I think there is a world market for maybe five computers!
To err is human but in order to mess-up big time you need a computer!
Dr Alan Hind
School of Computing
You can never have too much memory!
GIGO – Garbage
In Garbage Out
There is no reason that anyone in the right state of mind will want a computer in their home.
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• It wasn’t until the early 1940s that electrical devices were first referred to as computers .
• Over the years, a rough definition of a computer has evolved to this:
– It must take some sort of input .
– It must produce some sort of output .
– It must process information/data .
– It must have some sort of memory/storage .
– It must control what it does.
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• This is the universal model for conventional computing developed independently by John
Von Neumann and Alan Turing.
• It separates the data I/O (data in memory), from the hardware and the program (also in memory).
Input
Devices
CPU Output
Devices
Memory
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The variations over the last few years are in control devices
(mouse/trackball/gamepad) and memory storage (USB sticks/CD/DVD/Blu-ray).
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• At the heart of any computer you will find a component called a processor .
• More formally, it is described as a
Central Processing Unit , or CPU .
• Today’s processors are constructed as a single electronic circuit - called an Integrated Circuit ( IC ) - on one single chip of silicon.
• Hence the names computer chip or microprocessor .
• What we often call a computer these days is more accurately a microprocessor-based computer system or microcomputer .
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• What are the smallest number of components we need to build a simple computer?
– Processor
• to process information and to control the whole system.
– Memory
• for storing data and instructions.
– Input device(s)
• to enter data into the system: a keyboard say.
– Output device(s)
• usually a monitor so we can see what our computer is up to.
• Together input and output devices are known as I/O .
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• In fact, a computer can happily work without the
I/O devices (not very usefully though) but does need three other vital components.
– Power
• From a power supply which converts mains AC at 240 Volts to 5 Volts DC or less.
– Clock signal
• To coordinate everything – over the years we moved from kHz through MHz to GHz (clock speed= 1 /
T
).
– Bus (plural busses )
• A set of parallel communication connections.
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• What does it do?
• Fetch an instruction from memory and execute it.
• … then do it again, and again, and again … ad infinitum …
• This is the Fetch-execute cycle .
• When it’s first switched on/powered-up the processor fetches the instruction from the first location in the memory, unless designed and configured to do otherwise.
• N.b. This is location zero not location one.
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• How can the cycle be speeded-up?
– Note that there are at least two limiting factors influencing its performance.
• CPU execution rate of instructions.
• Read/write access to the system memory.
• The so-called Von Neumann bottleneck .
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• Increase the Clock Speed .
– Decrease the time period (T) for each cycle (F= 1 /
T
)
– 1 MHz in 1980  1 GHz in 2000  7.03 GHz in 2013
• This has been achieved through:
– An increase in VLSI * density using ever smaller electronic devices.
– Advanced multi-chip modular packaging.
– Faster memory systems.
(*VLSI – Very Large-Scale Integrated circuit)
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• Increase the instruction size/complexity
– I.e. increase the amount of work done per cycle
• More data at a time.
• More complex instructions.
• More specialised instructions.
• This hasn’t been entirely successful as added complexity limited the clock speed hence the move to
RISC (Reduced Instruction Set Computer architecture).
• The above is then termed CISC (Complex Instruction
Set Computer architecture).
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• Parallelism !
– Have more than one fetch-execute cycle running at a time.
– Turns out to be tricky as most problems are inherently sequential*.
– This has led eventually to multicore processors though.
(*Look up Amdahl’s Law)
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IF 0
• Pipelining & Superscalar
ID 0
IF 1
DF 0
ID 1
IF 2
IE 0
DF 1
ID 2
IF 3
SR 0
IE 1
DF 2
ID 3
SR 1
IE 2
DF 3
SR 2
IE 3
– Pipelining
• involves using special circuitry to allow the fetchexecute cycle to be streamlined into something like an assembly-line.
• E.g. Instruction Fetch (IF), Instruction Decode (ID), Data
Fetch (DF), Instruction Execution (IE), Store Result (SR).
– The Superscalar approach
• uses separate specialised functional units for different instructions.
• E.g. Mathematics, graphics, communications.
SR 3
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• These techniques have been used concurrently along with advances in VLSI technology to speed-up processor operation.
– Smaller transistors  Increased clock speed
– Increasing instruction complexity (CISC)  More power.
– Decreasing instruction complexity (RISC)  Faster clock speed.
– Parallelism  Distributed and Multicore processors.
– Pipelining/Superscalar  Greater integration  More power.
• Along with this the advances have given us another opportunity.
– Smaller transistors  Reduced power consumption.
– Greater integration  More power on one chip.
– Hence, mobile, hand-held, battery powered computing devices.
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• Parallelism is hard to do within programs or within an operating system – Trust me, I’m a doctor and did my PhD in this area!
