//-------------------------------------------------------------------
//	vm86blue.cpp
//
//	This short demo program shows the essential steps needed in 
//	order for Linux to execute 16-bit code in Virtual-8086 Mode 
//	(provided no "i/o-sensitive" instructions are encountered).  
//
//	This demo is intended to be run from the text-mode console;
//	no effect will be seen if executed from a graphics console.	
//
//	Since write-access to the '/dev/mem' special file is likely
//	restricted to privileged users, we recommend compiling this 
//	program using:   root# make vm86blue ; chmod a+s vm86blue  
//
//	NOTE: Written and tested using Linux kernel version 2.4.20.
//
//	programmer: ALLAN CRUSE
//	written on: 18 JUL 2003
//	revised on: 23 JUL 2003 -- to delete references to 'errno' 
//-------------------------------------------------------------------

#include <stdio.h>		// for printf(), perror() 
#include <fcntl.h>		// for open() 
#include <stdlib.h>		// for exit() 
#include <unistd.h>		// for lseek(), read() 
#include <sys/mman.h>		// for mmap()
#include <asm/vm86.h>		// for struct vm86_struct
#include <linux/linkage.h>	// for asmlinkage attribute

#define ELF_LOAD_ADDRESS 0x08048000	// where Linux puts this 

asmlinkage void blue( void );		// no C++ "name-mangling"   
asm("	.code16			");	// generate 16-bit codes
asm("	.align	16		");	// "paragraph" alignment
asm("blue:			");	// label procedure entry
asm("	movw	$0xB800, %ax	");	// address screen memory
asm("	movw	%ax, %es	");	//   using ES register
asm("	xorw	%di, %di	");	// point es:di to screen
asm("	movb	$0x17, %al	");	// white-upon-blue color
asm("	movw	$0x4000, %cx	");	// number of screen elts 
asm("nxpel:			");	// main loop begins here
asm("	movb	%al, %es:1(%di)	");	// write color attribute
asm("	addw	$2, %di		");	// point to next element
asm("	loop	nxpel		");	// process entire screen
asm("	hlt			");	// exit Virtual8086 mode   
asm("	.code32			");	// generate 32-bit codes

int vm86( struct vm86_struct *vm )
{
	int	retval;
	asm(" movl $113, %eax ");	// system-call ID-number
	asm(" movl %0, %%ebx " : : "m" (vm) );	// struct address
	asm(" int $0x80	");		// enter the Linux kernel
	asm(" movl %%eax, %0 " : "=m" (retval) ); // store retval 
	return	retval;			// return function status
}


struct vm86_struct	vm;		// virtual machine struct


int main( int argc, char **argv )
{
	int	fd, prot, flag;
	
	// setup a memory-mapping for accessing video display memory 	
	void	*vram = (void*)0x000B8000;   // address of 32KB window
	prot = PROT_READ | PROT_WRITE;	// read-write access is needed
      	flag = MAP_FIXED | MAP_SHARED;	// with a fixed-shared mapping 
	if ( ( fd = open( "/dev/mem", O_RDWR ) ) < 0 )
		{ perror( "open /dev/mem" ); exit(1); }
	if ( mmap( vram, 0x8000, prot, flag, fd, (int)vram ) == MAP_FAILED )	
		{ perror( "mmap /dev/mem" ); exit(1); }

	// setup 64KB memory-mapping for access to VM86 code and stack  	
	void	*prog = (void*)0x00010000;   // address of 64KB region
	prot = PROT_READ | PROT_WRITE;	// read-write access is needed
      	flag = MAP_FIXED | MAP_SHARED;	// with a fixed-shared mapping 
	if ( ( fd = open( "/dev/zero", O_RDWR ) ) < 0 )
		{ perror( "open /dev/zero" ); exit(1); }
	if ( mmap( prog, 0x10000, prot, flag, fd, 0 ) == MAP_FAILED )	
		{ perror( "mmap /dev/zero" ); exit(1); }

	// now "load" our program code into this memory-mapped region  
	if ( ( fd = open( argv[0], O_RDONLY ) ) < 0 ) 
		{ perror( argv[0] ); exit(1); }
	int	progsize = lseek( fd, 0, SEEK_END );
	if ( lseek( fd, 0, SEEK_SET ) != 0 ) { perror( "lseek" ); exit(1); }
	if ( read( fd, prog, progsize ) != progsize ) 
		{ perror( "read" ); exit(1); }
	
	// compute offset-address for the 16-bit code's entry-point
	int	initial_IP = (int)blue - ELF_LOAD_ADDRESS;
	int	initial_CS = ((int)prog) >> 4;	// segment-address
	
	// initialize data-structure for entering to Virtual-8086 Mode
	vm.regs.eip = initial_IP;	// initial instruction offset 
	vm.regs.cs = initial_CS;	// segment-address for code
	vm.regs.eflags = (1<<9);	// IF=1, IOPL=0
	vm.regs.ss = initial_CS;	// segment-address for stack
	vm.regs.esp = 0x10000;		// initial stacktop offset
	vm.cpu_type = CPU_386;		// specify required processor

	// execute in Virtual-8086 Mode (until 'hlt' is encountered)
	vm86( &vm );			// execute the 'blue' routine	
	printf( "\nReturned successfully from Virtual-8086 Mode\n\n" );
}