when running a word processor； you don’t want the spell checker slowing down your typing。 The operating system will still allocate time slices in this case and use preemptive multitasking。 Figure 13…3 illustrates how the same application executes on a multiple…core micropro cessor。 Did you notice what happened？ The operating system； in a bid to make more efficient use of the microprocessor； has put one task on one core and another task on the other core。 Now both tasks are running in parallel。 And now it’s possible that both tasks would want to manipulate the same piece of data at the same time。 In a single…core microprocessor； that is not physically possible。 However； with a single processor； it is still possible for one task to be interrupted mid…flight while accessing some data； and for the other task to step in and use inconsistent data。 Figure 13…3。 Multiple…core microprocessor running two tasks If you have a multitasking application； you must be careful how you manipulate global state in the application。 If you are not careful； you could corrupt the state of your program。 ■Note As panies like Intel and AMD introduce microprocessors with more and more cores； it is your job as a software developer to write software that can make use of those cores。 This is not always easy and requires some forethought； since you need to work out the logic。 For example； if you are processing data that is sent to a file； you can’t read the file before the data has been processed。 Using Threads Tasks can run in two ways on the Windows operating system： via threads and via processes。 A process is when you start a program； causing the operating system to instantiate resources …………………………………………………………Page 367…………………………………………………………… 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 345 and time slices for the program。 A thread can be seen as a lightweight process that executes in the context of a process。 All processes will start a single thread。 The thread that is executed as part of the process is the main thread； and when it exits； so does your application。 If the main thread creates other threads； when the main thread exits； so do the created background threads。 From an architec tural perspective； when multitasking； the main thread will execute and coordinate the threads that run your code。 Two processes cannot reference each other。 This is so that if one process crashes； it does not cause the other process to crash。 Threads executing in the context of a process have the ability to bring down an entire process。 ■Note You could run multiple processes and then use interprocess munication mechanisms to muni cate。 However； I can’t remend that solution； since you can get the same effect of a process by using a application domain （AppDomain）。 For the most part； you don’t need to concern yourself with AppDomains。 You will use threads； since they are lightweight （pared to a process）； easy to manage； and easy to program。 Creating a New Thread You can create a new thread that will run independently of its originating thread。 In the following example； the main thread creates two other threads。 Each of the threads outputs some text to the console。 Note that the type Thread and other related types used in this chapter’s examples are in the System。Threading namespace。 Imports System。Threading Module Module1 Sub Main（） SimpleThread（） End Sub Sub SimpleThreadTask1（） Console。WriteLine（〃hello there〃） End Sub Sub SimpleThreadTask2（） Console。WriteLine（〃Well then goodbye〃） End Sub Private Sub SimpleThread（） Dim thread1 As New Thread（AddressOf SimpleThreadTask1） Dim thread2 As New Thread（AddressOf SimpleThreadTask2） thread1。Start（） thread2。Start（） End Sub End Module …………………………………………………………Page 368…………………………………………………………… 346 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 To create a thread； you instantiate the Thread type and call the Start（） method。 The Thread type is a class that contains all of the functionality necessary to start and control a multitasking thread。 When running a thread； the Thread type needs code to execute。 The solution used by Thread is a delegate； which is passed to Thread via the constructor。 The name of the delegate type is ThreadStart。 The example here does not use the ThreadStart type explicitly； because both threads use programming constructs that do not need a delegate declaration。 Here’s the explicit syntax for pleteness： Dim thread1 As New Thread（New ThreadStart（AddressOf SimpleThreadTask1）） Dim thread2 As New Thread（New ThreadStart（AddressOf SimpleThreadTask2）） Running the thread example； you may see this output： well then goodbye hello there Notice how hello there is after well then goodbye。 The output implies that the second thread （thread2） starts before the first thread （thread1）。 However； your output might be the opposite； which demonstrates the true nature of threading； concurrency； and why threading is so difficult。 Imagine for a moment that the sample thread code were not multithreaded； but executed in a serial manner。 Calling thread1。Start（） and then thread2。Start（） results in the text hello there being first