#include "k_defines.h"

inline void outportb(unsigned int port,unsigned char value);

unsigned int panic(char *message);

void k_clear_screen();
unsigned int k_printf(char *message, unsigned int line);
void update_cursor(int row, int col);

k_main() // like main
{
	k_clear_screen();

	k_printf("Welcome to Simplified OS.", 0);
	update_cursor(1, 0);
	k_printf("Right now the kernel is loaded at 1MB physical,/nbut mapped at FF800000 hex.", 3);
	update_cursor(5, 0);
};

void k_clear_screen() // clear the entire text screen
{
	char *vidmem = (char *) 0xb8000;
	unsigned int i=0;
	while(i<(80*2*25))
	{
		vidmem[i]=' ';
		i++;
		vidmem[i]=WHITE_TXT;
		i++;
	};
};

unsigned int k_printf(char *message, unsigned int line)
{
	char *vidmem = (char *) 0xb8000;
	unsigned int i=0;

	i=(line*80*2);

	while(*message!=0) // 24h
	{
		if(*message==0x2F)
		{
			*message++;
			if(*message==0x6e)
			{
				line++;
				i=(line*80*2);
				*message++;
				if(*message==0)
				{
					return(1);
				};
			};
		};
		vidmem[i]=*message;
		*message++;
		i++;
		vidmem[i]=0x7;
		i++;
	};

	return(1);
};

/* ==============  */

inline void outportb(unsigned int port,unsigned char value)  // Output a byte to a port
{
    asm volatile ("outb %%al,%%dx"::"d" (port), "a" (value));
};

/* ==============  */

/* by DF */
void update_cursor(int row, int col)
{
	USHORT	position=(row*80) + col;
	// cursor LOW port to vga INDEX register
	outportb(0x3D4, 0x0F);
	outportb(0x3D5, (UCHAR)(position&0xFF));
	// cursor HIGH port to vga INDEX register
	outportb(0x3D4, 0x0E);
	outportb(0x3D5, (UCHAR)((position>>8)&0xFF));
};

unsigned int panic(char *message) // just a modified version of k_printf
{
	char *vidmem = (char *) 0xb8000;
	unsigned int i=0;
	unsigned int line=0;

	i=(line*80*2);

	while(*message!=0) // 24h
	{
		if(*message==0x2F)
		{
			*message++;
			if(*message==0x6e)
			{
				line++;
				i=(line*80*2);
				*message++;
				if(*message==0)
				{
					return(1);
				};
			};
		};
		vidmem[i]=*message;
		*message++;
		i++;
		vidmem[i]=0x9; // use blue text
		i++;
	};
	return(1);
};

kernel.c


#define WHITE_TXT 0x7
#define BLUE_TXT 0x09

#define UCHAR unsigned char
#define USHORT unsigned short
 

k_defines.h



[BITS 32]

[global start]
[extern _k_main] ; this is in the c file

start:
  ; stop using bootloader GDT, and load new GDT
  lgdt [gdt_ptr]

  mov ax,LINEAR_DATA_SEL
  mov ds,ax
  mov es,ax
  mov ss,ax
  mov fs,ax
  mov gs,ax

  call _k_main

  jmp $ ; crash

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

SECTION .data

gdt:
; NULL descriptor
	dw 0		; limit 15:0
	dw 0		; base 15:0
	db 0		; base 23:16
	db 0		; type
	db 0		; limit 19:16, flags
	db 0		; base 31:24

; unused descriptor
	dw 0
	dw 0
	db 0
	db 0
	db 0
	db 0

LINEAR_DATA_SEL	equ	$-gdt
	dw 0FFFFh
	dw 0
	db 0
	db 92h		; present, ring 0, data, expand-up, writable
	db 0CFh		; page-granular (4 gig limit), 32-bit
	db 0

LINEAR_CODE_SEL	equ	$-gdt
	dw 0FFFFh
	dw 0
	db 0
	db 9Ah		; present,ring 0,code,non-conforming,readable
	db 0CFh		; page-granular (4 gig limit), 32-bit
	db 0
gdt_end:

gdt_ptr:
	dw gdt_end - gdt - 1
	dd gdt


K_entry.asm


OUTPUT_FORMAT("binary")
ENTRY(start)
SECTIONS
{
  .text  0xFF800000 : {
    *(.text)
  }
  .data  : {
    *(.data)
  }
  .bss  :
  { 					
    *(.bss)
  }
}
kernel.ld


gcc -c kernel.c -o kernel.o
nasmw -f aout k_entry.asm -o k_entry.o
ld -T kernel.ld kernel.o k_entry.o
rename a.out kernel.bin

build.bat