Function Requests Specification
There is a need for access to features from the MSCDEX redirector that transend DOS
capabilities. This proposal documents a means that the application can use to talk directly
to MSCDEX to request information or set parameters that only MSCDEX can provide. This
document outlines some of the features I think MSCDEX should support. Comments and
suggestions are welcome.
Access to these functions is provided through an INT 2Fh interface. AH contains 15h which
is what MSCDEX will use to tell its requests from those of other INT 2Fh handlers. AL will
contain the code of the function to be performed.
Function Request Command Codes:
Contents of AL Function
00h Get Number of CD-ROM Drive Letters
01h Get CD-ROM Drive Device List
02h Get Copyright File Name
03h Get Abstract File Name
04h Get Bibliographic Doc File Name
05h Read VTOC
06h Turn Debugging On
07h Turn Debugging Off
08h Absolute Disk Read
09h Absolute Disk Write
0Ah Reserved
0Bh CD-ROM Drive Check
0Ch MSCDEX Version
0Dh Get CD-ROM Drive Letters
0Eh Get/Set Volume Descriptor Preference
0Fh Get Directory Entry
10h Send Device Request
11h-0FFh Reserved
Get Number of CD-ROM Drive Letters
AX 1500h
BX Number of CD-ROM drive letters used
CX Starting drive letter of CD-ROM drive letters (A=0, B=1, ...Z=25)
MSCDEX will return the number of CD-ROM drive letters in BX and the starting drive letter
in CX. The first CD-ROM device will be installed at the starting drive letter and subsequent
drives will be assigned the next greater drive letter. A single device driver may be assigned
to more than one drive letter, such as the case of a device driver that supports multiple
units. MSCDEX keeps track of which sub-unit a particular drive letter is assigned to.
───────────────────────────────────────────────────────────────────────────
NOTE: This function can be used to determine if MSCDEX is installed by setting BX to zero
before executing INT 2Fh. MSCDEX is not installed if BX is still zero on return.
───────────────────────────────────────────────────────────────────────────
Also, in a networking environment, one cannot assume that drive letters will always be
assigned contiguously beginning with the starting drive letter. Use function Get CD-ROM drive
letters instead.
Get CD-ROM Drive Device List
AX 1501h
ES:BX Transfer address; pointer to buffer to copy drive letter device list
The buffer must be large enough to hold the device list. By calling function Get Number of
CD-ROM Drive Letters, one can find out the number of CD-ROM drive letters and the buffer size
will be a multiple of that. This will be an absolute maximum of 26. Each drive letter device
entry will consist of one byte for the sub-unit followed by 4 bytes for the address of the
device header assigned to that drive letter. This byte for the sub-unit takes care of the
problem of distinguishing which unit is assigned to which drive letter for device drivers
that handle sub-units.
For example: Suppose there are two installed CD-ROM device drivers, FOO, which supports 1
sub-unit, and BAR, which supports two sub-units, on a system with 2 floppy drives (A=0 and
B=1) and a hard disk (C=2). Then asking for the number of CD-ROM drive letters will report
that there are 3 drive letters used starting at drive letter D=3. ES:BX must point to a
buffer that is at least 3 * 5 = 15 bytes long. The buffer will be filled as follows:
ES:BX = Buffer
Buffer DB 0 ; sub-unit of FOO on drive letter D:
DD <far addr of FOO device header>
DB 0 ; sub-unit of BAR on drive letter E:
DD <far addr of BAR device header>
DB 1 ; sub-unit of BAR on drive letter F:
DD <far addr of BAR device header>
Get Copyright File Name
AX 1502h
ES:BX Transfer address; pointer to a 38 byte buffer
CX CD-ROM drive letter (A=0, B=1, ... Z=25)
MSCDEX will copy the name of the copyright file in the VTOC for that drive letter into the
buffer space provided. The copyright filename is presently restricted in the High Sierra
proposal to 8.3 but we require 38 bytes here for the possibility at a later date of handling
31 character file names plus 6 bytes for a ';' and 5 digit version number and 1 byte for a
NULL at the end. Carry will be set if the drive letter is not a CD-ROM drive and
error_invalid_drive (15) will be returned in AX.
