4. Examples

/* multiboot.h - the header for Multiboot */
/* Copyright (C) 1999, 2001  Free Software Foundation, Inc.
   
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2 of the License, or
   (at your option) any later version.
   
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.
   
   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */

/* Macros. */

/* The magic number for the Multiboot header. */
#define MULTIBOOT_HEADER_MAGIC          0x1BADB002

/* The flags for the Multiboot header. */
#ifdef __ELF__
# define MULTIBOOT_HEADER_FLAGS         0x00000003
#else
# define MULTIBOOT_HEADER_FLAGS         0x00010003
#endif

/* The magic number passed by a Multiboot-compliant boot loader. */
#define MULTIBOOT_BOOTLOADER_MAGIC      0x2BADB002

/* The size of our stack (16KB). */
#define STACK_SIZE                      0x4000

/* C symbol format. HAVE_ASM_USCORE is defined by configure. */
#ifdef HAVE_ASM_USCORE
# define EXT_C(sym)                     _ ## sym
#else
# define EXT_C(sym)                     sym
#endif

#ifndef ASM
/* Do not include here in boot.S. */

/* Types. */

/* The Multiboot header. */
typedef struct multiboot_header
{
  unsigned long magic;
  unsigned long flags;
  unsigned long checksum;
  unsigned long header_addr;
  unsigned long load_addr;
  unsigned long load_end_addr;
  unsigned long bss_end_addr;
  unsigned long entry_addr;
} multiboot_header_t;

/* The symbol table for a.out. */
typedef struct aout_symbol_table
{
  unsigned long tabsize;
  unsigned long strsize;
  unsigned long addr;
  unsigned long reserved;
} aout_symbol_table_t;

/* The section header table for ELF. */
typedef struct elf_section_header_table
{
  unsigned long num;
  unsigned long size;
  unsigned long addr;
  unsigned long shndx;
} elf_section_header_table_t;

/* The Multiboot information. */
typedef struct multiboot_info
{
  unsigned long flags;
  unsigned long mem_lower;
  unsigned long mem_upper;
  unsigned long boot_device;
  unsigned long cmdline;
  unsigned long mods_count;
  unsigned long mods_addr;
  union
  {
    aout_symbol_table_t aout_sym;
    elf_section_header_table_t elf_sec;
  } u;
  unsigned long mmap_length;
  unsigned long mmap_addr;
} multiboot_info_t;

/* The module structure. */
typedef struct module
{
  unsigned long mod_start;
  unsigned long mod_end;
  unsigned long string;
  unsigned long reserved;
} module_t;

/* The memory map. Be careful that the offset 0 is base_addr_low
   but no size. */
typedef struct memory_map
{
  unsigned long size;
  unsigned long base_addr_low;
  unsigned long base_addr_high;
  unsigned long length_low;
  unsigned long length_high;
  unsigned long type;
} memory_map_t;

#endif /* ! ASM */

/* boot.S - bootstrap the kernel */
/* Copyright (C) 1999, 2001  Free Software Foundation, Inc.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2 of the License, or
   (at your option) any later version.
 
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.
 
   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */

#define ASM     1
#include <multiboot.h>
        
        .text

        .globl  start, _start
start:
_start:
        jmp     multiboot_entry

        /* Align 32 bits boundary. */
        .align  4
        
        /* Multiboot header. */
multiboot_header:
        /* magic */
        .long   MULTIBOOT_HEADER_MAGIC
        /* flags */
        .long   MULTIBOOT_HEADER_FLAGS
        /* checksum */
        .long   -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)
#ifndef __ELF__
        /* header_addr */
        .long   multiboot_header
        /* load_addr */
        .long   _start
        /* load_end_addr */
        .long   _edata
        /* bss_end_addr */
        .long   _end
        /* entry_addr */
        .long   multiboot_entry
#endif /* ! __ELF__ */

multiboot_entry:
        /* Initialize the stack pointer. */
        movl    $(stack + STACK_SIZE), %esp

        /* Reset EFLAGS. */
        pushl   $0
        popf

        /* Push the pointer to the Multiboot information structure. */
        pushl   %ebx
        /* Push the magic value. */
        pushl   %eax

        /* Now enter the C main function... */
        call    EXT_C(cmain)

        /* Halt. */
        pushl   $halt_message
        call    EXT_C(printf)
        
loop:   hlt
        jmp     loop

halt_message:
        .asciz  "Halted."

