sign in sign up
<<
Home , Microcomputer, Minicomputer

Inside the

Classification of Systems: • / . – Microcomputer • Desktop machine, including portables. – • Used for small, individual tasks - such as – Mainframe simple desktop publishing, small business – accounting, etc.... • Typical cost : £500 to £5000. • Chapters 1-5 in Capron • Example : The PCs in the labs are .

Minicomputer Mainframe

• Medium sized • Large server / Large Business applications • Desk to fridge sized machine. • Large machines in purpose built rooms. • Used for distributed data processing and • Used as large servers and for intensive multi-user server support. business applications. • Typical cost : £5,000 to £500,000. • Typical cost : £500,000 to £10,000,000. • Example : Scarlet is a minicomputer. • Example : IBM ES/9000, IBM 370, IBM 390.

Supercomputer

• Scientific applications • Large machines. • Typically employ parallel architecture (multiple processors running together). • Used for VERY numerically intensive jobs. • Typical cost : £5,000,000 to £25,000,000. • Example : Cray supercomputer

1 What's in a Computer System? Software

• The Onion Model - layers. • Divided into two main areas • Hardware • • BIOS • Used to control the hardware and to provide an interface between the user and the hardware. • Software • Manages resources in the machine, like • Where does the operating system come in? • Memory • Disk drives • Applications • includes games, word-processors, databases, etc....

Interfaces Hardware • The chunky stuff! •CUI • If you can touch it... it's probably hardware! • Command Line Interface • The mother board. •GUI • If we have motherboards... surely there must be • Graphical fatherboards? right? •WIMP • What about sonboards, or daughterboards?! • Windows, Icons, Mouse, Pulldown menus • Hard disk drives • Monitors • Keyboards

BIOS

• Basic Input Output System • Directly controls hardware devices like UARTS (Universal Asynchronous Receiver-Transmitter) - Used in COM ports. Central Peripherals Processing Memory • Stored in the ROM of the machine. Unit (CPU) • What's ROM? - Read only memory • Preserved while the computer is turned off. • How? • WHY?!

2 Basics Memory Central Peripherals Processing Memory • Stores the program to be executed and the Unit (CPU) data that this manipulates as bits • Volatile - contents are lost when the computer is switched off • Memory consists of series of cells, each of which holds one word of information • Each cell has a unique (a number) that can also be written as bits

Memory Store and fetch

Memory cells Memory addresses • Two key properties of a cell are its value 0101100111000010 00000001 1001100101111000 00000010 and address 0000110000111100 00000011 • Memory hardware performs to 1101110100110000 00000100 1111111110111110 00000101 operations

Cell 1010101010101010 00000110 – store ( value address ) containing 0001100011101011 00000111 one word of 0001000100010001 00001000 – fetch (address value information 1101101110010110 00001001 1000100111011010 00001010

Types of memory ROM • Read Only Memory • Random Access Memory (RAM) • Contains crucial start-up information for • Read Only Memory (ROM) PCs. • Erasable Programmable Read Only • Typically only 48K. Memory (EPROM) • Non-volatile - information is preserved when the power is switched off. • Relatively slow - sometimes copied to RAM for faster execution. • Also located on memory chips.

3 RAM EPROM • Random access memory • Volatile - information is lost when the • Using a ROM Burner the instructions power is switched off. within the chips can be changed

• Used for updates • Early computer had around 4K of RAM. • These-days 128Mb RAM is around £30 • Also seen in DVD players and videos to change/update functions • Read about SIMM, DIMM, SRAM, DRAM

Measuring memory Memory

• Memory not usually measured in words – Every 0 or 1 in binary is called a bit • Measured instead in: – (0 means off, 1 means on) • – 8 bits = 1 = one character (A,B,,$,£,1,2) • kilobytes • megabytes • kilobytes (Kb) = 210 = 1024 bytes •gigabytes • megabytes (Mb) = ~1,000,000 bytes • International word for a byte is an octet • gigabytes (Gb) = ~1 billion bytes

Where are we ?? CPU

A Arithmetic General and B purpose Logic Unit register C (ALU) D IR PC Special MBR MAR purpose register memory 0101100111000010