Get Abstract File Name
AX 1503h
ES:BX Transfer address; pointer to a 38 byte buffer
CX CD-ROM drive letter (A=0, B=1, ... Z=25)
MSCDEX will copy the name of the abstract file in the VTOC for that drive letter into the
buffer space provided. The abstract filename is presently restricted in the High Sierra
proposal to 8.3 but we require 38 bytes here for the possibility at a later date of handling
31 character file names plus 6 bytes for a ';' and 5 digit version number and 1 byte for a
NULL at the end. Carry will be set if the drive letter is not a CD-ROM drive and
error_invalid_drive (15) will be returned in AX.
Get Bibliographic Documentation File Name
AX 1504h
ES:BX Transfer address; pointer to a 38 byte buffer
CX CD-ROM drive letter (A=0, B=1, ... Z=25)
───────────────────────────────────────────────────────────────────────────
NOTE: This function is provided in advance of the ISO standard. For discs complying with the
May 28th draft from the High Sierra Group, this function will return a null string as though
the field is blank on the disc.
───────────────────────────────────────────────────────────────────────────
MSCDEX will copy the name of the bibliographic documentation file in the VTOC for that drive
letter into the buffer space provided. The bibliographic documentation filename is presently
restricted in the High Sierra proposal to 8.3 but we require 38 bytes here for the
possibility at a later date of handling 31 character file names plus 6 bytes for a ';' and
5 digit version number and 1 byte for a NULL at the end. Carry will be set if the drive
letter is not a CD-ROM drive and error_invalid_drive (15) will be returned in AX.
Read VTOC
AX 1505h
ES:BX Transfer address; pointer to a 2048 byte buffer
CX CD-ROM Drive letter
DX Sector index
This function is provided to scan the Volume Descriptors on a disc. A sector index of 0 will
read the first volume descriptor, 1 reads the second, etc. If there is no error, then AX will
return 1 if the volume descriptor read was the standard volume descriptor, 0FFh if it was
the volume descriptor terminator and there are no more volume descriptors to be read, and
0 for all other types.
If there is an error in processing the request, the Carry Flag will be set and AL will
contain the MS-DOS error code. These will be either error_invalid_drive (15) or
error_not_ready (21).
Turn Debugging On
AX 1506h
BX Debugging function to enable
This is used for development and is reserved. It will be non-functional in the production
version of MSCDEX.
Turn Debugging Off
AX 1507h
BX Debugging function to disable
This is used for development and is reserved. It will be non-functional in the production
version of MSCDEX.
Absolute Disk Read
AX 1508h
ES:BX Disk Transfer Address; pointer to a buffer to copy data to
CX CD-ROM Drive letter (A=0, B=1, ... Z=25)
DX Number of sectors to read
SI:DI Starting sector
This function corresponds to INT 25h. It will be converted directly into a READ_LONG device
driver request and sent to the correct device driver. There are no requirements for this call
to pop flags as there are with INT 25h. SI holds the high word and DI the low word for the
starting sector to begin reading from.
If there is an error in processing the request, the Carry Flag will be set and AL will
contain the MS-DOS error code. These will be either error_invalid_drive (15) or
error_not_ready (21).
Absolute Disk Write
AX 1509h
ES:BX Disk Transfer Address; pointer to buffer to copy data from
CX CD-ROM Drive letter
DX Number of sectors to write
SI:DI Starting sector
This function corresponds to INT 26h. It is not supported at this time and is reserved. It
is intended to be used by authoring systems.
CD-ROM Drive Check
AX 150Bh
BX Signature word
CX CD-ROM Drive letter (A=0, B=1,...Z=25)
This function returns whether or not a drive letter is a CD-ROM drive supported by MSCDEX.
