machine architecture · • any location in the memory can be accessed very fast just by specifying...

ITNP23 - Autumn 2014 Lectures 2&3, Slide 1

Machine Architecture Prof Leslie Smith

Lectures 2 & 3

… or what’s in the box?


Basic Machine Architecture

§  understand the basic architecture of a simple computer.

§  understand the components: CPU, Main Memory, I/O Controllers.

§  understand concepts of stored program, machine cycle, memory mapped I/O.

In these lectures we aim to:


What is a computer ? … wikipedia

§  A computer is a programmable machine that receives input, stores and manipulates data or information, and provides output in a useful format.

§  While a computer can, in theory, be made out of almost anything, and mechanical examples of computers have existed through much of recorded human history, the first electronic computers were developed in the mid-20th century (1940–1945).

§  http://www.youtube.com/watch?v=0anIyVGeWOI&NR=1&feature=fvwp §  http://www.youtube.com/watch?v=KL_wy-CxBP8&feature=related

ITNP23 - Autumn 2014

Background: computers and digital hardware §  Digital computers consist of

large number of gates •  AND, OR, NOT, NAND

§  And other circuitry that (e.g.) generates digital signals •  Clocks, lines carrying data

§  These all need switching circuitry

§  And switching circuitry can be built from may different technologies.

Machine hardware: April 2016 4


Konrad Zuse’s 1st computer 1941


Zuse (1910-1995) Built the Z3 in 1941 (inspecting rebuilt Z3 at Deutsches Museum, 1991)


IBM 7090 series: from 1959

§  For “large scale scientific and technological applications”

§  $2.9 million (equivalent to $23 million now)

§  50,000 Germanium transistors •  6x faster than IBM 709,

much lower power consumption: world’s most powerful computer in 1960


IBM7090, from1965: credit Australian news and information bureau


The era of the minicomputer


§  Data General NOVA minicomputer, early 1970’s


Structure of a simple computer

Main Memory

Registers

ALU

Control Unit

CPU

Bus

Keyboard controller

Disk controller

Screen controller


Microcomputers, smartphones, …

§  The basic architecture of all of these is the same

Lectures 2&3, Slide 9


Basic architecture of a computer

§  Memory unit contains both programs and data

§  CPU runs the program by executing instructions using ALU.

§  Conceptually, this is a von Neumann computer

§  There are alternative designs, e.g. Harvard architecture: separate memory for data and instructions.

CSCU9A1: (c) LSS


Execution of a program

§  A program is a sequence of instructions.

§  A machine-code program is a sequence of machine-code instructions.

§  To be executed, a machine-code program must be stored in main memory.

§  Execution takes place within the CPU.

§  Execution follows ‘Fetch-Decode-Execute’ cycle


The CPU

The Central Processing Unit (CPU) is where the work takes place. It has three components:

•  Control Unit organises the actions of the CPU, and sequences the actions of the other components to execute programs

•  Arithmetic/Logic Unit (ALU) acts as a calculator, under the direction of the control unit. All data alteration takes placec in the ALU.

•  Registers: individual high-speed storage locations, internal to the CPU, used to hold transient data. Some are dedicated for particular purposes, others are used as temporary stores during computations


The CPU : control unit

It is the task of the control unit to :

1.  Fetch from memory the next instruction to be executed

2.  Decode it, that is, determine what should be done

3.  Execute it by issuing the appropriate command to the ALU, memory, and I/O Controllers

These 3 steps are repeated over and over again until we reach the last

instruction of a program (usually HALT, STOP, or QUIT).


The Fetch-Execute Cycle

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The CPU : ALU

§  The subsystem that performs such mathematical and logical operations as addition, subtraction, and comparison for equality.

§  In modern machines, the ALU, control unit and registers are fully integrated into a single component called the processor. For reasons of clarity and convenience, the functions of each are described separately.


Main Memory : RAM

Main Memory is also called random-access memory (RAM) §  It traditionally has the following characteristics:

•  It is divided into fixed-sized units called cells. Each cell is associated with a unique identifier called an address.

