//-------------------------------------------------------------------
//	newzero.cpp
//
//	This program temporarily alters some register-values within   
//	the VGA's Graphics Controller and Timer-Sequencer, in order
//	to gain access to the portion of Video Display Memory where 
//	the VGA character-generator's glyph-tables reside while the
//	system is executing in text mode, so it can install there a 
//	new image for the zero chartacter ('0') whose ascii-code is
//	equal to 0x30.  Thus for proper operation this demo-program 
//	must be executed from a text-mode console, rather than from
//	a window in the graphical desktop environment.  Note that a
//	separate task must run to restore the normal-looking zero.) 
//
//	  compile using:  $ g++ newzero.cpp int86.cpp -o newzro
//
//	Reference:  Richard Wilton, "Programmer's Guide to PC Video 
//	Systems (Second Edition)," Microsoft Press (1994), 294-299.
//
//	programmer: ALLAN CRUSE
//	written on: 04 SEP 2003
//	revised on: 07 SEP 2005 - replaced 'mmap()' by 'init8086()'
//-------------------------------------------------------------------

#include <stdio.h>	// for printf()  
#include <sys/io.h>  	// for outw() 
#include "int86.h"	// for init8086(), int86()

#define TSEQ_PORT 0x3C4		// for i/o to the VGA Timer-Sequencer
#define GRAF_PORT 0x3CE		// for i/o to the Graphics Controller
#define VRAM_BASE 0x000A0000	// address of the VGA graphics memory

unsigned char	*vram = (unsigned char*)VRAM_BASE;
struct vm86plus_struct	vm;

unsigned char newglyph[ 16 ] = 	{
				0x00,	// 00000000
				0x00,	// 00000000
				0x7C,	// 01111100
				0x82,	// 10000010
				0x82,	// 10000010
				0x82,	// 10000010
				0x82,	// 10000010
				0x82,	// 10000010
				0x82,	// 10000010
				0x82,	// 10000010
				0x82,	// 10000010
				0x7C,	// 01111100
				0x00,	// 00000000
				0x00,	// 00000000
				0x00,	// 00000000
				0x00	// 00000000
				};




int main( int argc, char **argv )
{
	// display some sample text that has a few zeros within it
	printf( "\e[H\e[J\n\n" );	// start with a clear screen
	printf( "\nOK, a few facts every computer student should know:\n" );
	printf( "\nTest message #1: one kilobyte equals 1024 bytes" );
	printf( "\nTest message #2: one megabyte equals 1024 kilobytes" );
	printf( "\nTest message #3: one gigabyte equals 1024 megabytes" );
	printf( "\nTest message #4: one terabyte equals 1024 gigabytes" );
	printf( "\n\nNow, to see a new glyph for zero, press <ENTER> " );
	getchar();
	
	// now setup 1-MB memory-mapping and direct I/O privileges
	init8086();			

	// prologue (makes character ram accessible to the cpu)
	outw( 0x0100, TSEQ_PORT );	// enter synchronous reset
	outw( 0x0402, TSEQ_PORT );	// write only to map 2
	outw( 0x0704, TSEQ_PORT );	// use sequential addressing
	outw( 0x0300, TSEQ_PORT );	// leave synchronous reset
	outw( 0x0204, GRAF_PORT );	// select map 2 for reads
	outw( 0x0005, GRAF_PORT );	// disable odd-even addressing
	outw( 0x0006, GRAF_PORT );	// map starts at 0xA000:0000

	// load our new image for '0' into character generator ram 
	int		zlocn = 0x30 * 32;
	for (int i = 0; i < 16; i++) vram[ zlocn + i ] = newglyph[ i ];
	
	// epilogue (makes character ram inaccessible to the cpu)
	outw( 0x0100, TSEQ_PORT );	// enter synchronous reset
	outw( 0x0302, TSEQ_PORT );	// write to maps 0 and 1
	outw( 0x0304, TSEQ_PORT );	// use odd-even addressing
	outw( 0x0300, TSEQ_PORT );	// leave synchronous reset
	outw( 0x0004, GRAF_PORT );	// select map 0 for reads
	outw( 0x1005, GRAF_PORT );	// enable odd-even addressing
	outw( 0x0E06, GRAF_PORT );	// map starts at 0xB800:0000
}