If the extensions are installed, BX will be set to ADADh. If the drive letter is supported
by MSCDEX, then AX is set to a non- zero value. AX is set to zero if the drive is not
supported. One must be sure to check the signature word to know that MSCDEX is installed and
that AX has not been modified by another INT 2Fh handler.
MSCDEX Version
AX 150Ch
BX MSCDEX Version
This function returns the version number of the CD-ROM Extensions installed on the system.
BH contains the major version number and BL contains the minor version. Values returned are
binary. For example, BX would contain 0x020a for version 2.10. This function does not work
on versions earlier than 2.00 so if BX is zero before and after this function is called, an
earlier version of MSCDEX is installed.
Get CD-ROM Drive Letters
AX 150Dh
ES:BX Transfer address; pointer to buffer to copy drive letter device list
The buffer must be large enough to hold a list of drive letters. The buffer size will be a
multiple of the number of drives returned by the Get Number of CD-ROM Drive Letters function.
There are a maximum of 26 drive letters. Each drive letter entry is a single byte (0=A:, 1=B:
.. 25=Z:) that exactly corresponds each respective entry returned by the command Get CD-ROM
Drive Device List. This command is included to allow applications to locate CD-ROM drives
supported by MSCDEX. CD-ROM drive letters may sometimes be noncontiguous so this command is
necessary.
For example: Suppose there is an installed CD-ROM device driver FOO supporting 3 sub-units
on a system with 2 floppy drives (A=0 and B=1), a hard disk (C=2) and a network drive (E=4).
Note the network drive occupies one of the drive letters normally taken by a CD-ROM drive.
MSCDEX assigns that CD-ROM drive to the next available drive letter. Asking for the number
of CD-ROM drive letters reports there are 3 drive letters used starting at drive letter D=3.
ES:BX must point to a buffer that is at least 3 bytes long and will be filled as follows:
ES:BX = Buffer
Buffer DB 3 ; drive letter for CD-ROM (D=3)
DB 5 ; drive letter for CD-ROM (F=5)
DB 6 ; drive letter for CD-ROM (G=6)
Get/Set Volume Descriptor Preference
AX 150Eh
BX 0 - Get Preference. 1 - Set Preference
CX CD-ROM Drive letter (A=0, B=1,...Z=25)
DX if BX = Get Preference
DX = 0
MSCDEX will return preference settings in DX
if BX = Set Preference
DH = volume descriptor preference
1 - PVD - Primary Volume Descriptor
2 - SVD - Supplementary Volume Descriptor
DL = Supplementary Volume Descriptor Preference
if DH = PVD
DL = 0
if DH = SVD
1 - shift-Kanji (an unregistered ISO coded character set)
Normally, MSCDEX will scan for the PVD (Primary Volume Descriptor) when initializing a CD-
ROM. This behavior can be altered for each individual drive to scan for a SVD (Supplementary
Volume Descriptor) instead. A CD-ROM drive set to scan for an SVD will use the PVD if there
is no SVD present. There can be more than one SVD on a CD-ROM but at present, MSCDEX will
only recognize SVDs for shift-Kanji CD-ROMs. Carry will be set, AX will be set to
error_invalid_function (1) and DX will be set to 0 if the coded character set is not
recognized.
If BX contains Get_Preference, MSCDEX will report the present setting for that drive. If DX
is still zero on return, that version of MSCDEX does not support this function or reading
SVDs. Otherwise DX will contain the setting.
If the drive letter is not a CD-ROM drive, carry will be set and error_invalid_drive (15)
will be returned in AX. If BX is anything other than Get/Set_Preference, AX will be set to
error_invalid_function (1) and carry will be set.
Get Directory Entry
AX 150Fh
CX CD-ROM Drive letter (A=0, B=1,...Z=25)
ES:BX Pointer to buffer with null-terminated path name
SI:DI Pointer to buffer to copy directory record information
AX 0 is returned if the disc is High Sierra, 1 is returned if the
disc is ISO-9660
The pathname expected is a null-terminated string e.g. char far *path = "\\a\\b\\c.txt";
(note: the "\\" characters map to a single '\' character in C so this would be '\a\b\c.txt'
if printed). The path must consist only of valid High Sierra or ISO-9660 filename characters
and must not contain any wildcards nor may it include entries for '.' or '..'.