•  Any location in the memory can be accessed very fast just by specifying the address of the location

•  The time it takes to fetch or store a cell is the same for all cells in memory

•  Programs (and data) are stored in main memory when they are running

•  RAM is linked to CPU in such a way that data can be moved from one to the other very quickly


§  A small part of main memory is non-volatile, read-only memory (ROM)

§  Simply RAM where the ability to store information has been disabled

§  It is only possible to fetch information: it cannot be overwritten

§  In most computers, a section of RAM is set aside as ROM to store essential system instructions which the user cannot overwrite

Main Memory : ROM


Main Memory

Registers

ALU

Control Unit

CPU

Bus

Keyboard controller

Disk controller

Screen controller

Buses §  Buses: Each bus consists of several signal wires, so a large number of

bits (perhaps 64 bits or more) can be sent simultaneously between the components, making communication between them very fast


I/O controllers

Main Memory

Registers

ALU

Control Unit

CPU

Bus

Keyboard controller

Disk controller

Screen controller


I/O controllers

§  Input and output operations are slow in comparison with computations done in the CPU

§  Communication with a peripheral is done via a special hardware controller. This can be nearly as complex as the CPU.

§  An I/O controller is like a special-purpose computer with a responsibility to handle the details of input/output, and to compensate for any speed differences between I/O devices and other parts of the computer.

§  It has a small amount of memory known as an I/O buffer and enough I/O control and logic processing capability to handle mechanical functions of the I/O device, such as the read/write head (on a disk), paper feed mechanism (on a printer), and screen display.

§  It is also able to transmit an interrupt signal (see later) when an I/O operation is complete.


1. Transfer the 80 characters from their current location in memory to the I/O buffer storage within the I/O controller (takes place at high speed ~ 10s or 100s of millions of chars per second)

2. The processor then instructs the I/O

controller to begin the output operation. 3. The control logic of the I/O controller

handles the actual transfer of the 80 chars to the screen (this takes place perhaps at a much slower rate ~100s /1000s of chars per second, depending on the display type)

4. When all 80 characters have been displayed,

the I/O controller sends an interrupt signal to the processor

5. The appearance of this special signal

indicates to the processor that the I/O operation is complete.

I/O example: display 1 line of text on a display

Note that the processor continues to execute instructions while this is going on


§  Each type of processor (e.g., Core i7, PowerPC, ARM) has a repertoire of instructions that the CPU is built to recognise and execute

§  This is the instruction-set of the processor

§  A program that is expressed in this instruction-set is in machine code

§  In the first days of computers, programmers wrote in machine code (because there wasn’t’ anything else)

§  Now, we write programs in languages that other programs translate into machine code

Machine Instruction Set


Machine Code

§  Machine code is binary code •  not readable by people (without extreme difficulty or training) •  very readable by machines

§  Binary •  literally only 1s and 0s •  like 11100111100000011110010111110000

§  Internally held as a pattern of voltages representing 1’s and 0’s


Instructions in this language have a typical format: •  operation code field (or op code) •  address field (s)

Operation code:

A unique number assigned to each machine language operation recognised by the hardware

Address field (s):

Memory addresses of the values on which the operation will work Machine code (lines of numbers!) is very hard to read, so instead

programmers use a set of mnemonics called assembly language to represent the numerical instructions.

Machine Code Instructions


§  Some examples from the Intel 8021 instruction set:

•  MOV A, 4 “Copy contents of memory location 4 to register A” •  DEC A “Take away 1 from (decrement) the contents of register A” •  MOV B, A “Copy contents of register A to register B” •  ADD A, B “Add contents of register B to register A” •  MOV 4, A “Copy contents of register A to memory location 4”

Example Let’s try this out with the following values: A : initial value 2 B : initial value 4 Memory location 4: initial value 3

§  In a modern programming language this could be written as x = (x-1)*2 !

Assembly language instructions


§  Each processor (of any type) has two special registers: •  The Program Counter (PC): this holds the address in memory of the next

instruction to be obeyed •  The Instruction Register (IR): holds the instruction currently being obeyed

§  At the dawn of time (when your computer is switched on) •  There is a program sitting in memory, starting at some location in RAM (or ROM,

at the dawn of time) •  The address of this location (probably “zero”, at the dawn of time) is in the PC •  The control unit starts the machine cycle (see later) •  and the machine executes the program (ditto)

§  At the dawn of time, this program will be the bootstrap (it loads the operating system program from disk to memory when you start up your computer), but as the machine continues, the programs will be user programs

Running a Program


§  Also known as the fetch/decode/execute cycle §  Also known as the Von Neumann cycle (see later slide for further

information). The processor (all types) endlessly executes the following loop §  Repeat

Fetch instruction from location in PC Increment PC by instruction length Decode instruction Execute instruction

§  The point of the Increment step: the next time round the cycle it fetches the next instruction in the program

§  The machine (unless it’s switched off) is always executing this loop (even if apparently idle)

§  It does it at a fixed rate: the clock speed §  When we say a processor is “running at” (say) 3 GHz, we mean it executes the

cycle 3,000,000,000 times a second ... or faster with later “versions”!

