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\chapter{Overview of the Program}
\label{overview_of_program}
The MPX Operating System is designed as an exercise to examine various concepts within the theory of operating systems (rather than as a consumer-usable system). It is specially constructed to be started from within MS-DOS, at which time it will ``take over'' the operating system to perform many of its tasks. As MS-DOS is interrupt-driven, it uses interrupt handler routines to perform many low-level services, such as performing I/O, keeping time using the CPU clock, etc. Thus, we can replace the functionality of the interrupt handlers with code we have written by modifying the interrupt vector table.
MPX is written in C. The following files are used:
\begin{enumerate}
\item {\tt mpx.h} --- A C header file containing all of the functions implemented, global variables, and {\tt \#DEFINE}d macros.
\item {\tt clock.c} --- Functions for enabling, configuring, and disabling the clock interrupt, as well as the function that is referenced in the interrupt vector table that is called when a clock interrupt occurs.
\item {\tt com.c} --- Functions for enabling, configuring, and disabling the {\tt COM} interrupts, as well as the function that is referenced in the interrupt vector table and is called when a clock interrupt occurs.
\item {\tt comhan.c} --- The code for the command handler, which is the main system process that prints a prompt and gets a user-inputted command.
\item {\tt dcb.c} --- Functions for dealing with Device Control Blocks (see page \pageref{device_control_block}).
\item {\tt direct.c} --- Functions for accessing MS-DOS directory listings. This code was provided by the project.
\item {\tt io.c} --- Functions for managing the queueing of processes with regard to I/O operations.
\item {\tt main.c} --- Contains {\tt main()}, which is called to perform setup tasks when MPX-OS first boots.
\item {\tt pcb.c} --- Functions for dealing with Process Control Blocks (see page \pageref{process_control_block}), including their queueing operations.
\item {\tt printer.c} --- Functions for enabling, configuring, and disabling the printer interrupts, as well as the function that is referenced in the interrupt vector table and is called when a clock interrupt occurs.
\item {\tt sys\_req.c} --- Contains the code for generating a software interrupt for when a system request is made from within MPX. This code was provided by the project.
\item {\tt sys\_sppt.c} --- System support, including initializing devices, closing devices, and handling system calls.
\end{enumerate}
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