Hardware (Part B)

Reading Materials:
 Chapter 5 of [SG]: The Computer System
 Optional: Chapter 2 of [Brookshear]
OUTLINE
1. Organization of a Digital Computer
2. Major Components of a Computer System
3. von Neumann architecture: how it fits Together
4. The Future: non von Neumann architectures
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Introduction

Computer organization examines the
computer as a collection of interacting
“functional units”

Functional units may be built out of the
circuits already studied

Higher level of abstraction assists in
understanding by reducing complexity
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Figure 5.1
The Concept of Abstraction
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Digital Computer: Logical Organization

Organization of a Computer
Input
Devices
CPU
Output
Devices
Memory
Unit
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Figure 5.2 Components of the Von Neumann Architecture
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The Components of a Computer System

Functional units in Von Neumann architecture:
 Memory
 Input/Output
 Arithmetic/Logic unit
 Control unit

Sequential execution of instructions
 One instruction at a time
 Fetched from memory to the control unit

Concept of a stored program
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Memory and Cache

The functional unit that stores
 instructions (programs / “software”) and
 Data / information

Primary Memory Types:
 ROM (Read Only Memory)
 Read only, permanent
 RAM (Random Access Memory)
 Read/Write, Volatile
 Cache memory
 keeps values currently in use (with faster memory)
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Figure 5.3
Structure of Random Access Memory
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Memory and Cache (continued)

RAM (Random Access Memory)
 Memory made of a large array of addressable
“cells” (each of the same size)
 Maps “addresses” to memory locations (cells)
 Current standard cell size is 8 bits
 All memory cells accessed in equal time
 Memory address
 Unsigned binary number N long
 Address space is then 2N cells
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Memory Unit -- Operations

Data Transfers:
 Need Instructions:
 FETCH – instr to read content of a memory location
(fetch some data from memory)
 STORE – instr to write a value to a memory location
(store some data into memory)
 Need Special Registers:
 MAR – for the address of the memory location;
 MDR – for data to be written to / read from memory
 implemented via digital circuitry
 Using decoders to select individual cells
 Fetch / Store decoder
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Registers (or Memory Registers)

Memory register
 Examples: MAR, MDR
 Very fast memory location (1 cycle)
 Given a name, not an address
 Serves some special purpose
 Modern computers have dozens or hundreds
of registers
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The Fetch Operation

To “read” data/info from memory

fetch (address)
1. Load the address of the desired memory cell
into the MAR
2. Decode the address in the MAR
3. Copy content of that memory location into
MDR
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The Store Operation

To store information into memory

Store (address, value)
1. Load the address into the MAR
2. Load the value into the MDR
3. Decode the address in the MAR
4. Store the contents of MDR into that memory
location.
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Figure 5.7
Overall RAM Organization
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Cache Memory

Memory access is much slower than
processing time

Faster memory is too expensive to use for all
memory cells

Locality principle
 Once a value is used, it is likely to be used
again

Small size, fast memory just for values
currently in use speeds computing time
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Peripheral Devices (Overview)

Other Devices that augments the CPU+memory
 I/O: Keyboard, mouse, monitor, printers, speakers,
 Storage: Cache, Disk drives, CD-drive, Zip-drive, tapes
 Communication: Network cards, modems,

Devices communicate with CPU via controllers
 usually some kind of circuit board (eg: sound cards)

Also,
 I/O devices vary greatly
 Can Dynamically added/removed devices
 Flexible Design needed to allow easy addition / removal
/ upgrading
 Design may be sub-optimal.
 Flexibility often more important than optimality.
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Input/Output and Mass Storage

Communication with outside world and
external data storage
 Human interfaces: monitor, keyboard, mouse
 Archival storage: not dependent on constant
power

External devices vary tremendously from
each other
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Input/Output and Mass Storage…

Volatile storage
 Information disappears when the power is
turned off
 Example: RAM

Nonvolatile storage
 Information does not disappear when the
power is turned off
 Example: mass storage devices such as harddisks, thumb-drives, and tapes
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Input/Output and Mass Storage…

Mass storage devices
 Direct access storage device
 Hard drive, CD-ROM, DVD, etc.
 Uses its own addressing scheme to access
data
 Sequential access storage device
 Tape drive, etc.
 Stores data sequentially
 Used for backup storage these days
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Input/Output and Mass Storage…

Direct access storage devices
 Data stored on a spinning disk
 Disk divided into concentric rings (sectors)
 Read/write head moves from one ring to
another while disk spins
 Access time depends on:
 Time to move head to correct sector
 Time for sector to spin to data location
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Figure 5.8
Overall Organization of a Typical Disk
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Input/Output and Mass Storage…

I/O controller
 Intermediary between central processor and
I/O devices
 Processor sends request and data, then goes
on with its work
 I/O controller interrupts processor when
request is complete
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Figure 5.9: Organization of an I/O Controller
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The Arithmetic/Logic Unit

Actual computations are performed

Primitive operation circuits
 Arithmetic (ADD, etc.)
 Comparison (CE, etc.)
 Logic (AND, etc.)

Data inputs and results stored in registers

Multiplexor selects desired output
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Figure 5.12
Using a Multiplexor Circuit to Select the Proper ALU Result
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The Arithmetic/Logic Unit (continued)

ALU process
 Values for operations copied into ALU’s input
register locations
 All circuits compute results for those inputs
 Multiplexor selects the one desired result from
all values
 Result value copied to desired result register
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Recap…

Have seen how
 logic gates and flip-flops can be used to form
combinational and sequential circuits;
 Any logic/arithmetic functions (operations) can be
implemented this way;
 But, then the “functions” will be “hard-wired”.
 Need a “different computer” for each new job!!

