1ITCS 3181 Logic and Computer Systems 2014 B. Wilkinson Slides12.ppt Modification date: Nov 11, 2014

Memory

Key component of a computer system is its memory system to store programs and data.

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Requirements

1. Able to store accessible binary words (patterns of 0’s and 1’s) by electrical, magnetic or other means.

2. For storing programs currently being executed and storing associated data, need to be able to access any storage location in the memory at will in any order with high speed, i.e. memory must be random access memory (RAM).

3. For data, need capability of both reading and writing information.

4. For program execution, the capability of reading sufficient.

5. Usually some of the memory at least must be non-volatile, i.e. the information is not lost when the power is removed.

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Memory Hierarchy

Historically, memory organized in levels of decreasing speed but decreasing cost/bit:

• Cache memory relatively small semiconductor memory operating at a speed that matches the processor

• Main memory – semiconductor random access memory

• Disk memory – not random access but not volatile.

• CD drives, tape drives, … – not random access but not volatile.

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Semiconductor RAM

These are very old memories – new ones are similar in concept.Above actually shows the two organizations, see next.

Memory busMemory chips

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Semiconductor Memory Organizationsx 1 organization

Each chip organized to hold one bit of each word.Memory cell

(Stores one bit)Memory chip

Ro wselectlin es

Co lumnselectlin es

Data(in/out)

n to 2 n

decoder

Me moryadd ress

lines

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Semiconductor Memory Organizationsx n organization

Each chip organized to hold n bits of each word, where n is usually 4 or 8.

Memory chip

Rowselectlines

Columnselectlines

Data(n data in/out lines)

Memoryaddresslines

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Types of Semiconductor (RAM) Memory Cells

Two basic types devised:

• Static RAM (SRAM)• Dynamic RAM (DRAM)

Static RAM faster operation than dynamic RAM (around 10 times faster) but requires more internal components (see next).Hence DRAM has more memory cells in chip (around 4-8 times capacity).

Usually main memory is dynamic and cache memory usually static.

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Static RAMEach cell - an S-R flip-flop formed with pair of cross-coupled gates.

Memory cell

Row i

Column j Column j

Transistor

S-R flip-flop

Q Q

Two complementary column lines used also to read stored value (Q/ Q) and write new values (Q/Q operating as S/R).

Transistor operates as

a switch

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Memory cell

Row i

Column j Column j

Transistor

Static RAM Memory Cell Circuit

Design requires 4 or 6 transistors and two column select lines per column. Do not worry about its operation. Key point is number of components.

Transistor operates as

a switch

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Dynamic RAM cell circuit

Each memory cell uses a capacitor to store charge. Two states, charged and not charged represent the two binary values 0 and 1. Less components, higher memory density. Usual design:

Row i

Column j

C

Memorycell

One column line

This capacitor isextremely small(< 0.04 pF)

Because charge will decay, memory cell must be refreshed periodically, typically every 2-4 ms. Refresh consists of reading data and rewriting it. Usually done automatically within chip.

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Asynchronous dynamic random access memory

Original (1970’s) DRAM were “Asynchronous” - emory operation not synchronized to processor clock - processor makes request for a memory location and waits whatever time it takes to access location.

Asynchronous dynamic random access memory (DRAM) operates with access times of 50-70 ns. (originally 100-150 ns).

Access time - time to select memory location and obtain stored information or write new information.1ns = 10-9seconds

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Synchronous DRAM (SDRAM)

Introduced in the last few years replacing original asynchronous DRAM but still using same basic dynamic memory cell design. Access time of individual memory cell essentially the same.

Speed improvement obtained by synchronizing memory operation with processor and a burst mode in which a group of sequential locations accessed one after the other at high speed after an initial read/write latency.

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Other Types of Semiconductor MemoryRead-Only Memory (ROM)

Memory cells store binary values which can only be read during normal operation. Binary values established either during manufacture of device or subsequently depending upon type of ROM. This memory is still random access.

Key feature - memory is non-volatile, that is, information not lost when power removed and can be obtained after power returned.

Very useful for bootstrap programs - small necessary programs that are executed when the computer is turned on to start the computer.

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General Arrangement of ROMs

Connection* made

Row

Column

Data in/out

lines

select

or not made(say correspondsto 1 or 0)

Various ways connectioncan be made dependingupon type of ROM

Memory cell

Column lines

* Connection has to be such to allow current to flow in one direction only.

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Types Read-Only MemorySeveral types:

1. Fixed Read-Only Memory (ROM)

Memory cells store fixed binary values permanently - as defined during manufacture.

Memory cell

Column line

Row line

Direct connection* made during

manufacture if a 1 is required, no

connection if a 0. (Could be other way

round.)

* has to be such to allow current to flow in one direction only - typical transistor used.

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2. Erasable Programmable Read-Only Memory (EPROM)

Memory cells stores fixed binary values but can be altered by user.

Loosely based upon previous dynamic cell design using very small capacitors that are charged to represent one binary state and unchanged for the other binary state, but the semiconductor construction allows the charge to remain for many years. In early designs, charge remains for maybe 10 years if not released.

Can be re-programmed with different data. Normally not altered while used as normal memory of the computer.

First need to erase information and then re-write required information.

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UV EPROMOriginal EPROM (1970’s) -- Erasure done by shining ultra violet light thro window in chip which releases charge in entire memory.

Then, writing done electrically.

UV light

Memory package

Electrically Erasable Programmable Read-Only Memory (EEPROM)

More recent EPROM in which the information can be removed by electrical means as well as writing. Still this is not done in normal computer operation.

Flash memory - a version of EEPROM in which the entire memory is erased electrically rather than individual bytes.

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Questions