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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。）
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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。
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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
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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