Memory 1

©Paul GodinCreated March 2008

Edit April 2011

Memory

◊ Digital systems often require the ability to retain and access binary values. Programmable devices and devices that access programs require memory systems to store values for, and after, processing.

◊ Memory systems are comprised of D Flip-Flop or D Latch configurations that are addressed with a decoder.

◊ This presentation addresses basic concepts associated with memory devices.

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Memory Devices

Memory

Read Only Random Access

Storage

Masked ROM

PROM

EPROM

EEPROM

CD-ROMDrives

Flash

SRAM

DRAM

DIMM

SIMM

SDRAM

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Memory

◊ Memory comes in 2 general varieties:

◊ RAM (Random Access Memory)◊ Often considered “volatile” because the data is lost when the

system is turned off.◊ RAM is used by systems to store values that are intended to

change regularly.◊ RAM is fast.

◊ ROM (Read Only Memory)◊ Considered permanent or semi-permanent memory and “non-

volatile” because data is not lost when the system is turned off.◊ ROM is used by systems to store values that are not intended to

change or that change only rarely.◊ ROM is slow for writing operations.

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READ ONLY MEMORY

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ROM Varieties – MP-ROM

◊ Mask-Programmed ROM (MP-ROM): ◊ A mask programmed ROM has had the values of the data

bits built into the device on manufacturing it and is therefore very application-specific.

◊ Have very high initial set up costs for manufacturing. Selected only when a large production run is required.

◊ These devices cannot be re-programmed.◊ Examples of Mask-Programmed ROM include game

cartridges and operating systems for smart cards.

◊ Today the availability, variety and lower costs of other ROM types means the use of MP-ROM is somewhat rare.

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ROM Varieties - PROM

◊ Programmable Read-Only Memory (PROM):◊ Memory devices that contain fusible links that can be

custom programmed one-time (OTP). During the programming process the undesired links are burned open.

◊ Once programmed, PROMs are much less susceptible to data loss than most other ROM devices.

◊ PROMs are inexpensive and come in a variety of shapes and sizes based on the application desired.

◊ Examples of PROMs include operating systems for automotive computers and other portable devices. Proms are also used in space vehicles due to their ruggedness.

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ROM Varieties - EPROM

◊ Erasable Programmable ROM (EPROM)◊ Devices that can be custom programmed by the user. ◊ Once programmed, the data is non-volatile. ◊ EPROMs can be erased by exposing the circuit to UV light

through a small glass window (typically about 20 minutes). Window should be covered with a sticker.

◊ Can be re-programmed as often as desired. ◊ EPROMs are more expensive than most other ROM

devices (except MP-ROM) and have one of the lowest densities. They are much less popular today than the past.

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ROM Varieties - EEPROM

◊ Electrically Erasable ROM (EEPROM)◊ Provide the user with the ability to custom program ROM

with the option of erasing the data electrically. ◊ Some EEPROMS can be selectively erased, where only

portions of the memory may be cleared and re-written. ◊ More expensive and has lower density than other ROM

devices.◊ Very popular for many applications. Permits user to

update code without physically handling the device. Used in many computer-based applications.

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ROM Varieties - Flash

◊ FLASH◊ Flash memory incorporates the advantages of erasable

non-volatile memory with a fast erase capability.◊ Permits selective write capability where specific

addresses may be re-written.◊ Flash ROM is very popular due to its relatively high

speed, ease of use, portability and cost reductions. Used extensively in portable digital electronic applications.

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Flash memory: K9GAG08U0Mhttp://www.flash-extractor.com/

RANDOM ACCESS MEMORY

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RAM (Random Access Memory)

◊ Referred to as “Read/Write Memory”, this memory is utilized for temporary storage. This is a volatile memory, meaning the data is present as long as power is applied to the IC.

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RAM Varieties – SRAM

◊ SRAM◊ Static RAM basically consists of flip-flop memory devices.

These individual flip-flops will retain data as long as power is maintained to the device.

◊ Very fast and is often used in applications where speed is of primary concern.

◊ The primary disadvantage is the amount of continuous power it requires and the amount of heat it generates.

◊ Typically used for microprocessor registers and cache.

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RAM Varieties - DRAM

◊ DRAM◊ Dynamic RAM utilized capacitive charges to maintain

binary values. ◊ has a lower power consumption than SRAM. ◊ requires frequent recharging of the capacitors otherwise

the data will be lost. ◊ Slower than SRAM but continues to improve.◊ DRAM is static sensitive.