The buffer to copy the directory record to can be a maximum of 255 bytes long including all
system use information. The directory record is a direct copy from the directory file and
it is up to the application to choose what fields to use.
Carry will be set and an error code returned if there were problems with the request. The
error codes will be error_invalid_drive (15) if the drive letter is incorrect,
error_not_ready (21) if the disc didn't initialize correctly, error_file_not_found (2) if
the file was not found and error_no_more_files (18) if the pattern fails to find a match or
if mscdex failed to allocate buffers.
The format of the directory record for High Sierra discs is:
/* High Sierra directory entry structure */
typedef struct hsg_dir_entry {
uchar len_dr; /* length of this directory entry */
uchar XAR_len; /* length of XAR in LBN's */
ulong loc_extentI; /* LBN of data Intel format */
ulong loc_extentM; /* LBN of data Molorola format */
ulong data_lenI; /* length of file Intel format */
ulong data_lenM; /* length of file Motorola format */
uchar record_time[6];/* date and time */
uchar file_flags_hsg;/* 8 flags */
uchar reserved; /* reserved field */
uchar il_size; /* interleave size */
uchar il_skip; /* interleave skip factor */
ushort VSSNI; /* volume set sequence num Intel */
ushort VSSNM; /* volume set sequence num Motorola*/
uchar len_fi; /* length of name */
uchar file_id[...]; /* variable length name upto 32 chars */
uchar padding; /* optional padding if file_id is odd length*/
uchar sys_data[...] /* variable length system data */
} hsg_dir_entry;
The format of the directory record for ISO-9660 discs is:
/* ISO-9660 directory entry structure */
typedef struct iso_dir_entry {
uchar len_dr; /* length of this directory entry */
uchar XAR_len; /* length of XAR in LBN's */
ulong loc_extentI; /* LBN of data Intel format */
ulong loc_extentM; /* LBN of data Molorola format */
ulong data_lenI; /* length of file Intel format */
ulong data_lenM; /* length of file Motorola format */
uchar record_time[7];/* date and time */
uchar file_flags_iso;/* 8 flags */
uchar il_size; /* interleave size */
uchar il_skip; /* interleave skip factor */
ushort VSSNI; /* volume set sequence num Intel */
ushort VSSNM; /* volume set sequence num Motorola*/
uchar len_fi; /* length of name */
uchar file_id[...]; /* variable length name upto 32 chars */
uchar padding; /* optional padding if file_id is odd length*/
uchar sys_data[...] /* variable length system data */
} iso_dir_entry;
The difference between the two forms is the file flag byte moved to account for an additional
byte of date and time used for a Greenwich mean time offset. See the May 28th draft of the
High Sierra proposal or ISO-9660 for a more complete explanation of the fields. Note that
the C structs above are not syntactically correct; C does not allow variable length arrays
as struct elements.
Send Device Driver Request
AX 1510h
CX CD-ROM drive letter (A=0, B=1, ... Z=25)
ES:BX Address of CD-ROM device driver request header
This function has been added to simplify communication with CD-ROM drivers and help prevent
contention between applications that wish to communicate with the device driver. It is highly
recommended that all applications communicate with device drivers through this function
request. Applications using this function will not have to locate the device driver. The
format of the request header is specified by the Microsoft MS-DOS CD-ROM Extensions Hardware-
Dependent Device Driver Specification.
Hardware-Dependent Device Driver Specification
Intent
This document (Document Number: 000080010-100-O00-1186) describes the CD-ROM hardware-
dependent device driver and its interface with MSCDEX.EXE, the MS- DOS CD-ROM Extensions
resident program. Differences between CD-ROM drives and hard- or floppy-disk drives account
for the differences in this device driver specification from the normal MS-DOS block and
character device driver specification. The chapters on device drivers in the MS-DOS
Programmer's Reference Manual (MS-PRM) provide more information.
