5 4 3 2 1

Position

Number1100 1011 1010 1001 1000 0111 0110 0101 0100 0011 0010 0001

Data Bit D8 D7 D6 D5 D4 D3 D2 D1

Check Bit C8 C4 C2 C1

Figure 5.9 Layout of Data Bits and Check Bits

• FYI: on pp. 170-171 it is explained how to figure out the exact number of parity bits for a code that corrects 1 error.

• However, the method of construction of the Hamming code explained on the prev. slide already has this number built-in automatically!

—We simply stop inserting bits when we’ve run out of original bits

Back to our 8-bit example …

Bit

position12 11 10 9 8 7 6 5 4 3 2 1

Position

number1100 1011 1010 1001 1000 0111 0110 0101 0100 0011 0010 0001

Data bit D8 D7 D6 D5 D4 D3 D2 D1

Check

bitC8 C4 C2 C1

Word

stored as 0 0 1 1 0 1 0 0 1 1 1 1

Word

fetched

as

0 0 1 1 0 1 1 0 1 1 1 1

Position

Number1100 1011 1010 1001 1000 0111 0110 0101 0100 0011 0010 0001

Check

Bit0 0 0 1

Figure 5.10 Check Bit Calculation

Each check bit is calculated as an even-parity bit over those data bits whose

position numbers contain the respective power of 2 (it’s a lot easier in binary!)

8-bit example (continued)

• Syndrome word is 0110

• This is the binary code of position 6 where the error has occurred!

Compare new and

old check bits to

create syndrome

SEC vs. SEC-DED

The price to pay: capacity overhead

Table 5.2 Increase in Word Length with Error Correction/Detection

SEC-DED is considered adequate for memory organizations with 1 bit per chip

Can you think of a reason why it wouldn’t be adequate if there are more bits per chip?

(Hint: Errors generally come in localized bursts)

Single-Error Correction

Single-Error Correction/

Double-Error Detection

Data Bits Check Bits % Increase Check Bits % Increase

8 4 50 5 62.5

16 5 31.25 6 37.5

32 6 18.75 7 21.875

64 7 10.94 8 12.5

128 8 6.25 9 7.03

256 9 3.52 10 3.91

3.3 Advanced DRAM Organization

• Basic DRAM same since first RAM chips

• New ideas:

—Synchronous DRAM

—Enhanced DRAM

– Contains small SRAM used as a prefetch buffer

—Cache DRAM

– Larger SRAM component, used as cache or serial buffer

Synchronous DRAM (SDRAM)

• Access is synchronized with an external clock

• Address is presented to RAM

• RAM finds data (CPU waits in conventional DRAM)

• Since SDRAM moves data in time with system clock, CPU knows when data will be ready

• CPU does not have to wait, it can do something else

• Burst mode allows SDRAM to set up stream of data and fire it out in block

• DDR-SDRAM sends data twice per clock cycle (leading & trailing edge)

A0 to A13 Address inputs

CLK Clock input

CKE Clock enable

Chip select

Row address strobe

Column address strobe

Write enable

DQ0 to DQ7 Data input/output

DQM Data mask

CS

RAS

CAS

WE

Table 5.4 SDRAM Pin Assignments

(The ones with bar are active low)

How large is this

memory chip?

SDRAM Read Timing

IBM 64 Mb SDRAM

We have covered pp.169-176.

Read and take notes!

Solve problem 5.10!

Lab week 7

End of chapter problems

• 2

• 3

• 9

• 10

• 11

Quiz

Why does the text say that the latency here is 2 cycles?

Doesn’t it look more like 3?

Check the explanation at the bottom of p.176.

Simplified DRAM Read Timing

RAMBUS DRAM

• Adopted by Intel for Pentium & Itanium

• Main competitor to SDRAM

• Vertical package – all pins on one side

• Data exchange over 28 wires < 12 cm long

• Bus addresses up to 320 RDRAM chips at 1.6Gbps

• Asynchronous block protocol

—480ns access time

—Then 1.6 Gbps

Why a distance

limitation?

Attenuation!

RAMBUS Diagram

No more RAS, CAS, etc. A more complex request is sent over the address + control

part of the bus (RC = Rambus Control).

Electrical termination

(as in the old Ethernet

cables!)

18 = 16 data + 2 parity

RAMBUS in Real Life

Did Intel’s choice of RAMBUS ―cripple‖ the Pentium 4?

Read this:

SystemLogic.net - Why Intel Is Twofaced

DDR SDRAM

• SDRAM can only send data once per clock

• Double-data-rate SDRAM can send data twice per clock cycle

—Rising edge and falling edge

—Note: RAMBUS has DDR, too!

DDR SDRAM

Read Timing

Cache DRAM

• Mitsubishi

• Integrates small SRAM cache (16 kb) onto generic DRAM chip

• Used as true cache

—64-bit lines

—Effective for ordinary random access

• To support serial access of block of data

—E.g. refresh bit-mapped screen

– CDRAM can prefetch data from DRAM into SRAM buffer

– Subsequent accesses solely to SRAM

Table 5.3 Performance Comparison of Some DRAM Alternatives

Clock

Frequency

(MHz)

Transfer Rate

(GB/s)

Access Time

(ns)

Pin Count

SDRAM 166 1.3 18 168

DDR 200 3.2 12.5 184

RDRAM 600 4.8 12 162

Homework for Ch.5 Due Tue, Oct 26

End of chapter problems

• 6

• 12

• 13

• 14

Term Paper: DRAM Mysteries

• Covers two DRAM topics not mentioned in text:

— Fast Page Read

— 2k vs. 4k refresh

• Good starting point: Dynamic random access memory – Wikipedia

—Specifically, in the References section, make sure you download Micron 4 Meg x 4 EDO DRAM data sheet , which has nice timing and block diagrams

• To do: For each of the topics above, find at least two more references by Thursday’s lab