Parameter Initialization

When a program starts, many parameters need to be initialized. This document describes the sequence of steps taken by a program, belonging to the VMB project, to initializes all its parameters.

Static Defaults

The variables get initial values when the program is loaded into memory. These values are specified in the variable declaration. For example:


		int port = 9002;

Options Table

Some parameters can be set in configuration files and on the command line. These parameters are specified in a table called options. This table specifies default values for all these parameters, and these defaults are assigned next.

Program Name

Before reading the configuration files and checking the windows registry for parameter settings, the program's name has to be determined. The program name, along with the path to the directory where the program is located, is determined from the first entry in the argument vector argv[0].

Defined Program Name

Because it might be necessary to start the same program, for example a button, with different parameters (color, label, interrupt number, ...), the second entry in the argument vector, argv[1], might give the program a new "defined" name. So you can start the button program two times like this:


		button.exe start
		button.exe stop

In this example, the first invocation will replace the program name "button" by the defined program name "start" and the second invocation will replace "button" by "stop". This new program name will then be used in the following steps to find the right parameters for the program.

Windows Registry

For programs running under the windows operating system, the registry is used to store and load parameter values in order to make them persistent. For example, the position of the program window on the screen is stored in the registry before the program terminates and is restored when the program starts. This improves the usability of the program, because the user does not need to reposition the windows each time the program is started. The parameters and their values are stored in the registry under HKEY_CURRENT_USER/Software/VMB/program name/parameter name. The program name used here is the real or defined program name as described before.

By convention, only parameters that pertain to user interface and user preferences are stored in the registry. Parameters that are essential for the correct working of a device configuration must be specified in the configuration file, which we consider next.

Configuration Files

The details about the content of configuration files can be found in the section "Configuration Files".

Configuration files can be specified on the command line using the -c or --config option. If the command line does not specify a configuration file, the file default.vmb is loaded before processing the command line. If the command line specifies a configuration file, default.vmb is not loaded because loading a configuration file is then part of processing the command line as described in the next step.

Unless the configuration file is given by an absolute file name, the program tries to find the file in the following directories in the order given:

the current working directory,
the directory where the last configuration file was located (could be absolute),
the directory where the executable is located.

Command Line

Last not least, parameters can be set using options on the command line. The possible options are listed in the option table (see above). Since the same option table is used for the command line and for the configuration file, it is possible to change options given in the configuration files by options given on the command line. For each option there is a short form like "-v" and a long form like "--verbose". To get a list of all options you can run the program with the "-?" option (short form) or with the "--help" option (long form). If the command line uses an option to specify a configuration file, options preceeding it are processed first, then the configuration file is processed as if all its options were part of the command line, and finaly the processing of options continues with the command line options following the configuration file option.

Some options, notably -x and -y (to set the position on the screen), can be used to change parameter settings taken normaly from the windows registry. This makes sense for devices (like buttons or LEDs) that are part of the user interface of the simulated assembly; the position on the screen is then more part of the simulation than a user preference.

Final Checks

After processing the command line, the program might perform final checks, to ensure that all parameters have reasonable values. For example, the x and y position of windows is usually checked to make sure the window is not placed "off screen", which would make it difficult to interact with the program.