C++ Program to write to the boot sector of a USB Pendrive

1. The program must be run with administrative access. Either run visual studio as admin. OR run the compiled exe as admin. WARNING!!! DATA ON THE PEN DRIVE WILL BE REMOVED.
2. Use a thumb drive less than 32GB. This is a safety limit to prevent accidental writes to hard disks.
3. We demonstrate how a byte array of 512 bytes can be written to the boot sector of a pen drive.
4. The array is a BIOS routine that prints HELLO when a PC is booted from the pen drive.
5. After running this program boot your computer from the pen drive. The monitor will show HELLO.
6. This program may be of real use to those who want to understand how a pc can be run without an operating system - direct from a pen drive - and with bare metal assembly code.

The BIOS routine to write

We shall write the following BIOS routine to the boot sector of our thumb drive. For details of this routine please refer Write an OS from scratch - Nick Blundell

MOV AH, 0x0E
MOV AL, 'H'
int 0x10
MOV AL, 'E'
int 0x10
MOV AL, 'L'
int 0x10
MOV AL, 'L'
int 0x10
MOV AL, 'O'
int 0x10
JMP $
TIMES 510 - ($ - $$) DB 0
DW 0xAA55

The Source Code

The code can be copy-pasted to any .cpp file and compiled with a C++ compiler.

#include <cstdio>

#include <windows.h>

using namespace std;

// COMPILE WITH A WINDOWS COMPILER 
// SUCH AS VISUAL STUDIO C++ 2005 
// OR LATER. PROGRAM TESTED ON 
// WINDOWS 10, 8 and 7 
int main()
{

  // the boot code for the above 
  // assembly instructions 
  // see the referenced pdf at the 
  // end of this page 
  BYTE mac_code[512] =
  {

    0xb4, 0x0e, 0xb0, 0x48, 0xcd, 0x10,
    0xb0, 0x65, 0xcd, 0x10, 0xb0, 0x6c,
    0xcd, 0x10, 0xb0, 0x6c, 0xcd, 0x10,
    0xb0, 0x6f, 0xcd, 0x10, 0xe9, 0xfd,
    0xff, 0x0
  };

  // boot signature 
  mac_code[510] = { 0x55 };

  mac_code[511] = { 0xAA };

  // for safety we restrict to 
  // USB less than 32 GB so that 
  // accidental access to hdd 
  // can be avoided 
  const int MAX_USB_GB = 32;

  // get the drive letter 
  // from the user 
  printf("Drive letter of USB?: ");

  // scanf used for readability 
  // and simplicity only. 
  char path[] = "\\\\.\\?:";

  scanf_s("%c", path + 4, 1);

  // open the volume 
  HANDLE hVol = CreateFileA(
  path,
  GENERIC_READ | GENERIC_WRITE,
  FILE_SHARE_READ | FILE_SHARE_WRITE,
  NULL,
  OPEN_EXISTING, 0,
  NULL);

  // if handle fails 
  if (INVALID_HANDLE_VALUE == hVol)
  {

    printf("Check drive letter.");

    return -1;

  }

  // get the physicaldrive number 
  // and the capacity of the disk 
  VOLUME_DISK_EXTENTS vde = { 0 };

  DWORD dw;

  DeviceIoControl(
  hVol,
  IOCTL_VOLUME_GET_VOLUME_DISK_EXTENTS,
  NULL,
  0,
  (LPVOID)&vde,
  (DWORD)sizeof(VOLUME_DISK_EXTENTS),
  &dw, NULL
  );

  // check disk size 
  if (
  vde.Extents[0].ExtentLength.QuadPart
  >

  LONGLONG(MAX_USB_GB * 1000000000LL)
  )
  {

    printf("USB too large\n");

    printf("Use < %d GB", MAX_USB_GB);

    CloseHandle(hVol);

    return -2;

  }

  // lock the volume 
  DeviceIoControl(
  hVol,
  (DWORD)FSCTL_LOCK_VOLUME,
  NULL,
  NULL,
  NULL,
  NULL,
  &dw,
  NULL);

  // we should be able to write 
  // to every sector from 
  // start till the end 
  DeviceIoControl(hVol,
  (DWORD)FSCTL_ALLOW_EXTENDED_DASD_IO,
  NULL,
  NULL,
  NULL,
  NULL,
  &dw,
  NULL);

  // open the disk now by using 
  // the disk number 
  // path format is \\.\PhysicalDriveN 
  // so a buffer of 20 sufficient 
  char diskPath[20] = { 0 };

  // use sprintf to construct 
  // the path by appending 
  // disk number to \\.\PhysicalDrive 
  sprintf_s(diskPath,
  sizeof(diskPath),
  "\\\\.\\PhysicalDrive%d",
  vde.Extents[0].DiskNumber
  );

  // open a handle to the disk 
  HANDLE hUsb = CreateFileA(
  diskPath,
  GENERIC_READ | GENERIC_WRITE,
  FILE_SHARE_READ | FILE_SHARE_WRITE,
  NULL,
  OPEN_EXISTING, 0,
  NULL);

  // if handle fails 
  if (INVALID_HANDLE_VALUE == hUsb)
  {

    printf("Run as admn.");

    // unlock the volume 
    DeviceIoControl(hVol,
    (DWORD)FSCTL_UNLOCK_VOLUME,
    NULL,
    NULL,
    NULL,
    NULL,
    &dw,
    NULL);

    CloseHandle(hVol);

    return -3;

  }

  // write the sector! 
  WriteFile(hUsb,
  (void*)mac_code,
  sizeof(mac_code),
  &dw,
  NULL);

  // read back to verify now 
  // set read pointer to start 
  SetFilePointer(hUsb,
  0,
  NULL,
  FILE_BEGIN);

  // file pointer begins at 0 
  // boot sector also starts at 0 
  // so read now... 
  ReadFile(hUsb,
  mac_code,
  sizeof(mac_code),
  &dw, NULL);

  printf("Boot sector: \n");

  for (int i = 0; i < 512; i++)
  {

    // create a row after every 16 
    // columns so that display 
    // looks good 
    if (0 == i % 16)
    {

      printf("\n");

    }

    BYTE b = mac_code[i];

    printf("%02x ", b);

  }

  // unlock the volume 
  DeviceIoControl(hVol,
  (DWORD)FSCTL_UNLOCK_VOLUME,
  NULL,
  NULL,
  NULL,
  NULL,
  &dw,
  NULL);

  // release handles 
  CloseHandle(hVol);

  CloseHandle(hUsb);

  return 0;

}