BS2911 Week 2 1

Networking and the Internet (2) Last Week:

» Why does Networking matter?» Some thoughts about e-Business» Hardware foundation – Computer architecture

Week 2 Focus» Computer architecture – working through operations» Software – Operating Systems, File storage» Operating system foundations

– Interrupts, concurrency and multi-programming– Scheduling and Dispatching– Time-sharing and Online systems

» Graphical Operating Systems, including Windows

Useful text-book: Coope, S et al (2002)

Computer Systems McGraw Hill

BS2911 Week 2 2

Computer Architecture

ProcessorProcessorMemoryMemory

Disk StorageDisk StorageOther

long-term Storage

Other long-term Storage

Processor executes instructions from memory, and works on data in memory Other data flows through the bus

1234567890-=QWERTYUIOP[]#ASDFGHJKL;’ZXCVBNM,./

Input (Data)

Output (Information)Bus

BS2911 Week 2 4

Computer Processor & Machine code

Arithmetic and Logic Unit

ClockInstruction Control

Unit(starts at 310)

Instruction Register

Memory

310 1 1 600314 1 2 602318 5 1 231A 2 1 604

1 = Load

2 = Store

3 = Add

4 = Subtract

5 = Multiply

Memory

600 4602 6604 4660606 4608608 466060A -14

Registers

12

Central Processor Unit

...

BS2911 Week 2 6

Operating Systems

How we control this hardware

BS2911 Week 2 7

Operating Systems Though the processor is simple and serial, we want to do

more complex things, often several at once An operating system is a program that provides the

building blocks of complex systems» Some simply encapsulate function to save every application

from having to include a copy» Others handle specific hardware, presenting a generic

interface that hides behaviour unique to that hardware» Sometimes the interface is so generic that it has little to do

with the hardware – file structures are the best example Modern operating systems make it look as if the computer

is doing several things at the same time» Our operating system is Windows XP

BS2911 Week 2 9

Concurrent Operations

To give the appearance of doing several things at once:» OS must stay ready to accept work:

– keystrokes, mouse clicks, signals from modem, printer ready to receive another buffer of data

– These can interrupt a computation already being run» It then does a bit of the required work,» then goes back to an interrupted task, and so on.

We say the machine is doing things “concurrently” – they’re not simultaneous, but they look it!

The key is switching the CPU between logical processes» In theory, you could go round “polling” – high overhead» In practice, concurrency depends on hardware interrupts

BS2911 Week 2 10

Essentials of an operating system

Controls the hardware» Lets applications be less hardware-specific by abstracting

operations (who cares how big a track is!) » Reduces havoc that can be done by rogue programs

by restricting use of risky instructions (such as those giving direct access to hardware)

» Allows processes to update files with integrity Encapsulates commonly-used functions Manages resources, including storage Supports concurrent operations Success judged by performance, in terms of

Availability, Reliability, Response-time, Throughput

BS2911 Week 2 11

In the beginning...

21 June 1948: First stored-program computer (Manchester University “Baby”) ran its first program» Program keyed directly into memory

» Results displayed as dots on a CRT

» When program finished, it stopped

» Next machines used tape or card for I/O Monitors developed in 1950s

» encapsulate standard functions (for example, Input/Output)

» automate running of programs one after another

» still one program at a time

» then added SPOOLING to overlap input and output

Too much investment to let it sit around

waiting for humans to press buttons

BS2911 Week 2 13

True Operating Systems

Introduced with Ferranti Atlas and IBM System/360 Applied concurrency to user work as well as to SPOOL

» Potential to run complementary jobs alongside each other OS became a resource manager

» Sharing processor resource between jobs» Providing separate memory for use by each job» Controlling allocation of tapes and other hardware » Scheduling jobs to fit resources available

Used interrupts to switch control between processes» Need to be sure we understand how they work

Foundation for on-line systems with terminals

BS2911 Week 2 15

On-line Computing

Terminal attached to mainframe computer Operating system “time-shared” processor among users Developed initially with slow lines and typewriter-like

terminals or teleprinters, which sent a character at a time It was expensive to read every keystroke

» so switched to using “Block mode”:» User types into terminal buffer, presses Enter to transmit

Most transaction processing is done that way, even today Weaknesses of mainframe + terminal are

» poor bandwidth: can’t track mouse, write graphics» can’t take shortcuts based on every keystroke

BS2911 Week 2 17

Basic Concepts Covered So Far Faking concurrency

» multiprogramming: appearing to do several things at once » processes and threads» For multiple users –

or one user with several balls in the air Interrupts

There’s more to come on:» I/O buffering» Spooling (offline, online)» Multiprocessing

– Using multiple processors to get more power– symmetrical, clustering or master-slave

BS2911 Week 2 19

What Operating Systems Do

OS/360 and Batch SchedulingHow Online Computing is different

BS2911 Week 2 20

OS/360

“Betting the Business” for IBM in late 1960s Environment:

» Batch processing» Real memory only (Ferranti Atlas had paging, but the idea

hadn’t yet crossed the Atlantic)» Physical cards used for job entry» Line-printers used for output (up to 1000 lines/min)» Tapes and disks expensive but heavily used

Concepts» Job Control Language (JCL) to describe each job» Each program ran in a Job Step

BS2911 Week 2 21

Structure of a Job Must run steps in sequence Don’t run step if previous one failed Must have input available at start of

step Need somewhere to write results Usually generate spool output Normally expects the Program run in

each step to be on disk

Whole Job

Step 1, e.g. compile program

Step 2, e.g. link-edit program

Step 3, e.g. run program

Step 4, e.g. sort output

Step 5, e.g. run utility on result

Needs card input + workfile

Needs object deck + workfile

Needs module + input data

Needs output data

Needs sort output + target file

//ERIC JOB//COMP EXEC PGM=PLIOPT//SYSIN DD *//SYSPRINT DD *//SYSOUT DD DSN=“ERIC.PLI.OBJ”//LINK EXEC PGM=LKEDIT/* and so on

BS2911 Week 2 22

Scheduling Jobs Back in the days of monitors, scheduling was easy

» When a job finishes, load the next one and run it» If I/O is spooled, next one will be loaded from spool file that

contains images of cards read in earlier..» ..and CPU time needs to be shared between the running job

and spool I/O (which uses predictably little CPU)

Gets harder when more than one real job can run» Have to match resource requirements with availability» Need to be concerned with sequence of jobs» Optimization needs awareness of job type

(processor-heavy, I/O heavy, etc.)» Specified with complex Job Control Language

BS2911 Week 2 23

OS/360 Batch Scheduling Memory was critical resource

(>$1000 per kilobyte) Divided into partitions or

regions, each with a job initiator OS reads each job in and places

it on a (prioritized) queue When job completes in region,

initiator looks on queue for more work» Must fit in available memory» Necessary resources must be

available» Otherwise try continue looking

down queue

Print spool

Operating System

Card spool

Job 1

Job 2

Job 3 Job 4

Job 5

Job initiatorsJob

queue

Wasted fragment of storage m

emory

BS2911 Week 2 24

Batch Scheduling Problems

Storage» With fixed-size partitions, you can have long queues for a

big partition while one or more small ones are free» With variable-sizes (“regions”), you get fragmentation» Various steps may have different space requirements

Resource allocation» Need to collect together all resources needed by a step:

such as files and dedicated hardware» Can still waste run time (e.g. if you get a tape drive but

haven’t mounted the right tape on it)» Addressed by extensions to Job Control Language

(even more chances to make mistakes with it!)

BS2911 Week 2 25

Risks of Deadlock

OS/360 enforced pre-allocation to avoid deadlock Consider two jobs, A & B Job A

» is holding tape drive 1» can’t complete until it can update file X.Y.Z

Job B» is writing file X.Y.Z» can’t close the file until it’s written a log to tape» BUT there are no tape drives available

Therefore both jobs will stall, tying up both regions But you can’t pre-allocate with on-line users

BS2911 Week 2 26

Scheduling Today

We’re not usually concerned with resources on a home PC» Schedule an anti-virus upgrade and it’ll run quickly

(though running the virus checker might be much slower)

We may be concerned about sequence» Better to be sure the upgrade is complete before scanning

In business systems:» Duration can be longer (that’s why ITS constrains disk space;

otherwise the back-ups would take too long)» Sequence can be critical – don’t pay out before taking in» Every transaction must run once and once only

BS2911 Week 2 27

Scheduling and Dispatching

Once we’ve scheduled jobs to start, we have to divide machine cycles between the concurrent processes» With a small degree of multiprogramming (few regions), that

can be fairly simple» Whenever a region waits, we dispatch another one » Dispatcher is like a micro-level scheduler» Often have different dispatch and initiator priorities

Storage limitations addressed with Virtual Memory» Allowed increase in number of initiators» Increasing degree of multiprogramming ..» .. and need for more complex dispatching – there’s

no point in dispatching a process without (real) memory

BS2911 Week 2 28

Online Systems

Expands scale of allocation and dispatching problems “Jobs” become very long-running

» so we can’t pre-assign all resources and hold until job end» Total number of jobs grows greatly» Jobs need to sleep when user isn’t interacting» Each interaction is like a mini job step

Two basic approaches to managing this» Treat each user as a process and tune the dispatcher

» Unix, VM/370, MVS/TSO and VMS work this way » Treat each user as a thread on a “timesharing” process

» CICS and IMS work this way» High-performance web servers too

BS2911 Week 2 29

Time-sharing

Consider a system with 100 on-line users» Average think time 10 seconds

(so overall arrival rate is 10 interactions per second)» Average CPU demand of 50msec per interaction» Therefore load should be 50% plus overhead» If load is homogeneous, “round-robin” dispatching is OK

