//-------------------------------------------------------------------
//	cgademo.cpp
//
//	This program demonstrates early IBM-PC graphics programming
//	with 32KB of installed video memory (real address 0xB8000),
//	supporting either 2-colors or 4-colors using "packed-pixel"
//	memory organization, with "interlaced" scanline addressing.
//	(Supported by VGA/SVGA devices for backward compatibility.)
//
//	  compile using:  $ g++ cgademo.cpp int86.cpp -o cgademo
//
//	programmer: ALLAN CRUSE
//	written on: 01 SEP 2005
//-------------------------------------------------------------------

#include <stdio.h>	// for printf(), getchar() 
#include <string.h>	// for strlen(), strncpy()
#include "int86.h"	// for init8086(), int86() 

#define VRAM_BASE_FOR_CGA 0x000B8000

typedef unsigned char	u8;

u8	*vram = (u8*)VRAM_BASE_FOR_CGA;
void	(*set_pixel)( int x, int y, int color );
struct vm86plus_struct	vm;

void set_pixel_1bpp( int x, int y, int color )
{
	int	posn = (y % 2)*0x2000 + (y / 2)*80 + (x / 8); 
	int	mask = 0x80 >> (x % 8);

	color &= 1;
	color <<= 7;
	color >>= (x % 8);

	u8	temp = vram[ posn ];
	temp &= ~mask;
	temp |= color;
	vram[ posn ] = temp;
}

void set_pixel_2bpp( int x, int y, int color )
{
	int	posn = (y % 2)*0x2000 + (y / 2)*80 + ((2 * x) / 8); 
	int	mask = 0xC0 >> ((2 * x) % 8);

	color &= 3;
	color <<= 6;
	color >>= ((2 * x) % 8);

	u8	temp = vram[ posn ];
	temp &= ~mask;
	temp |= color; 	
	vram[ posn ] = temp;
}


int main( int argc, char **argv )
{
	void show_text_string( int bpp, int color, char *message );

	int	hres, vres, minx, midx, maxx, miny, midy, maxy;
	int	x, y, color, radius_squared;

	printf( "\e[H\e[J\n" );		// avoids getchar jerk
	init8086();			// enable BIOS service
	//--------------------------------------------------
	// Demonstrate CGA monochrome graphics (IBM mode 6)
	//--------------------------------------------------
	vm.regs.eax = 0x0006;		// Set Display Mode 6
	int86( 0x10, vm );		// invoke BIOS service
	hres = 640, vres = 200, set_pixel = set_pixel_1bpp;
	
	// draw screen border
	minx = 0, maxx = hres-1;
	miny = 0, maxy = vres-1;
	x = y = 0; 	color = 1; // white
	do { set_pixel( x, y, color ); ++x; } while ( x < maxx );
	do { set_pixel( x, y, color ); ++y; } while ( y < maxy );
	do { set_pixel( x, y, color ); --x; } while ( x > minx );
	do { set_pixel( x, y, color ); --y; } while ( y > miny );
	getchar();

	// draw explanatory title
	show_text_string( 1, color, "CGA monochrome graphics" );
	getchar();

	// fill circle (unrescaled)
	midx = hres/2; midy = vres/2; radius_squared = 60*60;
	for (y = 4; y <= maxy-4; y++) for (x = 4; x <= maxx-4; x++)
		{
		int	dx = x - midx;
		int	dy = y - midy;
		int	dist_squared = dx*dx + dy*dy;
		color = ( dist_squared < radius_squared ) ? 1 : 0;
		set_pixel( x, y, color );
		}	
	getchar();

	// fill circle (adjusted)
	radius_squared *= 60;
	for (y = 4; y <= maxy-4; y++) for (x = 4; x <= maxx-4; x++)
		{
		int	dx = (x - midx)*5;
		int	dy = (y - midy)*12;
		int	dist_squared = dx*dx + dy*dy;
		color = ( dist_squared < radius_squared ) ? 1 : 0;
		set_pixel( x, y, color );
		}	
	getchar();

	//---------------------------------------------
	// Demonstrate CGA color graphics (IBM mode 5)
	//---------------------------------------------
	vm.regs.eax = 0x0005;		// Set Display Mode 5
	int86( 0x10, vm );		// invoke BIOS service
	hres = 320, vres = 200, set_pixel = set_pixel_2bpp;
	
	// draw screen border
	color = 3;	// white
	minx = 0, maxx = hres-1;
	miny = 0, maxy = vres-1;
	x = y = 0;
	do { set_pixel( x, y, color ); ++x; } while ( x < maxx );
	do { set_pixel( x, y, color ); ++y; } while ( y < maxy );
	do { set_pixel( x, y, color ); --x; } while ( x > minx );
	do { set_pixel( x, y, color ); --y; } while ( y > miny );
	getchar();

	// draw explanatory title
	show_text_string( 2, color, "CGA color graphics" );
	getchar();

	// fill circle (unrescaled)
	midx = hres/2; midy = vres/2; 
	radius_squared = 60*60;
	for (y = 4; y <= maxy-4; y++) 
	for (x = 4; x <= maxx-4; x++)
		{
		int	dx = x - midx;
		int	dy = y - midy;
		int	dist_squared = dx*dx + dy*dy;
		color = ( dist_squared < radius_squared ) ? 1 : 2;
		set_pixel( x, y, color );
		}	
	getchar();

	// fill circle (adjusted)
	radius_squared *= 30;
	for (y = 4; y <= maxy-4; y++) 
	for (x = 4; x <= maxx-4; x++)
		{
		int	dx = (x - midx)*5;
		int	dy = (y - midy)*6;
		int	dist_squared = dx*dx + dy*dy;
		color = ( dist_squared < radius_squared ) ? 1 : 2;
		set_pixel( x, y, color );
		}	
	getchar();

	// return the display to standard text-mode
	vm.regs.eax = 0x0003;		// Set Display Mode 3
	int86( 0x10, vm );		// invoke BIOS service
	printf( "finished\n" );
}


void show_text_string( int bpp, int color, char *message )
{
	// compute message's length and screen-centering parameters
	int	msglen = strlen( message );
	int	col = (80/bpp - msglen)/2;
	int	row = 12;
	int	rowcol = (row << 8) | col;
	
	// copy the message to an 8086-addressable memory-location 
	strncpy( (char*)0x00018000, message, msglen );	
	
	// so a standard BIOS service can put that message on-screen
	vm.regs.eax = 0x1300;		// BIOS drawstring function
	vm.regs.es  = 0x1000;		// string's segment-address
	vm.regs.ebp = 0x8000;		// string's offset-address
	vm.regs.ecx = msglen;		// string's size (in bytes)
	vm.regs.ebx = color;		// string's foreground color
	vm.regs.edx = rowcol;		// string's screen-coordinates
	int86( 0x10, vm );  		// invoke ROM-BIOS service
}