4 • ALU is the sub-component that performs basic arithmetic and logic operations Program execution • Registers provide very fast but expensive storage (hence there are only a few) • The fetch-decode-execute cycle – General purpose registers – Fetch instruction at memory address PC from – Special purpose registers memory and put it into IR • IR put instructions here to execute them – Decode and execute the instruction in IR • PC memory address of the next instruction – Increment PC to point at the next instruction • MAR where addresses are placed for fetch or store • Each single instruction may involve many • MBR where values are placed for fetch or store smaller micro-instructions. such as moving words of information between registers Chapter 3 (Capron)

• For example, for the single instruction: “add the data at address X to the data at address Y The clock and store the result back at Y” : • The whole cycle is driven by the CPU’s – move contents of PC to MAR internal clock – fetch – move from MBR to IR • CPU speed is measured in Hertz (Hz) – Put address X into MAR (500MHz computer can handle 500 million machine cycles per second) – fetch – move MBR to general register A • Chunks of information have to be moved – put address Y into MAR – fetch between parts of the computer - carried – move MBR to general register B along an arrangement of parallel wires – add A to B, result in C – move C to MBR called a – store

Peripherals Peripheral Devices • Various peripherals are responsible for input, output and permanent storage • Input – programs and data have to get into memory • Mouse – results have to be displayed to users • Scanner – permanent storage is required • Light-pen • • Access to peripherals is orders of magnitude Keyboard • slower than the speed of the CPU of the • What does modem mean? speed of memory access • Interrupts are used to allow the CPU to get on with something else in the meantime

5 Peripheral Devices Peripheral devices

• Output •Storage • • Hard drive • Plotter • Floppy drive • Monitor • CD-ROM • Modem • Zip disk

• Some peripherals use special dedicated I/O Summary of simple model processors to increase performance • Each peripheral is controlled by its own • Memory - words, cells, addresses, fetch and instructions store, sizes – disk: seek, read, write … • CPU - registers, ALU, fetch-decode- – tape: wind, rewind, read, write ... execute, micro-programs, clock – printer: form feed, reset, newline … • Peripherals - interrupts, specialised – monitor: clear, refresh ... instructions Central Peripherals Processing Memory Unit (CPU)

Application development Source code software Source code: • The development of programming languages “The written commands in a , together with the comments that • Language Translators describe what they do.” • Software Tools

6 The development of Algorithms programming languages • An algorithm is a logical plan for solving a task • Algorithms and programs revisited that can be understood by humans. • Machine-code • There are many notations for expressing algorithms. They need to include: • – hierarchical decomposition of tasks • High-level languages – sequences of actions • Object oriented languages – choice – iteration – descriptions of objects and information

1st generation 2nd generation Machine code Assembly languages • execute binary instructions called machine code • Limited improvements over machine code • Typically a few simple instructions consisting of op-codes and addresses – mnemonics for op-codes improve readability • Very difficult for humans to work with algorithms – more powerful addressing styles such as named in machine code locations, relative addressing and indirect – hard to read addressing – easy to make but hard to correct mistakes •ADD A – minimal program or data structures • LOAD A + 1 – not portable •STORE (A)

3rd generation High level languages Important features of HLLs

• Developed in the 60s and 70s to be closer to • Readability algorithms in terms of notation, control – use of English words and phrases structures, data structures, and task – mathematical notation decomposition – block structure • Relatively portable – comments • Based around hierarchical task • Powerful instructions and control structures decomposition realised using – complex expressions and operators functions/procedures/sub-routines – choice and repetition – input and output

7 • Data structures – named variables and constants Object-orientated languages – typed variables – structured types (arrays and records) • Encapsulate data and operations into objects – dynamic linked types (trees, lists, queues and stacks) • Program and system structure • Classes provide templates for objects – functions – establish precise interfaces between system – libraries and modules components before implementation • Abstraction and encapsulation – encourage team-work, re-use and safety – functions support procedural abstraction – modules support data abstraction Internal data Public interface operations and functions

object

Language translators The role of language translators

• The role of language translators • Programming languages have evolved to • Interpreters and compilers make programming easier for humans • Computers still execute machine code • Syntax and semantics • Language translators are required to • Stages of interpretation translate between the two • A new translator is required for each combination of programming language and machine-code

