//-------------------------------------------------------------------
//	backward.cpp
//
//	This text-mode demo must be executed with 'root' privileges.
//
//	programmer: ALLAN CRUSE
//	written on: 04 SEP 2003
//-------------------------------------------------------------------

#include <stdio.h>	// for printf(), perror() 
#include <fcntl.h>	// for open() 
#include <stdlib.h>	// for exit() 
#include <sys/mman.h>	// for mman() 
#include <sys/io.h>  	// for iopl() 

int main( int argc, char **argv )
{
	int	fd = open( "/dev/mem", O_RDWR );
	if ( fd < 0 ) { perror( "/dev/mem" ); exit(1); }
	int	base = 0x000A0000, size = (128<<10); // 128K
	int	prot = PROT_READ | PROT_WRITE;
	int	flag = MAP_FIXED | MAP_SHARED;
	void	*mm = (void*)base;
	if ( mmap( mm, size, prot, flag, fd, base ) == MAP_FAILED )
		{ perror( "mmap" ); exit(1); }
	if ( iopl( 3 ) ) { perror( "iopl" ); exit(1); }

	// prologue
	outw( 0x0100, 0x3C4 );	// enter synchronous reset
	outw( 0x0402, 0x3C4 );	// write only to map 2
	outw( 0x0704, 0x3C4 );	// use sequential addressing
	outw( 0x0300, 0x3C4 );	// leave synchronous reset
	outw( 0x0204, 0x3CE );	// select map 2 for reads
	outw( 0x0005, 0x3CE );	// disable odd-even addressing
	outw( 0x0006, 0x3CE );	// map starts at 0xA000:0000
	
	// modify the contents of character generator ram
	unsigned char	*vram = (unsigned char*)base;
	for (int i = 0; i < 32*256; i++)
		{
		unsigned char	orig = vram[ i ], revs = 0;
		for (int j = 0; j < 8; j++)
			if ( orig & (1<<j) ) revs |= 1<<(7-j);
		vram[ i ] = revs;
		}

	// epilogue
	outw( 0x0100, 0x3C4 );	// enter synchronous reset
	outw( 0x0302, 0x3C4 );	// write to maps 0 and 1
	outw( 0x0304, 0x3C4 );	// use odd-even addressing
	outw( 0x0300, 0x3C4 );	// leave synchronous reset
	outw( 0x0004, 0x3CE );	// select map 0 for reads
	outw( 0x1005, 0x3CE );	// enable odd-even addressing
	outw( 0x0E06, 0x3CE );	// map starts at 0xB800:0000
}