The MS-DOS operating system reads CONFIG.SYS and installs the device. MSCDEX.EXE performs
an open system call on the device driver name in order to communicate with it and uses an
IOCTL call to ask the device driver for the address of its device header. From the device
header address, MSCDEX.EXE locates the device driver's interrupt and strategy routines. After
that, all requests the device driver receives come directly from MSCDEX.EXE, not MS- DOS.
To avoid reentrancy problems and allow MSCDEX to monitor all media changes, all other
applications that wish to communicate directly with CD- ROM device drivers should do so
through the Send Device Driver Request INT 2Fh function 10h. MSCDEX.EXE interfaces with MS-
DOS so that normal requests for I/O with files on a CD-ROM drive down to the MS-DOS INT 21h
service layer will work just as they would for a normal MS-DOS device.
Installation
The device driver will be installed in the same way as any other device with an entry in
CONFIG.SYS. The syntax is:
DEVICE=<filename> /D:<device_name> /N:<number of drives>
The following are examples:
DEVICE=HITACHI.SYS /D:MSCD001 /D:MSCD002
DEVICE=SONY.SYS /D:MSCD003 /N:2
The arguments will be the character device names that will be used on the command line when
starting MSCDEX.EXE so that it can find and communicate with the device driver.
A device driver may support one or more physical drives or logical disks. This may be done
by having multiple device headers in the device driver file (in which case it will be
necessary to have more than one device_name on the command line - one for each device header;
see the HITACHI.SYS example above) or through the use of subunits. Each disk handled by a
device driver that supports multiple disks using subunits is addressed by the subunit field
of the request header when a request is made for that disk. A device driver that supports
more than one disk can share code and data instead of requiring separate device drivers for
each disk. A "jukebox" CD-ROM system would be an example of a CD-ROM device that might wish
to support more than one drive or a disk pack using a single device driver.
Device drivers that use multiple subunits should use the optional switch /n:<number of
drives> to say how many drives are present. If not present, the default number of drives is
1. If the driver can tell how many drives are installed without a command line switch, then
this argument is not necessary. Unless there are special considerations, it is better
practice to support multiple drives using subunits than to have multiple device headers in
the same device driver file.
Device Header
The device header is an extension to what is described in the MS-PRM.
DevHdr DD -1 ; Ptr to next driver in file or -1 if last driver
DW ? ; Device attributes
DW ? ; Device strategy entry point
DW ? ; Device interrupt entry point
DB 8 dup (?) ; Character device name field
DW 0 ; Reserved
DB 0 ; Drive letter
DB ? ; Number of units
The following are the device attributes for MSCDEX.EXE device drivers:
Bit 15 1 - Character device
Bit 14 1 - IOCTL supported
Bit 13 0 - Output 'till busy
Bit 12 0 - Reserved
Bit 11 1 - OPEN/CLOSE/RM supported
Bit 10-4 0 - Reserved
Bit 3 0 - Dev is CLOCK
Bit 2 0 - Dev is NUL
Bit 1 0 - Dev is STO
Bit 0 0 - Dev is STI
MSCDEX.EXE device drivers will be character devices that understand IOCTL calls and handle
OPEN/CLOSE/RM calls.
The drive letter field is a read-only field for the device driver and is initialized to 0.
The field is for MSCDEX.EXE to use when it assigns the device driver to a drive letter (A
= 1, B = 2...Z = 26). It should never be modified by the device driver. For drivers that
support more than one unit, the drive letter will indicate the first unit, and each
successive unit is assigned the next higher drive letter. For example, if the device driver
has four units defined (0-3), it requires four drive letters. The position of the driver in
the list of all drivers determines which units correspond to which drive letters. If driver
ALPHA is the first driver in the device list, and it defines 4 units (0-3), they will be A,
B, C, and D. If BETA is the second driver and defines three units (0-2), they will be E, F,
and G, and so on. The theoretical limit to the number of drive letters is 63, but it should
be noted that the device installation code will not allow the installation of a device if
it would result in a drive letter > 'Z' (5Ah). All block device drivers present in the
standard resident BIOS will be placed ahead of installable device drivers in the list.
