Shared files specific for drivers
Driving an LCD display is not easy; you need to address ports, to send bytes in a certain order, to respect timing, and unfortunately no two operating system let you do this in the same way. But don't despair! There's hope! Someone in a galaxy far far away, has Already done the dirty job for you! This dirty job has been put in shared files. These shared files are full cross platform and are auto-magically configured by the configure script. You only need to include them and use their functions to benefit from them.
These files are provided only for drivers, others are provided for all of LCDproc. These files are located in the shared directory, they have a dedicated chapter in this book.

port.h : Parallel port I/O
The file port.h, located in the server/drivers/ directory provide Input/Output and port permissions for the PC compatible parallel port, also known as the LPT port.
Of course, these functions will only work if the computer where LCDproc runs has parallel port! In these situations, the configure script will see this and disable drivers that need a parallel port.
The functions in port.h are defined and as 'static inline'. Therefore each driver including this header file gets its own copy of the functions and they are inlined into the driver's code. As a result calls to port_in() and port_out() are directly translated to inb() or outb() or assembly code by the compiler. There is less to no overhead in using them.
port.h file defines 6 static inline functions for port I/O:

Read a byte from port
static inline int port_in unsigned short int port
Returns the content of the byte.

Write a char(byte) 'val' to port
static inline void port_out unsigned short int port unsigned char val
Returns nothing (void).

Get access to a specific port
static inline int port_access unsigned short int port
Returns 0 if successful, -1 if failed.

Close access to a specific port
static inline int port_deny unsigned short int port
Returns 0 if successful, -1 if failed.

Get access to multiple sequential ports
static inline int port_access_full unsigned short int port unsigned short int count
Returns 0 if successful, -1 if failed.

Close access to multiple sequential ports
static inline int port_deny_full unsigned short int port unsigned short int count
Returns 0 if successful, -1 if failed.

Example use

#include "port.h"

/*
 * Get access to these 3 ports:
 *  0x378 (CONTROL),
 *  0x379 (STATUS) and
 *  0x37A (DATA)
 */
if (port_access_multiple(0x378, 3) == -1) {
	/* Access denied, do something */
}

/* Write a 'A' to the control port */
port_out(0x378, 'A');

/* Read from the status port */
char status = port_in(0x379);

/* Close the 3 ports */
port_deny_multiple(0x378, 3);

adv_bignum.h : Write Big-Numbers
adv_bignum.h is the headerfile for libbignum.a (made from adv_bignum.c) which contains everything needed to show big-numbers, including the fonts for the different displays. (All files are located in the server/drivers/ directory.)
There are only a few requirements to the calling driver:
The following functions have to be implemented by the driver:
height() 	 to determine the display's height and thus the maximal height of the big numbers to be displayed.
get_free_chars() 	 to determine the number of user-definable characters that can be used in the generation of big numbers.
set_char() 	 to define a character necessary to write a big number. Of course this is only necessary if there really are user-definable characters, i.e. only if get_free_chars() returns a value greater 0.
chr() 	 to actually write the characters the big numbers consist of.
The display's cellwidth has to be 5 (6 works also in some cases) and the cellheight 7 or 8.
The custom-characters (if any) have to be at character positions offset+0, offset+1, offset+2, ... offset+ get_free_chars()-1,
offset+ get_free_chars()-1 must be less than 32,
The library determines the correct font, depending on the display size and the number of user-defined characters itself. So it is easy to integrate into the driver.

Provided Functions
void lib_adv_bignum Driver *drvthis int x int num int offset int do_init
The main thing the driver has to do is to call this function from its num() function with the parameters described below.
drvthis 	 the pointer pointing to the Driver structure passed to the driver's num() function.
x 	 the horizontal position of the top-left corner of the big-number (the big-numbers don't have a y position). The placing of the characters is done by the client, so the driver only has to forward the position to the lib. The bignumlib has no influence on the placing of the characters.
num 	 the number (legal: 0 - 9, and :) to be written.
offset 	 the character position where the user-definable characters start (usually 0). The user-definable characters (if any) are then expected to be at the character positions offset+0, offset+1, offset+2, ... offset+ get_free_chars()-1 and offset+ get_free_chars()-1 is required to be less than 32.
do_init 	 if not 0, lib_adv_bignum will set the custom characters of the display for the big-numbers.
The driver has to check if the custom-characters have to be set or if it is already done and tell it to the lib (using the do_init parameter). The common way is to use variable called p->ccmode or similar. In the different drivers there are some differences in the naming and handling of this variable. So the responsibility of checking and setting is left to the driver.

Calling lib_adv_bignum()

#include "adv_bignum.h"

MODULE_EXPORT void
myDriver_num( Driver * drvthis, int x, int num )
{
  PrivateData *p = drvthis->private_data;
  int do_init = 0;

  if (p->ccmode != CCMODE_BIGNUM){     // Are the custom-characters set up correctly? If not:
    do_init = 1;                       // Lib_adv_bignum has to set the custom-characters.
    p->ccmode = CCMODE_BIGNUM;         // Switch custom-charactermode to bignum.
  }

  // Lib_adv_bignum does everything needed to show the big-numbers.
  lib_adv_bignum(drvthis, x, num, 0, do_init);
}

All that's left to do is to add libbignum.a to the libs and adv_bignum.h sources of your driver in the Makefile (or the file that generates the Makefile).

Enabling adv_bignum support in Makefile.am

myDriver_LDADD: libLCD.a libbignum.a

myDriver_SOURCES: lcd.h lcd_lib.h myDriver.c myDriver.h adv_bignum.h

Internal Structure and Functions
The only purpose of lib_adv_bignum() is to determine the best display-dependent big-number function, based upon the values of the driver's height() and get_free_chars() functions, and call it.
The display-dependent functions are named adv_bignum_num_N_M(), where N is the display's height in lines and M the number of used user-definable characters. The bits of the user-characters are stored in static char bignum (take a look at the source and you will see what I mean). (On a display with a cellheight of 7 the lowest line stored is not shown.) While static char num_map defines the placing in the big number. (A big number is always 3 characters wide and 4 characters high. On a big number for 2 line displays the 2 lower lines are not in use.)
If user-definable characters have to be set, the driver's set_char() function will be called once for every user-definable character.
Now adv_bignum_write_num() is called. This function places the 6 or 12 characters the big-number consists of in the framebuffer using the drivers chr() function.