How Win32 Applications Can Read CD-ROM Sectors in Windows 95

• Microsoft Win32 Application Programming Interface (API) included with: Microsoft Windows 95 version 4.0

SUMMARY

Some Win32 applications, such as multimedia applications and games, need to read sectors directly from compact discs to implement custom access or read caching to optimize CD-ROM access for specific purposes. This article provides information and example code that demonstrates how Win32 applications can read sectors from compact discs on Windows 95.

MORE INFORMATION

The Windows 95 Compact Disc File System (CDFS) is a protected-mode CD-ROM driver that implements the V86-mode MSCDEX Interrupt 2Fh functions as its sole application-callable interface. This interface allows it to replace real-mode CD-ROM drivers and MSCDEX.EXE and still be completely compatible with existing applications for MS-DOS and Windows 3.x. In fact, applications for MS-DOS and Windows 3.x will not be able to tell the difference between CDFS and MSCDEX.EXE.

Windows 95 provides an interface for Win32 applications to read sectors from compact discs. This interface differs substantially from that provided by Windows NT. While Win32 applications on Windows NT use CreateFile() and ReadFile() to read sectors from compact discs, Win32 applications running on Windows 95 must use an indirect method that involves thunking to a 16-bit DLL that calls MSCDEX Int 2Fh functions.

Because the MSCDEX interface is callable only in V86-mode, Win32 applications must thunk to a 16-bit DLL, and the 16-bit DLL must use the DOS Protected Mode Interface (DPMI) Simulate Real Mode Interrupt function to call its functions. The Win32 application must pass a buffer to the 16- bit DLL. Inside the thunk, the 16-bit DLL obtains the sector data from MSCDEX, copies it into the Win32 application's buffer, and returns to the Win32 application.

The following code demonstrates how to read sectors from a compact disc using the method described above. Because the code implements a flat thunk, it is divided into three sections:

• Code that resides in a Win32 DLL (the 32-bit side of the thunk).
• Code that resides in the thunk script.
• Code that resides in the 16-bit DLL that actually calls MSCDEX.

  //--------------------------------------------------------------------
  // Code inside the Win32 DLL (the 32-bit side of the thunk)
  // ReadSectorsFromCD() is exported by the DLL so that it can be called
  // by Win32-based applications.
  
  #include <windows.h>
  
  #define CD_SECTOR_SIZE         2048
  
  

BOOL WINAPI ReadSectorsFromCD (BYTE   bDrive,
                               DWORD  dwStartSector,
                               WORD   wSectors,
                               LPBYTE lpBuff);

// Prototype for thunk function in 16-bit DLL.
BOOL FAR PASCAL ReadCDRomSectors (BYTE   bDrive,
                                  DWORD  dwStartSector,
                                  WORD   wSectors,
                                  LPBYTE lpBuffer);

/*-------------------------------------------------------------------
  ReadSectorsFromCD()

  Calls the thunked function, ReadCDRomSectors(). This function is
  exported by the Win32 DLL and thus is callable from Win32-based
  applications.

  Parameters:

     bDrive
        Drive letter of CD-ROM drive to read from. Specified as
        a character in the range 'A', 'B', 'C', ..., 'Z'.  May be
        upper- or lower-case.

     dwStartSector
        First sector to read.

     wSectors
        Number of sectors to read.

     lpBuffer
        Buffer to contain sector data. Must be large enough to
        accommodate all sectors being read. Because this buffer is
        provided to the 16-bit DLL via a thunk, its maximum
        length is limited to 64K.

  Example use:
     // Read 5 sectors from CD-ROM drive E: starting at sector 16.
     fResult = ReadSectorsFromCD ('E', 16, 5, lpBuff);

  Return Value
     Returns TRUE if successful, or FALSE if an error occurred
     in reading or if a parameter was invalid.

__declspec(dllexport)

BOOL WINAPI ReadSectorsFromCD (BYTE   bDrive,
                               DWORD  dwStartSector,
                               WORD   wSectors,
                               LPBYTE lpBuff)
{
   // Call 16-bit DLL to read sectors via a 32->16 thunk
   return ReadCDRomSectors (bDrive, dwStartSector, wSectors, lpBuff);

//--------------------------------------------------------------------
// Contents of the thunk script

typedef unsigned short WORD; typedef unsigned long DWORD; typedef unsigned char BYTE, *LPBYTE;

typedef bool           BOOL;

BOOL ReadCDRomSectors (BYTE   bDrive,
                       DWORD  dwStartSector,
                       WORD   wSectors,
                       LPBYTE lpBuffer)
{
   lpBuffer = inout;

//--------------------------------------------------------------------
// Code inside the 16-bit DLL. This code implements the 16-bit side of
// the thunk, and is where the calls to MSCDEX are made to read
// sectors from a compact disc.

#include <windows.h>
#include <string.h>
#include <ctype.h>

// Cooked-mode sector size in bytes
#define CD_SECTOR_SIZE         2048

// Maximum sector buffer size in bytes
#define MAX_BUFFER_LENGTH      65536

// Maximum number of sectors for each read request
#define MAX_CD_SECTORS_TO_READ ((MAX_BUFFER_LENGTH) / \
                                (CD_SECTOR_SIZE))

// Processor flags masks -- use for testing wFlags member of RMCS
#define CARRY_FLAG  0x0001

// Real-mode call structure for making DPMI Simulate Real Mode
// Interrupt calls.

