Microsoft KB Archive/137176

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PRB: DeviceIoControl Int 13h Does Not Support Hard Disks

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Q137176

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The information in this article applies to:


 * Microsoft Win32 Application Programming Interface (API), included with:
 * Microsoft Windows 95

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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. __declspec(dllexport) BOOL WINAPI CallReadPhysicalSector1 {   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

enablemapdirect3216 = true;

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))))


 * 1) define SECTOR_SIZE 512      // Size, in bytes, of a disk sector
 * 2) define CARRY_FLAG 0x0001

typedef BYTE FAR *LPBYTE;

typedef struct tagRMCS {  DWORD edi, esi, ebp, RESERVED, ebx, edx, ecx, eax; WORD wFlags, es, ds, fs, gs, ip, cs, sp, ss; } RMCS, FAR* LPRMCS;

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)); }