But what if a few users need 1 sec & the rest 10 msec?» Queue will build up if the long task is allowed to complete» OK, let’s preempt task on expiry of a time-slice;

suspend task (take the CPU away) if it takes too long, and make it wait until next time round

BS2911 Week 2 30

Life Cycle of an Interaction

Task Created

User hits ENTER

Terminated

Executable Task

FinishesRunningOn Ready Queue

Blocked

Dispatched

Requeued on Event

Time expires

Enters WAIT

Task runs until interrupted for some reason

BS2911 Week 2 31

Dispatching Tasks

Let’s assume a Ready Queue of tasks waiting to run» OS adds tasks to queue according to a priority pattern

(cheapest is FIFO*, but we may want to improve on that)» Dispatch the task at the head of the queue» When it gives up control, dispatch new head of queue» May want to maintain queue of blocked tasks too

What if task doesn’t give up control?» Need to interrupt it at end of time slice to let others run

(Windows 3.x didn’t do this except for DOS tasks)» Can return it directly to Ready Queue

(with risk that it’ll consume too much CPU time)» Maybe we should favour short interactions over long

* First In First Out

BS2911 Week 2 32

Graphical Operating Systems

Development of Windows

BS2911 Week 2 33

Graphical Operating Systems WIMP concepts

» Windows, Icons, Menus, Pointer» invented late 70s at Xerox Palo Alto Research Center

First marketed in Lisa machine – very expensive» Later in Xerox “Daisy” – still too costly to succeed

Apple made success of the idea in Macintosh – 1984» Massive advertising campaign» Succeeded first with designers and Desk Top Publishing

IBM & Microsoft followed with 16-bit OS/2 V.1 (1987)» Full WIMP approach in 1988» IBM Went it alone for OS/2 V2 – incompatible interface

BS2911 Week 2 34

Early Flavours of Windows Windows 1 and 2

» Never really made it – Lisa ideas done less well than Mac OS/2 Version 1 (Microsoft/IBM collaboration) Windows 386

» Exploited 80386 virtual machine code; Win2 interface Windows 3.0 (1990) – The breakthrough

» Picked up OS/2 V1 user interface, with simpler API» Added Windows 386 process mgt (multiple DOS boxes)» Still no pre-emption of ill-behaved Windows processes

Windows 3.1» Enhanced Win3.0 without major architecture changes» Added some new GUI controls

BS2911 Week 2 35

Flavours of Windows since 1993 Windows for Workgroups (3.1 and 3.11)

» Added networking capabilities» Introduced (some) 32-bit code, such as file I/O» You could bolt-on IP network support – free but not trivial

Windows 95 – New user interface» Integrated IP networking » Much more 32-bit code» Long file-names (and file-types, unlike OS/2)» Still not fully pre-emptive multitasking, but improved

capability to detect and abort ill-behaved processes Windows 98 – Minor changes from W95

» Adds Internet-like interface to Windows 95» Meant to be “last of the line of Windows 3.x successors”

BS2911 Week 2 36

Windows NT NT V3.x

» Microsoft’s OS/2 successor, with full Windows GUI» Kernel based on experiences with VMS (at Digital)» Data permissions RACF style, by user and group» 32-bit, with some limitations on use of 16-bit applications

NT V4.0» Enhanced from NT V3.51» Added Windows 95 user interface» Improved tolerance of 16-bit applications…» …providing they don’t try to access the hardware directly» Full pre-emptive multitasking – Windows processes get

time-sliced, so ill-behaved ones can’t hog the processor

BS2911 Week 2 37

Current Windows Flavours

Windows 2000 – Intended to unify NT and 9x families » To avoid duplicate development effort» Replaced NT for professionals and large & small Servers » But Domestic version didn’t run all W98 software, so…

Millennium Edition (of Windows 98)» Stopgap because 98/NT integration wasn’t complete

Windows XP – finally did unite NT and 9x families » Came in versions for different purposes

– XP Home edition, – Professional edition for corporate clients – Servers

Similar approach with Windows Vista (2007) and W7

Increasing price

BS2911 Week 2 38

Summary of Week 2

Computer does simple things, in sequence» Instruction counter contains address of next instruction to run

Operating systems package these into useful facilities» Including ability to run programs concurrently

Processor is a resource that the Operating System uses» OS treats each program as a task» Gives it a bit of time on the processor» Then passes the processor to another task

OS gets control back through interrupts» Voluntary (supervisor call by user program)» External (such as disk I/O completion, timer interrupt)

BS2911 Week 2 39

Checkpoint Questions

Describe differences between B2C & B2B e-Commerce» 1.

» 2.

What are the main functions of an Operating System?» 1. 2.

» 3. 4.

What does the “Instruction Counter” point to?