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

BOOL FAR PASCAL __export ReadCDRomSectors (BYTE   bDrive,
                                           DWORD  dwStartSector,
                                           WORD   wSectors,
                                           LPBYTE lpBuffer);

BOOL FAR PASCAL MSCDEX_ReadSector (BYTE   bDrive,
                                   DWORD  dwStartSector,
                                   LPBYTE RMlpBuffer);

void FAR PASCAL BuildRMCS (LPRMCS lpCallStruct);

/*-------------------------------------------------------------------
  ReadCDRomSectors()

  Reads a specified number of sectors from a CD-ROM.

  Parameters:

     bDrive
        Drive letter of CD-ROM drive to read from. Specified as
        a character in the range 'A', 'B', 'C', ..., 'Z'.  May be
        upper- or lower-case.

     dwStartSector
        First sector to read.

     wSectors
        Number of sectors to read.

     lpBuffer
        Buffer to contain sector data. Must be large enough to
        accommodate all sectors being read. Since this buffer is
        provided by the calling Win32 application, no more than
        the first 64K bytes will be used.

  Return Value
     Returns TRUE if successful, or FALSE if an error occurred
     in reading or if a parameter was invalid.

BOOL FAR PASCAL __export ReadCDRomSectors (BYTE   bDrive,
                                           DWORD  dwStartSector,
                                           WORD   wSectors,
                                           LPBYTE lpBuffer)
{
   BOOL   fResult;
   DWORD  cbOffset;
   DWORD  i;
   DWORD  gdaBuffer;     // Return value of GlobalDosAlloc().
   LPBYTE RMlpBuffer;    // Real-mode buffer pointer
   LPBYTE PMlpBuffer;    // Protected-mode buffer pointer


   // Convert drive letter into drive number for MSCDEX call.
   bDrive = toupper(bDrive) - 'A';

   /*
      Validate parameters:
         bDrive must be between 0 and 25, inclusive.
         lpBuffer must not be NULL.
         wSectors must be between 1 and the maximum number of
            sectors that can fit into a 64K buffer, inclusive.
   */
   if (bDrive > 25 || !lpBuffer)
      return FALSE;

   if (!wSectors || (wSectors > MAX_CD_SECTORS_TO_READ))
      return FALSE;

   /*
      Allocate buffer for MSCDEX call. This buffer must be below 1MB
      because the MSCDEX function will be called using DPMI. Like real-
      mode MSCDEX.EXE, CDFS implements the MSCDEX API as V86-mode
      Interrupt 2Fh functions.

      Free memory below 1MB is relatively scarce, so allocating a
      small sector buffer increases the chances that it can be
      allocated no matter how many other applications and MS-DOS
      device drivers are running. Also, a small sector buffer leaves
      more memory for other applications to use.
   */
   gdaBuffer = GlobalDosAlloc (CD_SECTOR_SIZE);

   if (!gdaBuffer)
      return FALSE;

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

   // Call MSCDEX to read each sector.
   for (i = cbOffset = 0;
        i < wSectors;
        i++, cbOffset += CD_SECTOR_SIZE)
      {
      if (fResult = MSCDEX_ReadSector (bDrive,
                                       dwStartSector + i,
                                       RMlpBuffer))
         _fmemcpy (lpBuffer + cbOffset, PMlpBuffer, CD_SECTOR_SIZE);
      else
         break;
      }

   GlobalDosFree (LOWORD(gdaBuffer));

   return (fResult);

/*-------------------------------------------------------------------
  MSCDEX_ReadSector()

  Calls MSCDEX to read a single sector from a CD-ROM compact disc.

  Parameters:

     bDrive
        Drive number of CD-ROM drive to read from. Expected to be
        a number in the following series: 0 = A, 1 = B, 2 = C, etc.

     dwStartSector
        First sector of read.

     RMlpBuffer
        Real-mode segment:offset pointer to a buffer that will receive
        sector data. Must be large enough to accommodate a single
        sector in cooked mode.

  Return Value
     Returns TRUE if successful, or FALSE if an error occurred in
     reading.

BOOL FAR PASCAL MSCDEX_ReadSector (BYTE   bDrive,
                                   DWORD  dwStartSector,
                                   LPBYTE RMlpBuffer)
{
   RMCS   callStruct;
   BOOL   fResult;

   /*
      Prepare DPMI Simulate Real Mode Interrupt call structure with
      the register values used to make the MSCDEX Absolute read call.
      Then, call MSCDEX using DPMI and check for errors in both the DPMI
      call and the MSCDEX call.
   */
   BuildRMCS (&callStruct);
   callStruct.eax = 0x1508;                 // MSCDEX Absolute read
   callStruct.ebx = LOWORD(RMlpBuffer);     // Offset of sect buffer
   callStruct.es  = HIWORD(RMlpBuffer);     // Segment of sect buffer
   callStruct.ecx = bDrive;                 // 0=A, 1=B, 2=C, etc.
   callStruct.edx = 1;                      // Read one sector
   callStruct.esi = HIWORD(dwStartSector);
   callStruct.edi = LOWORD(dwStartSector);

   if (fResult = SimulateRM_Int (0x2F, &callStruct))
      fResult = !(callStruct.wFlags & CARRY_FLAG);

   return fResult;

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

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

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

  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 Interrupt
         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 from PM to RM stack
         les  di, lpCallStruct  ; Real mode call structure
         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 by zeroing all its members.

   Parameters:

      lpCallStruct
         Points to a real-mode call structure

   Return Value
         None.

   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

DOS Protected Mode Interface Specification Version 0.9. Programmer's Guide to Windows 95. Win32 SDK Online Documentation.