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Microsoft Visual Basic provides resources for managing threads and semaphores. This module will give a basic overview of threads and semaphores in Visual Basic (VB). This module presents threads and semaphores in VB in a very simplistic manner. There are much better ways to use threads in VB but they will be presented here in a way that makes the programs much easier to write. The threads described here will be methods of the Form class. This is what makes the programming very simple. Threads should/could be classes on their own but it is not necessary. This module is not intended to make you proficient in using threads and semaphores in Visual Basic.
It would be helpful to have completed the TSK and SEM modules for the DSP/BIOS operating system but it is not necessary if you are familiar with tasks and semaphores.
A thread is simply a separate method or function that gets executed apart from the main program flow. There can be many threads executing simultaneously in a system. In VB the
System.Threading namespace needs to be imported into the program. This is done by putting the following at the very top of your code:
Imports System.Threading
If this is not included at the top of your program then it will not know what threads are.
The following is the basic structure of your program for this lab. Each part of the example code is described in the comments.
' This imports the threading namespace so the program knows about threads
Imports System.Threading' This is the class that executes the form for the programPublic Class Form1
' This is a definition of a thread. The thread object is thread1 and' it uses the method Task1() below as its code.
Dim thread1 As New Thread(AddressOf Task1)' When the form program starts or is loaded the following function is' called. This is where you can put the commands to set up our program.
' The code for starting up the threads can be put here.Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load' Make this a background thread so it automatically
' aborts when the main program stops.thread1.IsBackground = True
' Set the thread prioritythread1.Priority = ThreadPriority.Lowest
' This command starts the thread. The method Task1() then' starts to execute.
thread1.Start()End SubPrivate Sub Task1()
' This is where your code goes for this threadEnd SubEnd Class
The real-time operating system in Visual Studio is not a deterministic operating system. This means that you cannot be sure of how the threads will execute. Windows assigns time to threads at the same priority in a round-robin fashion. When threads at a particular level do not need to execute the operating system assigns time to lower priority threads.
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