Transcript Lecture 7 - e

Lecture 7
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Monday 20th Oct 2008
Today's Agenda
Hardware Concepts
 Main Memory
Recall from last lecture
 What's latest in microprocessor technology?
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Multiprocessors (e-g intel duo)
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Hyper threaded technology
 Examples of system software?
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Computer Hardware
Hard Disk
Input device
CPU
Output
Device
Main
Memory
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Main Memory
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For the purpose of storing data, a
computer contains a large collection of
circuits, each capable of storing a bit.
This bit reservoir is known as the
machines main memory.
The storage circuits in a machines
main memory are arranged in
manageable units called cells or
(words), with typical cell size being 8
bits.
Bit collections of size eight are known
as Byte.
The size of a machines main memory
is measured in terms of 1048,576 cell
units.( The value 1048,576 is a power
of 2 namely 220 , so it is more natural
as a unit of measure than an even
1000,000. The term Mega is used to
indicate this measure.
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Cell 1
10110101
Cell 2
10011111
Cell n
3
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The abbreviation MB is often used for the term Megabyte. Thus a memory of 4MB
contains 4194,394 (4*1,048,576) cells, each of which is one byte in size.
Other units of measuring are Kilobyte (KB) which is equal to 1024 bytes ( 210 bytes)
and Gigabyte (GB) which is equal to 1024 MB or ( 230 bytes).
To identify individual cells in the machines memory, each cell is assigned a unique
name called its address.
Memory cells addresses are entirely numeric.
To be more precise, we can envision all the cells being placed in a single row and
numbered in this order starting with the value zero.
The cells in a 4MB memory would be therefore addressed as 0, 1, 2……, 4194304
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Remember 4MB contains 4*1,048576=4194304 memory cells
Note that such an addressing system not only gives us a way of uniquely identifying
each cell but also associates an order to the cell, (refer to figure in the last slide)
giving us phrases such as “the next cell” or “the previous cell”.
To complete the main memory of the machine, the circuitry that actually holds the bits
is combined with the circuitry required to allow other circuits to store and retrieve data
from the memory cells.
In this way other circuits can get data from the memory by electronically asking for
the contents of a certain address (called read operation) or they could record
information in the memory by requesting that a certain bit pattern be placed in the cell
at a particular address (called the write operation).
An important consequence of machines main memory as small addressable cells is
that each cell can be accessed individually. That is, data stored in a machines main
memory can be processed in random order which explains why main memory is often
referred to as random access memory (RAM).
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This random accessibility of small data units is in stark contrast to the mass storage
systems (that will be discussed later) in which long strings of bits must be
manipulated as a block.
We envision the bits within a memory cells as being arranged in a row. We call one
end as the high-order end and the other the low-order end.
Although there is no left or right in a machine, we imagine the bits arranged in a row
from left to right with high-order end arranged on the left. The bit at this end is called
the most significant bit; similarly the bit at the lower end is called the least
significant bit.
We may represent the contents of a byte-size cell as shown below
Most significant bit
Least significant bit
01101101
High order end
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Low order end
5
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An important consequence of ordering of both the cells in the main memory and
the bits within each cell is that the entire collection of bits within a machines main
memory is essentially ordered in one long row.
Pieces of this long row can therefore be used to store bit patterns that may be
longer than the length of a single cell.
In particular if the memory is divided into byte size cells, we can still store a string
of 16 bits merely by using 2 consecutive memory cells.
Class exercise (12 min- collaboration allowed)
1.
How many bits would be in the memory of a computer with 4KB memory?
2.
If the memory cell whose address is 5 contains the value 8, what is the difference
between writing a value 5 into cell number 6 and moving the contents of cell
number 5 into cell number 6?
3.
Suppose you want to interchange the values stored in memory cells 2 and 3. What
is wrong with the following steps of sequence of steps:
Step1: Move the contents of cell number 2 to cell number 3
Step2: Move the contents of cell number 3 to cell number 2
Design a sequence of steps that correctly interchange the contents of these cells.
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Size of Main Memory (revisited)
(RRM slide 7 to 10)
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For the sake of discussion assume main memory to be 1MB.
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MAIN MEMORY= 1MB
1MB =2²° bytes.
=1048,576 bytes OR 1048,576B.
Where B=Bytes
Aside (another way of looking at it) :
1KB =1024B
1MB =1024KB
=1024*1024
= 1048,576B
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Conceptually its is best to imagine the main memory as consisting of a very long list
of individual bytes (each byte having its own address) – e-g a 32 MB PC has
=32*1048576
=33,554,432 or ( 225 ) bytes numbered from 0 to 33554,431 as in computers
counting starts from 0.
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Question : In 64MB of memory, calculate the actual bytes of memory and also indicate
the last address?
1MB
= 1048,576B
64 MB
= 64 * 1048576
= 67108864 bytes of memory
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Last location will be binary equivalent of 67108863
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Aside (another way of looking at it):
64MB
=64*1MB
=64*1024KB
=64*1024*1024B
=26 * 210 * 210 B
=226 B
=67108864 Bytes
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Main Memory (from the hardware perspective)
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The main memory is the storage space for data that is directly accessed by the CPU.
Main Memory
ALU
CPU
Bus
Registers
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Its typically consists of a set of single in-line memory modules (SIMMs) mounted on
the computers motherboard close to the CPU.
SIMMs could be of various sizes (e-g 1MB, 128 MB and so on)
The circuitry that performs operations such as additions and subtraction etc on data
is not directly connected to the storage cells in the machines main memory.
Instead this circuitry is isolated in a part of computer called the Central Processing
Unit or CPU. This unit consists of 2 parts: the arithmetic/logic unit (ALU) which
contains circuitry that performs data manipulation and the control unit which
contains circuitry for coordinating machines activities.
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CPU
Resides on a single chip called the microprocessor e-g Intel Pentium 2 is a popular
example
More on CPU later…
Speed of Computer
Microprocessors are usually rated in terms of their internal clock. A computers speed
is usually measured in MHz. A 300Mhz processor has a cock that ticks 300,000,000
times per second
This means that the computer can execute 300,000,000 instructions in 1 second
which in turn means that each instruction takes (3.3 nano sec or 1/300,000,000 sec
to execute).
 (1 nano sec is 1 billionth of a sec i-e 10-9 sec).
 In practice that speed is rarely attained because the processor has to wait for
other components to catch up.
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