//-------------------------------------------------------------------
//	bitfield.c
//
//	This program investigates the GNU compiler's arrangement of
//	bitfields within a 'struct', something outside the language 
//	standards for the C programming language, but important for
//	properly accessing several hardware-defined field-elements.   
//
//		compile using:  $ gcc bitfield.c -o bitfield
//		execute using:  $ ./bitfield
//
//	programmer: ALLAN CRUSE
//	written on: 07 APR 2008
//-------------------------------------------------------------------

#include <stdio.h>	// for printf()  


typedef struct	{
		unsigned char	hlen:4;		// bits 3..0
		unsigned char	vers:4;		// bits 7..4
		} IP_BYTE;


typedef struct	{
		unsigned int	desc_status:20;	// bits 0..19
		unsigned int	desc_errors:12;	// bits 20..31
		} TXD_EXT;


int main( int argc, char **argv )
{
	IP_BYTE	ipb;
	TXD_EXT	txd;	

	ipb.vers = 4;
	ipb.hlen = 5;

	txd.desc_status = 0x00001;	// DD-bit set
	txd.desc_errors = 0xFFF;	// all bits set

	printf( "IP_BYTE: 0x%02X \n", *(unsigned char *)&ipb ); 
	printf( "TXD_EXT: 0x%08X \n", *(unsigned int *)&txd ); 
}