you write your code。 For example； suppose the lighting controller introduced in Chapter 8 had to control 3；000 rooms？ To be able to process such a large number of rooms； you would use the same code； except run it in parallel。 And therein lies the problem of code that needs to multitask。 It’s more difficult to make code run efficiently in parallel。 A mon problem in multitasking applications is the dreaded deadlock。 A deadlock occurs when two tasks need data from each other to continue； which means neither can continue。 Running code in parallel requires coding discipline； as you will learn in this chapter。 We will use only a single project； named JugglingTasks； which is a console application that imple ments the techniques demonstrated in this chapter。 Understanding Multitasking One of the biggest jumps in puting history was when the puter went from a machine that carried out a single task to a multitasking machine。 The old modore 64 and VIC…20 were single…tasking machines。 You started the puter； and then the puter waited for you to do something。 If you decided to make the puter run a loop saying “hello” millions of times； that is all the puter would do。 The puter could not do one thing and then do something else while waiting for an answer from the executing task。 How could anyone get anything done？ （On the other hand； you could argue that people might be more productive if they could work in only a single program； and not also check their e…mail； look at the latest blog entry from their favorite author； and so on。） 341 …………………………………………………………Page 364…………………………………………………………… 342 CH AP T E R 1 3 ■ L E A R N I N G A B OU T M U L T I TH R E A DI N G Things changed quite dramatically when puters could multitask。 Server puters running operating systems like Unix were multitasking from day one。 Operating systems like Windows were not 100％ preemptive multitasking until Windows NT and Windows 95。 Notice that I used the term preemptive； which makes a big difference。 Plain…vanilla multitasking is multitasking where individual tasks cooperate and allocate resources as a collective。 It is essentially a dead…end idea； and the best example of how it worked is Windows 3。0 and Windows 3。1 （but these were full of hacks）。 Now let’s see what preemptive multitasking means。 Preemptive Multitasking Preemptive multitasking is when the operating system controls which application does what and when。 Realize that you can run an application because applications are treated as po nents。 You don’t believe me？ Create a console application and rename the method Main（） to Mains（）。 See what happens。 The reality is that the Main（） method； as it is declared； is an API used by the operating system to run your ponent； which masquerades as a program。 So we have this program called the operating system that runs ponents called programs。 The next question is how can multiple programs run at the same time？ The answer is that the operating system program is no ordinary program。 It is the foundation upon which applica tions can be launched and managed。 The operating system hooks into special features of the microprocessor that allow the operating system to time slice the individual programs。 Because your programs are ponents； they will never need to access the microprocessor directly。 Time Slicing Time slicing is when an operating system can dictate for how much time a program is allowed to execute。 Between the times of execution； the program is in a state of deep freeze and does nothing。 You； as a user； are not aware of the time slices； because a time slice operates on the order of microseconds。 Because time slicing is so fast； you think your program is running continuously。 To see the time slicing in action； open the Windows Task Manager and select the Processes tab。 Look closely at the Task Manager window shown in Figure 13…1； and you’ll see that explorer。exe has 6％ of the CPU。 You can say that the explorer。exe application is using the appropriate amount of time slices that equals 6％ of the CPU time。 How the CPU slices the 6％ is a detail of the operating system。 Say you have a program that runs two tasks： task 1 and task 2。 The microprocessor is a single core； and thus when running two separate tasks； there will be two time slices； as illustrated in Figure 13…2。 In the figure； the entire processing cycle is represented as a pie； and each time slice is a slice of the pie。 …………………………………………………………Page 365…………………………………………………………… C HA P TE R 1 3 ■ L E AR N IN G AB O U T M U L T IT HR E AD IN G 343 Figure 13…1。 Windows time slicing in action Figure 13…2。 Single…core microprocessor running two tasks …………………………………………………………Page 366…………………………………………………………… 344 CH AP T E R 1 3 ■ L E A R N I N G A B OU T M U L T I TH R E A DI N G Notice how task 1 and task 2 run in a serial manner； rather than concurrently。 This is because the microprocessor is a single…task device made to look like a multitask device。 You would run multiple tasks on a single…core microprocessor when you want to have application background tasks that should not affect the foreground task of the application。 For example； when running a word processor； you don’t want the spell