• But, whole programs can run on separate processors (cores) quite nicely as long as they don’t need to talk to each other – much.
• The problems relate to Amdahl’s law
– again.
Cores
1
2
3
4
5
6
7
8
9
10
Names single-core dual-core tri-core quad-core penta-core hexa-core hepta-core octa-core nona-core deca-core
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• An observation was made in 1965 by Gordon Moore , co-founder of Intel.
– ”The number of transistors per square inch on ICs has doubled every year since the integrated circuit was invented.”
• Moore predicted that this trend would continue for the foreseeable future.
• In subsequent years, the pace slowed down a bit, but chip density/chip complexity has doubled approximately every two years, and this is the current definition of Moore's Law.
• Most experts expect Moore's Law to hold for at least another two decades.
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• Memory devices are distinguished by:
– Access time
– Capacity
– Volatility
– Locality
– Cost
• Fast memory is expensive even in small amounts
• Large Capacity memory is relatively slow.
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• A memory that is fast enough not to keep the
CPU waiting during the fetch-execute cycle:
– has to be on the same chip (i.e. very close) …
– but the memory circuitry takes up lots of silicon realestate …
– so, it is therefore relatively expensive …
– and not very big! (kb at best)
• Modern processors have registers and cache memory on-chip with the processor.
• However, this memory is also volatile – the contents are lost at shutdown.
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• So, we have two categories of memory device:
– Immediate Access Storage (fast, expensive, volatile)
• Registers and cache based on SRAM .
• Main memory based on DRAM .
– Secondary Access Storage (slower, cheaper, non-volatile)
• Internal and External Hard disk based on magnetic disks.
• USB memory sticks based on EAROM (flash memory).
• Magneto-optical disks based on CD, DVD and blu-ray.
• SRAM – Static Random Access Memory – uses flip-flop circuits with 6 transistors – fast to read/write – large silicon area
• DRAM – Dynamic Random Access Memory – uses a capacitor and one transistor – slow to read/write – needs refresh - smaller silicon area
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• Booting up a computer takes time partly as the program code and data is moved (loaded) from slower secondary (disk) to faster immediate access storage
(main memory).
• Programs being executed and data currently being processed is stored in immediate access storage.
– If it’s not in a register (<1ns) then it’s in the cache (<5 ns), if it’s not in the cache it’s in main memory (<50 ns) , if it’s not there then it’s out on the hard disk (<1 ms) etc.
• Everything else is in secondary storage.
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• Modern operating systems (OS) are too big to completely fit in the computer’s main memory (even if you have 4GB installed).
• So the computer pretends it has more than it does by creating a Virtual
Memory space on the hard disk.
– If you want to use an OS function not loaded in main memory then the computer has to get it from the virtual memory on the hard disk.
• Modern applications have also become steadily larger and more sophisticated.
– The more applications you run, the more memory (real and virtual) you use, the more likely the computer will spend a great deal of time swapping things between main memory and the hard disk.
– A major culprit is opening lots of different web pages in separate pages
(or tabs) plus all that browser history you haven’t deleted for a while.
– The perfect storm is when you load lots of applications on a computer whose hard disk is almost full – things start well and then … slowly … grind … to … a … … …
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• Cache memory is all about making the whole system go faster!
• Access is faster than main store memory (DRAM) as it is local and uses SRAM. It is limited in size partly due to cost.
• Most processors have part of the cache memory on-chip: this is called on-die cache. Off-die cache is located in other SRAM chips
• Cache is also often organised as two levels for speed considerations.
Level-1 cache is smaller in capacity but faster than Level-2.
• Cache memory is used to pre-fetch program instructions and data prior to being needed by the processor. Sophisticated algorithms are needed to "guess" what instructions or data are needed next.
• It works independently to all the other memory systems and sits between the processor and main store memory.
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• If we look inside the simplest CPU we start to get an insight into it’s fetch cycle using Register Transfer
Language (RTL).
– [MAR]  [PC]
– [PC]  [PC] + 1
– [MBR]  [MS([MAR])]
– [IR]  [MBR]
– CU  [IR(op-code)]
• Key
– [ ] indicates "the contents of".
– The arrow (  ) indicates the direction.
– ( ) indicates “given by the”.
– Copies are non-destructive to the source
– The first line reads as:
• Copy the contents of the Program Counter into the Memory Address Register.
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• This depends on the instruction - obviously.
• Let’s use the simplest single address instruction format.
– E.g. ADD X ; add the contents of memory
; location X to the accumulator.
– Here’s the format. op-code operand
– where the op-code is a binary number indicating the operation to be carried out (e.g. ADD) and the operand is another binary number with the data or the address of the data.
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• The overall effect of the execution of the instruction ADD X in RTL is:
– [A]  [A] + [MS(X)]
• The execute sequence for ADD X is:
– [MAR]  [IR(address)]
– [MBR]  [MS([MAR])]
– ALU  [MBR] + [A]
– [A]  ALU
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• Each line of RTL happens in each clock cycle (or tick).