Interpreters • Translation involves several tasks – translating high level program statements into • One statement is translated and then executed combinations of machine code instructions at a time – translating high level data structures into • Start at the beginning of the program combinations of memory locations – linking to existing code (libraries, modules, REPEAT objects, system calls) to form a final program translate next program statement • Two broad approaches to translation if no translation error occurred then execute the statement – compilation and interpretation UNTIL the end of the program or an error occurs

8 Compilers Comparison • Whole program is translated, then executed • The writes the source program which • Interpretation and compilation both have is translated into the object program advantages and disadvantages • Start at the beginning of the source program REPEAT translate next source program statement • They are suited to different environments UNTIL the end of the source program and stages of the development process IF translation errors occurred THEN report errors ELSE execute the object program

Languages, syntax and semantics • Examples of syntax versus semantics – In English • Program languages are much simpler than • John has be a bad boys (incorrect syntax) ‘natural’ languages such as English, but do share • Flying tables sleep furiously (incorrect semantics) some similarities: – In a program – vocabulary of defined words • c + b = a; (incorrect syntax) • d = 0; b = c/d; (incorrect semantics) – punctuation • hyp = opp + ajd; (incorrect sematics) – names created by the programmer (identifiers) • Another important difference to a natural language • Programming languages have grammar that is ambiguity defines it’s syntax • Statements also have a semantic meaning

Scheduling and memory Errors management • Errors can show up at translation time • Scheduling – (syntax errors) or run-time (semantic errors) • Swapping • Run time errors might be due to the • Memory - memory management program or its environment •Paging • Software development

9 Goals of memory management Physical vs. Logical

• enable several processes to be executed (seemingly) at the same time; • Computer's memory size determined by • provide satisfactory level of performance for hardware. This is the physical memory size. users; • Usually smaller than the address space. • protect each process from each other; • The address space is what a user program • enable sharing of memory between processes; can access. This is the logical memory size • make memory addressing transparent to (sometimes known as size). programmer.

Running a process Scheduling

• First step must be to load executable image • Running a program results in a process (.EXE file in MS-DOS; a.out format file in • The basic rule is that the CPU can only be ) from secondary storage into allocated to one process at a time memory. • Multiprogramming is the illusion that the CPU is shared between many concurrent processes

Swapping When to swap ? • The CPU has to swap between processes: • How does the CPU know when to swap? – preserve the complete state of the outgoing process – process might run until it completes (program, data and registers) – process might run until it requests I/O – restore the state of the incoming process – the O/S might regularly interrupt the CPU – hand over control of the CPU • Whenever a swap needs to happen the scheduler • The state of a process is captured in its core- (part of the kernel) takes over the CPU and chooses the next process to run: image, a freeze-frame of it as it runs – round robin • At any time, several processes may have core – dynamic priority images in memory or on disk • This is supported by the process table

10 Relocation Memory allocation schemes

• Program with user address space starting at • Single process system 0 will not necessarily be loaded into a • Fixed partition system physical space starting at address 0. • Segmented systems • Relocation of addresses (but not data). • Problems • Load time relocation (sorted out once when program is loaded - program amended). • Runtime relocation via relocation registers (sorted out each time a logical address is accessed).

Memory allocation systems Memory allocation systems

Single process systems Fixed partition system

• Operating system • Several partitions each capable of holding one • User process • Unused space program. Partitions might or might not be of same size. • Relocation register needed to indicate base address of user process. • Largest partition needs to be large enough to hold largest program. • For protection, also need register to indicate limit address • Results in internal fragmentation of memory due to unused space (holes) in each partition

Memory allocation systems Memory allocation systems Segmented systems Problems • Program split into two or more (variable sized) segments. • Fragmentation. Cannot use space between • Most common division is code and data. Also possible segments unless there is another segment small to have programmer-defined segments (e.g. one for enough to fit. each module of source code). • Compaction is almost as inefficient as swapping • Segments need not be contiguous, thereby reducing (but at least frees up some useful space). problems of allocating free space. • Trade-off between more, smaller segments making • Need separate pairs of registers for each segment. memory fitting easier against need for more • Segment attributes: sharable; writeable. segment registers.