Instead, we want a general purpose computer
 computer runs a STORED program;
 “function” of the computer varies according to the
different STORED program;
 the stored program is arbitrary  general purpose
computer;
 Basic Architecture: Von-Neumann Architecture
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The Control Unit

Manages stored program execution

Task
 Fetch from memory the next instruction to be
executed
 Decode it: determine what is to be done
 Execute it: issue appropriate command to
ALU, memory, and I/O controllers
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Machine Language Instructions

Can be decoded and executed by control unit

Parts of instructions
 Operation code (op code)
 Unique unsigned-integer code assigned to
each machine language operation
 Address field(s)
 Memory addresses of the values on which
operation will work
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Figure 5.14
Typical Machine Language Instruction Format
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Machine Language Instructions…

Operations of machine language
 Data transfer
 Move values to and from memory and registers
 Arithmetic/logic
 Perform ALU operations that produce numeric
values
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Machine Language Instructions…

Operations of machine language (continued)
 Compares
 Set bits of compare register to hold result
 Branches
 Jump to a new memory address to continue
processing
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Control Unit Registers And Circuits

Parts of control unit
 Links to other subsystems
 Instruction decoder circuit
 Two special registers:
 Program Counter (PC)

Stores the memory address of the next
instruction to be executed
 Instruction Register (IR)

Stores the code for the current instruction
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Figure 5.16
Organization of the Control Unit Registers and Circuits
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Putting All the Pieces Together — the Von
Neumann Architecture

Subsystems connected by a bus
 Bus: wires that permit data transfer among
them

At this level, ignore the details of circuits that
perform these tasks: Abstraction!

Computer repeats fetch-decode-execute cycle
indefinitely
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Figure 5.18
The Organization
of a Von Neumann
Computer
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CPU (Central Processing Unit)

Components of a CPU:
 Control Unit: the “brain” of the CPU.
 decoding which operation is to be performed, and
 deciding the next operation to perform
 ALU (Arithmetic Logic Unit)
 consists of logic circuits for addition, multiplication,
and all other operations
 Buses: wire connecting
 wires connecting up different parts of CPU, and the
CPU to other components;

Each component is built using logic
circuits
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CPU Execution: Example: W = X + Y

To add two numbers stored in X and Y and
store the result in W

CPU performs the following steps:
1.
2.
3.
4.
5.
6.
7.
Place address of first number (X) in MAR;
Issue a “FETCH” command to Memory Unit;
Transfer content of MDR to Register R1;
Place address of second number (Y) in MAR:
Issue a “FETCH” command to Memory Unit;
Transfer contents of MDR to Register R2;
Issue a “ADD” command to ALU to perform addition
of numbers in registers R1 & R2 and place result
in register R3;
8. Transfer contents of R3 to MDR;
9. Place address of result (W) in MAR;
10. Issue a “STORE” instruction to Memory Unit;
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Functioning of a CPU

The steps above illustrates
 basic technique for CPU to execute simple instructions
 similar technique is used for all other instructions
ANALOGY: “If we have buttons for the CPU functions, then
a human can press the appropriate button to execute
the above step”

In real computers, the
 role of human is performed by the “Control Unit”,
 role of buttons by using control signals

Control Unit is also responsible for
 decoding the instruction,
 figuring out the next instruction, etc
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The Future: Non-Von Neumann Architectures

Physical limitations on speed of Von
Neumann computers

Non-Von Neumann architectures explored to
bypass these limitations

Parallel computing architectures can provide
improvements: multiple operations occur at
the same time
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The Future: Non-Von Neumann Architectures

SIMD architecture
 Single instruction/Multiple data
 Multiple processors running in parallel
 All processors execute same operation at one
time
 Each processor operates on its own data
 Suitable for “vector” operations
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Figure 5.21. A SIMD Parallel Processing System
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The Future: Non-Von Neumann Architectures

MIMD architecture
 Multiple instruction/Multiple data
 Multiple processors running in parallel
 Each processor performs its own operations
on its own data
 Processors communicate with each other
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Figure 5.22. Model of MIMD Parallel Processing
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Summary of Level 2

Focus on how to design and build computer
systems

Chapter 4
 Binary codes
 Transistors
 Gates
 Circuits
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Summary of Level 2 (continued)

Chapter 5
 Von Neumann architecture
 Shortcomings of the sequential model of
computing
 Parallel computers
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Summary

Computer organization examines different
subsystems of a computer:
memory, input/output,
arithmetic/logic unit, and control unit

Machine language gives codes for each primitive
instruction the computer can perform, and its
arguments

Von Neumann machine: sequential execution of
stored programs

Parallel computers improve speed by doing
multiple tasks at one time
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
If you are new to all these
 read the textbook carefully
 Do the practice problems and some exercises
in the book
 Do the tutorials in the course.
… The End …
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
Registers – clock speed of CPU

RAM – 20 ns (20 x 10-9 s);

Hard-disk (eg: 15,00rpm) – 2 ms (2 x 10-3s)

Factor of about 10-5 difference

Paper on desk – 1s

Drawer in office – 10s

Warehouse in Changi – 2 hrs (7200s)
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
Memory Hierarchy
 Registers – 1 cycle
 L1-cache – a few cycles
 DRAM – hundreds of cycles
 Hard-disk – millions of cycles

Analogy
 Calculator – immediate
 Paper on desk – few seconds
 Drawer in office – 1 minutes
 Warehouse in Changi – 2 hrs (7200s)
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References

http://en.wikipedia.org/wiki/Access_time

http://whatis.techtarget.com/definition/0,,sid9_gci523855,00.
html#regram

http://en.wikipedia.org/wiki/Processor_register

http://en.wikipedia.org/wiki/Memory_hierarchy
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