Writing a Linux Driver

The concept of an operating system (OS) must be well understood before any attempt to navigate inside it is made. Several definitions are available for an OS:

It is also very important to clearly distinguish a program from a process. A program is a block of data plus instructions, which is stored in a file on disk and is ready to be executed. On the other hand, a process is an image in memory of the program which is being executed. This difference is highly important, because usually the processes are running under OS control. Here, our program is the OS, so we cannot speak about processes.

We will use the term kernel to refer to the main body of the OS, which is a program written in the C language. The program file may be named vmlinuz, vmlinux or zImage, and has some things in common with the MS-DOS files COMMAND.COM, MSDOS.SYS and IO.SYS, although their functionality is different. When we discuss compilation of the kernel, we mean that we will edit the source files in order to generate a new kernel.

Peripheral or internal devices allow users to communicate with the computer. Examples of devices are: keyboards, monitors, floppy and hard disks, CD-ROMs, printers, mice (serial/parallel), networks, modems, etc. A driver is the part of the OS that manages communication with devices; thus, they are usually called device drivers.

Figure 1 shows the relation between user programs, the OS and the devices. Differences between software and hardware are clearly specified in this scheme. At the left side, user programs may interact with the devices (for example, a hard disk) through a set of high-level library functions. For example, we can open and write to a file of the hard disk calling the C library functions fopen, fprintf and close:

FILE *fid=fopen("filename", "w");
fprintf(fid, "Hello, world!");
fclose(fid);

The user can also write to a file (or to another device such as a printer) from the OS shell, using commands such as:

echo "Hello, world!" >
echo "Hello, world!" > /dev/lp

To execute this command, both the shell and the library functions perform a call to a low level function of the OS, e.g., open(), write() or close():

fid = open("/dev/lp", O_WRONLY);
write(fid, "Hello, world!");
close(fid);

Each device can be referred to as a special file named /dev/*. Internally, the OS is composed of a set of drivers, which are pieces of software that perform the low-level communication with each device. At this execute level, the kernel calls driver functions such as lp_open() or lp_write().

On the right side of Figure 1, the hardware is composed of the device (a video display or an Ethernet link) plus an interface (a VGA card or a network card). Finally, the device driver is the physical interface between the software and the hardware. The driver reads from and writes to the hardware through ports (memory addresses where the hardware links physically), using the internal functions out_p and in_p:

out_p(0x3a, 0x1f);
data = in_p(0x3b);

Note that these functions are not available to the user. Since the Linux kernel runs in protected mode, the low memory addresses, where the ports addresses reside, are not user accessible. Functions equivalent to the low-level functions in and out do not exist in the high-level library, as in other operating systems such as MS-DOS.