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Internal Memory

Raul Queiroz Feitosa

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Objective

To present a survey of semiconductor main

menory technology.

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Outline

Semiconductor Main Memory

Advanced DRAM Organization

Error Detection/Correction

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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

(EEPROM) Electrically, byte-

level

Flash memory Electrically, block-

level

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Semiconductor Memory Types

RAM

ROM

PROM

EPROM EEPROM

E2PROM

FLASH

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Memory Cell Operation

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Read Only Memory (ROM)

Permanent storage

Nonvolatile

Microprogramming (see later)

Library subroutines

Systems programs (BIOS)

Function tables

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Types of ROM

Written during manufacture

Very expensive for small runs

Programmable (once)

PROM

Needs special equipment to program

Read “mostly”

Erasable Programmable (EPROM)

Erased by UV

Electrically Erasable (EEPROM)

Takes much longer to write than read

Flash memory

Erase whole memory electrically

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Random Access Memory - RAM

Misnamed as all semiconductor memory is random

access

Read/Write

Volatile

Temporary storage

Static or dynamic

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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

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Stating RAM Structure

high low

off

off

on

on

state 1 state 1

low high

on

on

off

off

state 0

Read

value is on line B Write applies

value to B and

compliment to B

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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

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Dynamic RAM Structure

0/1 on

charge

transfered

to capacitor

Write

0/1 on

charge fed

through

line to

sensor

Read

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Refreshing

Refresh circuit included on chip

Disable chip

Count through rows

Read & Write back

Takes time

Slows down apparent performance

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SRAM 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

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Simplified DRAM Read Timing

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Typical 16 Mb DRAM (4M x 4)

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Packaging

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Outline

Semiconductor Main Memory

Advanced DRAM Organization

Error Detection/Correction

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Advanced DRAM Organization

Basic DRAM same since 70’s.

Enhanced DRAM

Contains small SRAM as well

SRAM holds last line read (c.f. Cache!)

Cache DRAM

Larger SRAM component

Use as cache or serial buffer

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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)

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SDRAM Read Timing

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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

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DDR SDRAM - Read Timing

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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

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Error Correction

Hard Failure

Permanent defect

Soft Error

Random, non-destructive

No permanent damage to memory

Detected using Hamming error correcting

code

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Outline

Semiconductor Main Memory

Advanced DRAM Organization

Error Detection/Correction

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Error Correcting Code Function

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Text Book References

The topics are covered in

Tanenbaum -

Stallings - chapter 3.

Parhami -

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Internal Memory

FIM