11 Paging Page addressing

• Memory divided up into fixed-size chunks • Each process has a page table - defines mapping from logical pages to physical pages (and any page called pages. attributes). • Typical page sizes: 512 bytes, 1K, 2K, 4K, … • Each logical address is split into two parts: page number and offset within page. Page number mapped • Relocation problem - effectively need a by page table, then offset added in. relocation register for each page. However • Solves most of fragmentation problem since a process since all pages are the same size, no limit can be fitted in to a number of separate holes. Some register is needed fragmentation still exists because of unused space within a page

Consequences Virtual memory • More processes can be held in memory (ready to run) thereby easing scheduling problems. In extreme we could hold 1 page of each process. Paging (and to a lesser extent segmentation) • Each process can be larger than the actual physical memory suggests that not all pages (or segments) of available. Illusion that physical memory is as big as logical a program need be in physical memory to memory. With 32-bit address spaces the illusion is a run the program. Only the bits containing necessity. the code being executed and the data being • The page table must record whether a page is in memory or on disk. accessed need be there. The remainder • If an address that is not in a page in memory (the resident could be on secondary storage. set) is accessed, we have a slight problem. This is called a page fault (usually an interrupt) - the requisite page must be fetched from disk (demand paging).

• If at any time a page is to be brought into memory and • Efficiency is dependent on few page faults. there are no free pages, another page must be swapped out Experiments have shown however that most processes to make room for it (page replacement). exhibit locality of reference (i.e. they are more likely to refer to addresses close by than those far away - e.g. • If the page table records whether the contents of a page access to local variables within a procedure). Thus a have changed (since it was read in from disk), then only working set of related pages will tend to generate few dirty pages need to copied back out to the disk. page faults. (It is consequential that good use of • Repeated swapping of pages is inefficient. Need to keep a modular programming strategies will improve substantial portion of the program in memory to reduce efficiency of program.) number of page faults to a minimum. This is the working • If a process's resident set is significantly smaller than set. Working set can be determined by observing which its working set, rather than continue and have pages are accessed over a period of time. (Information to increasing numbers of page faults (thrashing), it might record this can be stored in the page table.) Pages not be better to swap the whole process out (thus freeing accessed for a long time are good candidates to be first to memory for other processes) and bring a larger be swapped out proportion of it back later.

12 Software development The waterfall process

• The waterfall process Requirements • Functions •Objects Design • Classes Coding Testing

Maintenance

• In the "waterfall model" of software development, each stage should be completed before starting the next. • However, there is no reason why different parts of NOTE: There is an old joke that says: a program should not progress at different rates. • Also in practice, there is often some overlap between phases, e.g. one may need to implement a “In theory there no difference between theory small part of a proposed design in order to find out and practice, but in practice there is." whether it is feasible or not.

It applies here.

Structure in computer programs

• Software engineering suggests that you • You can use a function without knowing should divide programs into separate pieces how it is implemented…. so that you can understand a piece at a time • Consider the following examples: – Ask a chef to prepare a meal • A function is an early form of program – Calculate the square root of a decimal number division: A function has: – Send an email message – An interface which describes how you can use it – An implementation which carries out the task

13 Object-orientated structures • Similar to a function, an object has… • An alternative to dividing a program into – An interface which describes how the object functions is to use objects can be used • Objects are a natural way of thinking about – An implementation which represents the things object’s internal state – We are surrounded by objects (cars, , people) • An object’s state dictates how it responds to • A computer program which is written in terms of objects, closely represents the real requests world problem it is trying to solve

• An example object: a CD Player – Operations include: What the CD player illustrates •Play •pause • An object provides a set of operations •seek (Methods) that the user may request •forward • An object has an internal state, which • reverse should be hidden from the user •eject – Thus an object is a black box – The state: • Is there a disk in the player ? – You should only use an object through it’s • What track is the player currently at ? interface

A class of CD Players

• We have discussed a single CD Player – But there are millions of such players throughout the world • Most different makes and models have a lot in common – To capture this commonality we can define a class of CD players • A class would describe the operations and internal state which is common to all CD players

14