How Win32 Applications Can Read CD-ROM Sectors in Windows 95
ID: Q137813
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The information in this article applies to:
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Microsoft Win32 Application Programming Interface (API), included with:
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Microsoft Windows versions 95, 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
__declspec(dllexport)
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
enablemapdirect3216 = true;
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.
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 __export ReadCDRomSectors (BYTE bDrive,
DWORD dwStartSector,
WORD wSectors,
LPBYTE lpBuffer);
BOOL FAR PASCAL MSCDEX_ReadSector (BYTE bDrive,
DWORD dwStartSector,
LPBYTE RMlpBuffer);
BOOL FAR PASCAL SimulateRM_Int (BYTE bIntNum, LPRMCS lpCallStruct);
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.
Additional query words:
Windows 95 4.00 cdrom cd-rom absolute
Keywords : kbcode kbAPI kbKernBase MMCDROM kbGrpKernBase
Version : 4.00
Platform : WINDOWS
Issue type :