        /* Our stack area. */
        .comm   stack, STACK_SIZE
        

/* kernel.c - the C part of the kernel */
/* Copyright (C) 1999  Free Software Foundation, Inc.
   
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2 of the License, or
   (at your option) any later version.
   
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.
   
   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */

#include <multiboot.h>

/* Macros. */

/* Check if the bit BIT in FLAGS is set. */
#define CHECK_FLAG(flags,bit)   ((flags) & (1 << (bit)))

/* Some screen stuff. */
/* The number of columns. */
#define COLUMNS                 80
/* The number of lines. */
#define LINES                   24
/* The attribute of an character. */
#define ATTRIBUTE               7
/* The video memory address. */
#define VIDEO                   0xB8000

/* Variables. */
/* Save the X position. */
static int xpos;
/* Save the Y position. */
static int ypos;
/* Point to the video memory. */
static volatile unsigned char *video;

/* Forward declarations. */
void cmain (unsigned long magic, unsigned long addr);
static void cls (void);
static void itoa (char *buf, int base, int d);
static void putchar (int c);
void printf (const char *format, ...);

/* Check if MAGIC is valid and print the Multiboot information structure
   pointed by ADDR. */
void
cmain (unsigned long magic, unsigned long addr)
{
  multiboot_info_t *mbi;
  
  /* Clear the screen. */
  cls ();

  /* Am I booted by a Multiboot-compliant boot loader? */
  if (magic != MULTIBOOT_BOOTLOADER_MAGIC)
    {
      printf ("Invalid magic number: 0x%x\n", (unsigned) magic);
      return;
    }

  /* Set MBI to the address of the Multiboot information structure. */
  mbi = (multiboot_info_t *) addr;

  /* Print out the flags. */
  printf ("flags = 0x%x\n", (unsigned) mbi->flags);

  /* Are mem_* valid? */
  if (CHECK_FLAG (mbi->flags, 0))
    printf ("mem_lower = %uKB, mem_upper = %uKB\n",
            (unsigned) mbi->mem_lower, (unsigned) mbi->mem_upper);

  /* Is boot_device valid? */
  if (CHECK_FLAG (mbi->flags, 1))
    printf ("boot_device = 0x%x\n", (unsigned) mbi->boot_device);
  
  /* Is the command line passed? */
  if (CHECK_FLAG (mbi->flags, 2))
    printf ("cmdline = %s\n", (char *) mbi->cmdline);

  /* Are mods_* valid? */
  if (CHECK_FLAG (mbi->flags, 3))
    {
      module_t *mod;
      int i;
      
      printf ("mods_count = %d, mods_addr = 0x%x\n",
              (int) mbi->mods_count, (int) mbi->mods_addr);
      for (i = 0, mod = (module_t *) mbi->mods_addr;
           i < mbi->mods_count;
           i++, mod++)
        printf (" mod_start = 0x%x, mod_end = 0x%x, string = %s\n",
                (unsigned) mod->mod_start,
                (unsigned) mod->mod_end,
                (char *) mod->string);
    }

  /* Bits 4 and 5 are mutually exclusive! */
  if (CHECK_FLAG (mbi->flags, 4) && CHECK_FLAG (mbi->flags, 5))
    {
      printf ("Both bits 4 and 5 are set.\n");
      return;
    }

  /* Is the symbol table of a.out valid? */
  if (CHECK_FLAG (mbi->flags, 4))
    {
      aout_symbol_table_t *aout_sym = &(mbi->u.aout_sym);
      
      printf ("aout_symbol_table: tabsize = 0x%0x, "
              "strsize = 0x%x, addr = 0x%x\n",
              (unsigned) aout_sym->tabsize,
              (unsigned) aout_sym->strsize,
              (unsigned) aout_sym->addr);
    }

