Microsoft KB Archive/104790

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Using Int 21h Get/Set Media ID in the Windows Environment

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Q104790

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The information in this article applies to:


 * Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1

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SUMMARY
The Interrupt 21h Generic IOCTL for Block Devices functions Get Media ID and Set Media ID cannot be used directly from a protected mode application in the Windows operating environment. However, these functions can be used in conjunction with the DPMI function Simulate Real Mode Interrupt.

MORE INFORMATION
The following Interrupt 21h AX=440Dh Generic IOCTL for Block Devices functions are automatically translated from protected mode by the DOSMGR VxD:

  40h - Set Device Parameters 41h - Write Track on Logical Drive 42h - Format Track on Logical Drive 60h - Get Device Parameters 61h - Read Track on Logical Drive 62h - Verify Track on Logical Drive 68h - Sense Media Type The following are not automatically translated by the DOSMGR VxD:

  46h - Set Media ID   66h - Get Media ID To use the Get Media ID or Set Media ID functions from a protected mode application in the Windows operating environment, the DPMI function Simulate Real Mode Interrupt must be used to manually translate the buffer pointer from protected mode to real mode.

The sample code below illustrates how the Get Media ID function can be manually translated by:


 * 1) Allocating a real mode addressable buffer using GlobalDosAlloc.
 * 2) Initializing a DPMI real mode register structure with a real mode pointer to the buffer.
 * 3) Issuing the interrupt in real mode using the DPMI function Simulate Real Mode Interrupt.
 * 4) If successful, the content of the real mode addressable buffer is copied to the protected mode destination buffer.
 * 5) The real mode addressable buffer is freed.

To translate the Set Media ID function, the contents of the protected mode source buffer would be copied to the real mode addressable buffer before issuing the interrupt in real mode.

Sample Code
// This is a standard MS-DOS structure. typedef struct tagMEDIAID {   WORD  wInfoLevel; DWORD dwSerialNum;    // Serial number char VolLabel[11];    // ASCII volume label char FileSysType[8];  // File system type } MEDIAID, far *LPMEDIAID;

// This is a standard DPMI structure. typedef struct tagREALMODEREG { DWORD rmEDI, rmESI, rmEBP, Reserved, rmEBX, rmEDX, rmECX, rmEAX; WORD rmCPUFlags, rmES, rmDS, rmFS, rmGS, rmIP, rmCS, rmSP, rmSS; } REALMODEREG, FAR *LPREALMODEREG;

//******************************************************************** // RealInt // // Simulate an interrupt in real mode using DPMI function 0300h // When the interrupt is simulated in real mode, the registers will // contain the values in lpRealModeReg. When the interrupt returns, // lpRealModeReg will contain the values from real mode. // //********************************************************************

BOOL RealInt (BYTE intnum, LPREALMODEREG lpRealModeReg) {  BOOL bRetVal = TRUE;

_asm {      mov  ax, 0300h  // Simulate real mode interrupt mov bl, intnum // Interrupt number to simulate mov bh, 0      // Flags mov cx, 0      // Number of words to copy on stack les di, lpRealModeReg int 31h jnc Done mov bRetVal, FALSE Done: }  return bRetVal; }

//******************************************************************** // GetMediaID // // Get Media ID by simulating an Interrupt 21h, AX=440Dh, CX=0866h in // real mode. Set up RealModeReg to contain a real mode pointer to a // MediaID structure. //********************************************************************

BOOL GetMediaID (WORD     Drive,                 LPMEDIAID lpMediaID) {  REALMODEREG RealModeReg; DWORD      dwGlobalDosBuffer; LPMEDIAID  lpRMMediaID; BOOL       bRetVal;

// (1) Get a real mode addressable buffer for the MediaID structure. //   dwGlobalDosBuffer = GlobalDosAlloc(sizeof(MEDIAID)); if (dwGlobalDosBuffer == NULL) {      return FALSE; }

// (2) Initialize the real mode register structure. //   memset(&RealModeReg, 0, sizeof(RealModeReg)); (WORD)RealModeReg.rmEAX = 0x440D; // IOCTL for Block Device (WORD)RealModeReg.rmEBX = Drive;  // 0 = default, 1 = A, 2 = B   (WORD)RealModeReg.rmECX = 0x0866;  // Get Media ID   // Set the real mode DS:DX to a real mode pointer to the buffer. // The offset in DX is zero from the memset. RealModeReg.rmDS = HIWORD(dwGlobalDosBuffer);

// (3) Simulate the real mode interrupt. //   if (RealInt(0x21, &RealModeReg) &&         // Int simulation ok?       (RealModeReg.rmCPUFlags & 0x0001)==0)  // Carry clear? {      // (4) Copy the content of the real mode addressable buffer //    to the protected mode destination buffer. //       lpRMMediaID = (LPMEDIAID) MAKELP(LOWORD(dwGlobalDosBuffer), 0); *lpMediaID = *lpRMMediaID; // Structure copy bRetVal = TRUE; }  else {      bRetVal = FALSE; }

// (5) Free the real mode addressable buffer. //   GlobalDosFree(LOWORD(dwGlobalDosBuffer));

return bRetVal; } For more information on the DOS Protected Mode Interface (DPMI), see the DPMI version 0.9 specification, which is available on the Microsoft Developer Network CD Disk Four.

Additional query words: serial number volume label

Keywords : _IK kb16bitonly

Issue type : kbhowto

Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310