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RAM Structure

◊ DIMM, SIMM, DDR, SDRAM◊ Memory modules may take on various physical forms to

make them easier to handle. ◊ Single In-line Memory Module (SIMM) has a set on

contacts on one side of the board only and 32 bit access.◊ Dual In-line Memory Module has contacts on both sides of

the board and are designed for 64 bit access.◊ Double Data Rate is memory that performs an operation

on both the rising and the falling edge of the clock.◊ SDRAM, EDO, FPM and others offer various options of

handling data while maintaining the highest rates of speed.

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STORAGE

◊ Various storage devices are used for maintaining large amounts of information and are in a different category than Memory. These storage systems include:

◊ CD and DVD-ROMs◊ hard drives◊ floppy drives◊ tape storage ◊ other systems

◊ Storage devices are considered I/O (Input/Output) devices.

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MEMORY – BASIC CONCEPTS

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Basic Terminology

◊ Bit◊ A single binary digit (1 or 0) ◊ abbreviated with a small b

◊ Byte◊ 8 bits ◊ abbreviated with a capital B

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Terminology in use

◊ Bytes: Computer-based devices such as memory, hard drives, ROM drives, flash drives and other similar devices have capacity rated in Bytes. i.e. a 100 GB drive has a stated capacity of 100 Billion Bytes, equal to 800 Billion bits.

◊ Bits: Communication-based systems use bits as the standard unit of measure. i.e. USB 2.0 transfer rate is 480 Mbps (Megabits per second), and basic Ethernet is 10 Mbps.

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Memory Terminology

◊ Cell: the element that can store a single bit of data.

◊ Word: the number of bits that represent an instruction or data. Usually determined by the data bus width.

◊ Capacity: the total number of bits or bytes a memory device can store.

◊ Density: a ratio, describes the physical space occupied by memory.

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Memory Capacity

◊ Memory devices store bits of data from the data bus, therefore these bits are arranged with the same width as the data bus.

◊ An 8 bit bus has an 8 bit word.

D7 D6 D5 D4 D3 D2 D1 D0

This is a single 8-bit word

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Memory Capacity

◊ In turn, we can access each byte of information stored in a memory device by an individual address.

D7 D6 D5 D4 D3 D2 D1 D0

D7 D6 D5 D4 D3 D2 D1 D0

00

01

10

11

D7 D6 D5 D4 D3 D2 D1 D0

D7 D6 D5 D4 D3 D2 D1 D0

Ad

dre

ss

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Memory Capacity

D7 D6 D5 D4 D3 D2 D1 D0

D7 D6 D5 D4 D3 D2 D1 D0

00

01

10

11

D7 D6 D5 D4 D3 D2 D1 D0

D7 D6 D5 D4 D3 D2 D1 D0

Ad

dre

ss

This example has a 2-bit address (22)There are 4 storage locations for 8 bit wordsTotal capacity is 32 bitsThis memory unit is a “4 x 8” (4 x 8-bit words)

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◊ With more address bits comes increased capacity.

Memory Capacity

D7 D6 D5 D4 D3 D2 D1 D0

D7 D6 D5 D4 D3 D2 D1 D0

0000

0001

1110

1111 D7 D6 D5 D4 D3 D2 D1 D0

D7 D6 D5 D4 D3 D2 D1 D0

Ad

dre

ss

How many storage locations are there?

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Memory Notation

◊ Standard notation for memory is:# addressable locations x word size

◊ The number of addressable locations is somewhat “rounded”. It follows a pattern that is familiar to us:◊ 1, 2, 4, 8, 16, 32, 64, 128, 256, 512◊ Example: 128k is actually 131,072 addresses (217)

“32k x 8” means 32k of addresses (rounded) for 8 bit words

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Review Questions

Complete the following table.

4k x 8 64k x 16 256k x 4 512 x 32

Capacity (words)

Word Size

# address lines

# data lines

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Review Questions

◊ Which memory stores the greatest number of bits?◊ 64k 8◊ 32k 16◊ 16k 32◊ 256k 4

◊ Of the above, what are the differences between them?

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Review Questions (Answers)

◊ Which memory stores the greatest number of bits?◊ 64k 8 = 65,536 addresses 8 bits/address = 524,288 bits◊ 32k 16 = 32,768 addresses 16 bits/address = 524,288 bits◊ 16k 32 = 16,384 addresses 32 bits/address = 524,288 bits◊ 256k 4 = 262,144 addresses 4bits/address = 1,048,576bits

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©Paul R. Godinprgodin°@ gmail.com

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