  /* Is the section header table of ELF valid? */
  if (CHECK_FLAG (mbi->flags, 5))
    {
      elf_section_header_table_t *elf_sec = &(mbi->u.elf_sec);

      printf ("elf_sec: num = %u, size = 0x%x,"
              " addr = 0x%x, shndx = 0x%x\n",
              (unsigned) elf_sec->num, (unsigned) elf_sec->size,
              (unsigned) elf_sec->addr, (unsigned) elf_sec->shndx);
    }

  /* Are mmap_* valid? */
  if (CHECK_FLAG (mbi->flags, 6))
    {
      memory_map_t *mmap;
      
      printf ("mmap_addr = 0x%x, mmap_length = 0x%x\n",
              (unsigned) mbi->mmap_addr, (unsigned) mbi->mmap_length);
      for (mmap = (memory_map_t *) mbi->mmap_addr;
           (unsigned long) mmap < mbi->mmap_addr + mbi->mmap_length;
           mmap = (memory_map_t *) ((unsigned long) mmap
                                    + mmap->size + sizeof (mmap->size)))
        printf (" size = 0x%x, base_addr = 0x%x%x,"
                " length = 0x%x%x, type = 0x%x\n",
                (unsigned) mmap->size,
                (unsigned) mmap->base_addr_high,
                (unsigned) mmap->base_addr_low,
                (unsigned) mmap->length_high,
                (unsigned) mmap->length_low,
                (unsigned) mmap->type);
    }
}    

/* Clear the screen and initialize VIDEO, XPOS and YPOS. */
static void
cls (void)
{
  int i;

  video = (unsigned char *) VIDEO;
  
  for (i = 0; i < COLUMNS * LINES * 2; i++)
    *(video + i) = 0;

  xpos = 0;
  ypos = 0;
}

/* Convert the integer D to a string and save the string in BUF. If
   BASE is equal to 'd', interpret that D is decimal, and if BASE is
   equal to 'x', interpret that D is hexadecimal. */
static void
itoa (char *buf, int base, int d)
{
  char *p = buf;
  char *p1, *p2;
  unsigned long ud = d;
  int divisor = 10;
  
  /* If %d is specified and D is minus, put `-' in the head. */
  if (base == 'd' && d < 0)
    {
      *p++ = '-';
      buf++;
      ud = -d;
    }
  else if (base == 'x')
    divisor = 16;

  /* Divide UD by DIVISOR until UD == 0. */
  do
    {
      int remainder = ud % divisor;
      
      *p++ = (remainder < 10) ? remainder + '0' : remainder + 'a' - 10;
    }
  while (ud /= divisor);

  /* Terminate BUF. */
  *p = 0;
  
  /* Reverse BUF. */
  p1 = buf;
  p2 = p - 1;
  while (p1 < p2)
    {
      char tmp = *p1;
      *p1 = *p2;
      *p2 = tmp;
      p1++;
      p2--;
    }
}

/* Put the character C on the screen. */
static void
putchar (int c)
{
  if (c == '\n' || c == '\r')
    {
    newline:
      xpos = 0;
      ypos++;
      if (ypos >= LINES)
        ypos = 0;
      return;
    }

  *(video + (xpos + ypos * COLUMNS) * 2) = c & 0xFF;
  *(video + (xpos + ypos * COLUMNS) * 2 + 1) = ATTRIBUTE;

  xpos++;
  if (xpos >= COLUMNS)
    goto newline;
}

/* Format a string and print it on the screen, just like the libc
   function printf. */
void
printf (const char *format, ...)
{
  char **arg = (char **) &format;
  int c;
  char buf[20];

  arg++;
  
  while ((c = *format++) != 0)
    {
      if (c != '%')
        putchar (c);
      else
        {
          char *p;
          
          c = *format++;
          switch (c)
            {
            case 'd':
            case 'u':
            case 'x':
              itoa (buf, c, *((int *) arg++));
              p = buf;
              goto string;
              break;

            case 's':
              p = *arg++;
              if (! p)
                p = "(null)";

            string:
              while (*p)
                putchar (*p++);
              break;

            default:
              putchar (*((int *) arg++));
              break;
            }
        }
    }
}