The Machine Cycle


Fetch->Decode->Execute … a little more detail

Each instruction is executed in a number of small steps:

1.  Fetch the next instruction from memory into the Instruction Register

2.  Change the Program Counter to point to the next instruction

3.  Decode the current instruction

4.  If the instruction uses a word in memory, determine where it is

5.  Fetch the word (in a CPU register if needed)

6.  Execute the instruction … and repeat until the

instruction is ‘Halt’

M e m o r y

Instruction Register

Program Counter

1

2 Control Unit

3

4

5

Register

6

ALU Register

Register


§  So, to run a program: •  we arrange that its instructions are in contiguous locations in

memory •  place the start address in the PC

•  …. stand clear!

Running Programs


Machine Cycle Example

§  Here is a snapshot of the state of a simple computer showing the contents of main memory.

§  Trace the execution of the program stored in the main memory. At each step, show the contents of main memory and of all registers used in executing this program.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 150 MOV A 34 DEC A MOV B A ADD A B MOV 34 A1632 4


§  In 1946 he proposed a radically different computer design based on a model called the stored program computer.

§  Until then, all computers were programmed externally using wires, connectors, and plugboards. The memory unit stored only data, not instructions.

§  To solve a different problem on these computers, the user had to rewire virtually the entire machine. For example, the plugboards contained 6000 different switches, and reprogramming involved specifying the new settings for all these switches.

§  Von Neumann proposed that the instructions that control the operation of the computer be encoded as binary values and stored internally in memory along with the data.

§  Now to solve a different problem you would simply rewrite the sequence of instructions, that is, create a new program.

§  Von Neumann invented programming as it is today!

von Neumann


Clock Speed versus Instruction Rate Clock speed: §  Measured in either millions of cycles per second (megahertz (MHz)) or billions of

cycles per second (gigahertz (GHz)) §  The actions of every computer are controlled by a central clock, and the “tick”

rate of this clock is one possible speed measure. Typical clock speeds of today’s computers - from 500MHz through to 2-3 GHz

§  but… this figure can be misleading as a machine’s capability is also how much work it can do between ticks, not just how fast it ticks.

Machine Speed Instruction Rate: §  ..is a better measure than clock speed alone §  It’s measured in MIPS (millions of instructions per second) §  measures how many machine language instructions can be fetched, decoded, and

executed in one second.


§  Not included in our machine architecture diagram, but certainly plays a part in program execution. It is located on or near the processor.

§  Computer designers observed that when a program fetches a piece of data or instruction, there is a high likelihood that: 1. It will access that same instruction or piece of data in the very near

future 2. It will likely access the instructions or data that are located near this

one.

§  This is known as the principle of locality.

Cache - 1


§  To exploit this, when the computer references a piece of data, it should move

the data from RAM to cache. It should also move data in the memory cells located near this item into cache.

§  Cache memory is typically 10 times faster than RAM, but is much smaller (100s of Kilobytes of storage, rather than 100s or Megabytes or Gigabytes .. See later for more on bytes)

Cache - 2


§  When the computer needs a piece of information, it does the following:

1. Looks first in cache to see if the information is there. 2. If it is not, then access the information in RAM 3. Copy the data just fetched into cache along with the “k” immediately

following memory locations. If the cache is full, discard some of the older items that have not been accessed recently.

§  A good analogy to cache is a fridge! Without one, we’d have to go to the supermarket every time we needed an item (that usually requires cool storage). With a fridge, we can store food for now, and food we will need in the immediate future, reducing trips to the supermarket (Schneider & Gersting, 2004)

§  Cache size on CPUs and devices (e.g. hard drives) can have a large impact on speed

Cache - 3


Summary

we have discussed… §  the structure of a simple computer

§  its basic components (CPU, Main Memory, I/O controllers)

§  its operation (stored program concept, program execution).


end of lecture

machine architecture · • any location in the memory can be accessed very fast just by specifying...

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