PRB: DeviceIoControl Int 13h Does Not Support Hard Disks

• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows 95
  

SYMPTOMS

Win32 applications running under Windows 95 use CreateFile to open VWIN32 and then use DeviceIoControl with the VWIN32_DIOC_DOS_INT13 flag to perform low-level BIOS disk functions. The functions work on floppy disks but always fail on hard disks.

CAUSE

Windows 95, like previous versions of Windows, does not support calling BIOS disk functions to gain access to hard disks from Win16 and Win32 applications. The reason is that BIOSXLAT.VXD does not translate BIOS requests to hard disks from protected-mode into V86 mode, and this causes the ROM BIOS to be called with an invalid address.

RESOLUTION

To work around this problem, Win32 applications must thunk to a Win16 DLL and have that DLL call the DOS Protected Mode Interface (DPMI) Simulate Real Mode Interrupt function to call Int 13h BIOS disk functions on hard disks. When you use DPMI to call Int 13h BIOS disk functions, you bypass BIOSXLAT.VXD and call the real-mode BIOS. Note that you cannot call DPMI from 32-bit code.

STATUS

This behavior is by design.

MORE INFORMATION

The following example code shows how a Win32 application running under Windows 95 can read the first physical sector of a hard disk, which contains the disk's partition tables. This code requires a flat thunk, so parts of it must be placed in a Win32 DLL and other parts must be placed in a Win16 DLL. The relevant code for each DLL is labeled.

Execution starts from a Win32 application calling the Win32 DLL and proceeds as follows. The Win32 DLL function, CallReadPhysicalSector1, calls the Win16 DLL's ReadPhysicalSector1 function by way of a thunk. Then ReadPhysicalSector1, which is exported by the 16-bit DLL, calls DPMI to call the BIOS Read Track function to read the first sector of the specified hard disk. After ReadPhysicalSector1 returns, the contents of the first sector of the hard disk, including the hard disk's partition table and bootstrap program, will be in the buffer pointed to by lpBuff.

Code Sample

//--------------------------------------------------------------------
// Code in the 32-bit DLL

// Prototype for function in 16-bit DLL.
BOOL FAR PASCAL ReadPhysicalSector1 (BYTE   bDrive,
                                     LPBYTE lpBuffer,
                                     DWORD  cbBuffSize);

/*--------------------------------------------------------------------
 CallReadPhysicalSector1()

 Reads the first sector of the first hard disk.

 Return Value
    Returns TRUE if the first sector was read, and FALSE if it
    wasn't.

    char lpBuff[512];
    BOOL fResult;

    // Read the first sector of the first hard disk.
    fResult = ReadPhysicalSector1 (0x80, lpBuff, 512);

    if (fResult)
       {
        // lpBuff contains the sector data.  Use it here
       }

    return fResult;

//--------------------------------------------------------------------
// Thunk Script

typedef unsigned long   DWORD;
typedef unsigned char   BYTE, * LPBYTE;
typedef bool            BOOL;

BOOL ReadPhysicalSector1 (BYTE   bDrive,
                          LPBYTE lpBuffer,
                          DWORD  cbBuffSize)
{
   lpBuffer = inout;

//--------------------------------------------------------------------
// Code in the 16-bit DLL

// Converts two BYTEs into a WORD.  This is useful for working with
// a RMCS, but was not provided in WINDOWS.H.

//#define MAKEWORD(low, high) \
           ((WORD)((((WORD)(high)) << 8) | ((BYTE)(low))))

#define SECTOR_SIZE 512       // Size, in bytes, of a disk sector
#define CARRY_FLAG  0x0001

typedef struct tagRMCS {

   DWORD edi, esi, ebp, RESERVED, ebx, edx, ecx, eax;
   WORD  wFlags, es, ds, fs, gs, ip, cs, sp, ss;

BOOL FAR PASCAL SimulateRM_Int (BYTE bIntNum, LPRMCS lpCallStruct);

void FAR PASCAL BuildRMCS (LPRMCS lpCallStruct);
BOOL FAR PASCAL __export ReadPhysicalSector1 (BYTE   bDrive,
                                              LPBYTE lpBuffer,
                                              DWORD  cbBuffSize);

/*--------------------------------------------------------------------
  ReadPhysicalSector1()

  Calls DPMI to call the BIOS Int 13h Read Track function to read the
  first physical sector of a physical drive. This function is used to
  read partition tables, for example.

  Parameters
     bDrive
        The Int 13h device unit,
           0x00 for floppy drive 0
           0x00 for floppy drive 1
           0x80 for physical hard disk 0
           0x81 for physical hard disk 1
           etc.

     lpBuffer
        Pointer to a buffer that receives the sector data.  The buffer
        must be at least SECTOR_SIZE bytes long.

     cbBuffSize
        Actual size of lpBuffer.

  Return Value
     Returns TRUE if the first sector was read into the buffer pointed
     to by lpBuffer, or FALSE otherwise.

  Assumptions
     Assumes that sectors are at least SECTOR_SIZE bytes long.