• So, this instruction takes 9 clock cycles on our simple processor.
• At the beginning of each cycle the Control Unit (CU) has to output the right binary signals to each part of the CPU.
• N.b. all fetch cycles will be identical and execute cycles will vary in length with complexity.
• Fetch
– [MAR]  [PC]
– [PC]  [PC] + 1
– [MBR]  [MS([MAR])]
– [IR]  [MBR]
– CU  [IR(op-code)]
• Execute
– [MAR]  [IR(address)]
– [MBR]  [MS([MAR])]
– ALU  [MBR] + [A]
– [A]  ALU
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Op-code Assembly
Instructions
RTL Operation Comment
000 LDA X [A]  [MS(X)] Load A with contents of location X
001 STA X
010 ADD X
011 SUB X
100 INC X
101 DEC X
110 JMP X
111 BRA X
[MS(X)]  [A] Store contents of A in location X
[A]  [A] + [MS(X)] Add the contents of location X to A
[A]  [A] - [MS(X)] Subtract contents of location X from A
[MS(X)]  [MS(X)] + 1 Increment value in location X
[MS(X)]  [MS(X)] - 1 Decrement value in location X
[PC]  X Jump to instruction given by the value in location X
IF Z = 1 THEN [PC]  X Conditional branch if Z is true*
(* Z is the Zero flag which is set to true if the result of the last instruction was zero)
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• A simple assembly language program.
– LDA 20
– ADD 21
– STA 22
• Which does the following.
– Load the accumulator with contents of location 20.
– Add contents of location 21 to the accumulator.
– Store the result in location 22.
• The binary executable machine code in main store at locations 0, 1 & 2 will be:
– 000 10100
2
– 010 10101
2
– 001 10110
2
(14
H
)
(55
H
)
(36
H
)
• N.b. the numbers we want to add must be pre-loaded in to locations 20 and 21.
• N.b. the 3-bit op-code and the 5-bit operand giving an
8-bit instruction.
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• We started by inputting instructions and data into computers in binary using switches ! (I kid you not!)
• We then wrote programs to read hexadecimal shorthand – a Hex loader – one instruction at a time.
• We then wrote programs to read holes in paper tape or cards to input programs of instructions and data.
• However, we could then move on to low-level programming written in Assembly Language .
– We typed the instructions into a card/tape punch machine which produced the right holes to give the machine code.
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A University Computer Centre in 1980
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• Once computers became interactive with a monitor and keyboard, we could use a text editor program to write an Assembly Language text file.
• An Assembler program would check this for syntax errors and then, if there weren’t any, would produce a new text file with all the absolute memory locations calculated.
• A Linker program would then be used to create a binary executable file, linking with any other necessary or requested files at the same time.
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• High-level programming allows us to write more sophisticated programs that are processor-independent .
• Assembly language is specific to the processor.
• We don’t need to worry about how many wossnames it has, how big or where they are!
• Our simple program to add two numbers together gets much simpler and interactive!
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• A Compiler is a program which takes a text-based high-level language program file and converts it into a binary executable file.
• It does this in three “passes”.
– Lexical analyser – takes out the irrelevant stuff.
– Syntax analyser – makes sure the language makes sense (no speeling errods!)
– Semantic analyser – makes sure that the program means something valid.
– Finally, it produces binary executable code but can also produce Assembly Language text.
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• An Interpreter …
• A Cross-compiler …
• An Emulator …
• An IDE …
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• An Interpreter produces executable code line-by-line rather than allin-one-go like a compiler.
– This is much slower but useful for interactive development, debugging as-you-go-along and learning programming.
• A Cross-compiler allows a PC to produce binary executable code for another device or platform.
– E.g. A mobile phone, tablet computer, games console etc.
• An Emulator is a program which allows you to run an application compiled for another device.
– E.g. Running an iPhone app on a Mac, running a PS2 game on a PC.
• An IDE ( Integrated Development Environment ) combines an intelligent editor, compiler and runtime debugging environment.
– E.g. Microsoft’s Visual Studio
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• Dr Alan Hind
– School of Computing
– Teesside University
– Borough Road, Middlesbrough, TS1 3BA.
• Email: a.hind@tees.ac.uk
• Telephone: (01642) 342641
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• Arduino: http://arduino.cc/en/
• Raspberry Pi: http://www.raspberrypi.org/
• What goes on inside the CPU: http://www.pcauthority.com.au/Feature/290164, what-goes-on-inside-the-cpu.aspx
• Computer Tutorial: http://www.eastaughs.fsnet.co.uk/index.htm
• Teach-ICT: http://www.teach-ict.com/
• Computer Organization and
Architecture: http://alanclements.org/index.html
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