BOOL FAR PASCAL __export ReadPhysicalSector1 (BYTE   bDrive,
                                              LPBYTE lpBuffer,
                                              DWORD  cbBuffSize)
{
   BOOL   fResult;
   RMCS   callStruct;
   DWORD  gdaBuffer;     // Return value of GlobalDosAlloc().
   LPBYTE RMlpBuffer;    // Real-mode buffer pointer
   LPBYTE PMlpBuffer;    // Protected-mode buffer pointer

   /*
     Validate params:
        bDrive should be int 13h device unit -- let the BIOS validate
           this parameter -- user could have a special controller with
           its own BIOS.
        lpBuffer must not be NULL
        cbBuffSize must be large enough to hold a single sector
   */

   if (lpBuffer == NULL || cbBuffSize < SECTOR_SIZE)
      return FALSE;

   /*
     Allocate the buffer that the Int 13h function will put the sector
     data into. As this function uses DPMI to call the real-mode BIOS, it
     must allocate the buffer below 1 MB, and must use a real-mode
     paragraph-segment address.

     After the memory has been allocated, create real-mode and
     protected-mode pointers to the buffer. The real-mode pointer
     will be used by the BIOS, and the protected-mode pointer will be
     used by this function because it resides in a Windows 16-bit DLL,
     which runs in protected mode.
   */

   gdaBuffer = GlobalDosAlloc (cbBuffSize);

   if (!gdaBuffer)
      return FALSE;

   RMlpBuffer = (LPBYTE)MAKELONG(0, HIWORD(gdaBuffer));
   PMlpBuffer = (LPBYTE)MAKELONG(0, LOWORD(gdaBuffer));

   /*
     Initialize the real-mode call structure and set all values needed
     to read the first sector of the specified physical drive.
   */

   BuildRMCS (&callStruct);

   callStruct.eax = 0x0201;                // BIOS read, 1 sector
   callStruct.ecx = 0x0001;                // Sector 1, Cylinder 0
   callStruct.edx = MAKEWORD(bDrive, 0);   // Head 0, Drive #
   callStruct.ebx = LOWORD(RMlpBuffer);    // Offset of sector buffer
   callStruct.es  = HIWORD(RMlpBuffer);    // Segment of sector buffer

   /*
      Call Int 13h BIOS Read Track and check both the DPMI call
      itself and the BIOS Read Track function result for success.  If
      successful, copy the sector data retrieved by the BIOS into the
      caller's buffer.
   */

   if (fResult = SimulateRM_Int (0x13, &callStruct))
      if (!(callStruct.wFlags & CARRY_FLAG))
         {
         _fmemcpy (lpBuffer, PMlpBuffer, (size_t)cbBuffSize);
         fResult = TRUE;
         }
      else
         fResult = FALSE;

   // Free the sector data buffer this function allocated
   GlobalDosFree (LOWORD(gdaBuffer));

   return fResult;

/*--------------------------------------------------------------------
  SimulateRM_Int()

  Allows protected mode software to execute real mode interrupts such
  as calls to DOS TSRs, DOS device drivers, etc.

  This function implements the "Simulate Real Mode Interrupt" function
  of the DPMI specification v0.9.

  Parameters
     bIntNum
        Number of the interrupt to simulate

     lpCallStruct
        Call structure that contains params (register values) for
        bIntNum.

  Return Value
     SimulateRM_Int returns TRUE if it succeeded or FALSE if it
     failed.

  Comments
     lpCallStruct is a protected-mode selector:offset address, not a
     real-mode segment:offset address.

BOOL FAR PASCAL SimulateRM_Int (BYTE bIntNum, LPRMCS lpCallStruct)

   {
   BOOL fRetVal = FALSE;      // Assume failure

   _asm {
         push di

         mov  ax, 0300h         ; DPMI Simulate Real Mode Int
         mov  bl, bIntNum       ; Number of the interrupt to simulate
         mov  bh, 01h           ; Bit 0 = 1; all other bits must be 0
         xor  cx, cx            ; No words to copy
         les  di, lpCallStruct
         int  31h                   ; Call DPMI
         jc   END1                  ; CF set if error occurred
         mov  fRetVal, TRUE
      END1:
         pop di
        }
   return (fRetVal);
   }

/*--------------------------------------------------------------------
  BuildRMCS()

  Initializes a real mode call structure to contain zeros in all its
  members.

  Parameters:
     lpCallStruct
        Points to a real mode call structure

  Comments:
     lpCallStruct is a protected-mode selector:offset address, not a
     real-mode segment:offset address.

void FAR PASCAL BuildRMCS (LPRMCS lpCallStruct)
   {
   _fmemset (lpCallStruct, 0, sizeof (RMCS));
   }

REFERENCES

Information on using DPMI may be found in the DOS Protected Mode Interface (DPMI) Specification Version 0.9.

Flat thunks are documented in "The Microsoft Programmer's Guide To Windows 95" topic in the Win32 Software Development Kit online documentation.