───────────────────────────────────────────────────────────────────────────
NOTE:
It is important that one set lastdrive=<letter> in CONFIG.SYS to accommodate the additional
drive letters that CD-ROM device drivers will require.
───────────────────────────────────────────────────────────────────────────
The number-of-units field is set by the device driver to the number of disks that are
supported. Normal character devices do not support more than one unit and MS-DOS does not
expect a character device to handle more than one unit or have a nonzero subunit value in
the request header. Since these device drivers are not called by MS-DOS directly, this is
not a problem. Nonetheless, the number of units returned by the device driver in the number-
of-units field during the INIT call must be 0, since MS-DOS makes the INIT call and does not
expect a nonzero value for a character device. MSCDEX.EXE will never see what is returned
anyway, and relies on the number- of-units field in the device header.
Sample device header:
HsgDrv DD -1 ; Pointer to next device
DW 0c800h ; Device attributes
DW STRATEGY ; Pointer to device strategy routine
DW DEVINT ; Pointer to device interrupt routine
DB 'HSG-CD1 ' ; 8-byte character device name field
DW 0 ; Reserved (must be zero)
DB 0 ; Drive letter (must be zero)
DB 1 ; Number of units supported (one or more)
As with other MS-DOS device drivers, the code originates at offset 0, not 100H. The first
device header will be at offset 0 of the code segment. The pointer to the next driver is a
double word field (offset/segment) that is the address of the next device driver in the list,
or -1 if the device header is the only one or the last in the list. The strategy and
interrupt entry points are word fields and must be offsets into the same segment as the
device header. The device driver is expected to overwrite the name(s) in each of its one or
more device headers with the <device_name> command line arguments during its initialization.
MSCDEX.EXE will call the device driver in the following manner:
1. MSCDEX.EXE makes a far call to the strategy entry.
2. MSCDEX.EXE passes device driver information in a request header to the strategy
routine.
3. MSCDEX.EXE makes a far call to the interrupt entry.
Request header
MSCDEX.EXE will call the device's strategy routine with the address of a request header in
ES:BX. The format of the request header is the same as what is described in the MS-PRM.
ReqHdr DB ? ; Length in bytes of request header
DB ? ; Subunit code for minor devices
DB ? ; Command code field
DW ? ; Status
DB 8 dup (?) ; Reserved
Status
The status word also has the same format as described in the MS-PRM. It is 0 on entry and
is set by the device driver.
Bit 15 - Error bit
Bit 14-10 - Reserved
Bit 9 - Busy
Bit 8 - Done
Bit 7-0 - Error code (bit 15 on)
Bit 15, the error bit, is set by the device driver if an error is detected or if an invalid
request is made to the driver. The low 8 bits indicate the error code.
Bit 9, the busy bit, should be set by the device driver when the drive is in audio play mode.
Device drivers should fail all requests to the physical device that require head movement
when the device is playing and return the request with this bit and the error bit set and
an error code. Requests that would not interrupt audio play may return without error but will
also have this bit set when the drive is in audio play mode. Play mode can be terminated
prematurely with a reset or STOP AUDIO request and a new request can be made at that point.
Monitoring this bit in each successive request, an Audio Q-Channel Info IOCTL for example,
will tell when play mode is complete.
Bit 8, the done bit, is set by the device driver when the operation is finished.
Error codes are the following:
0 Write-protect violation
1 Unknown unit
2 Drive not ready
3 Unknown command
4 CRC error
5 Bad drive request structure length
6 Seek error
7 Unknown media
8 Sector not found
9 Printer out of paper
A Write fault
B Read fault
C General failure
D Reserved
E Reserved
F Invalid disk change
Command Code Field
The following values are valid command codes:
0 INIT
1 MEDIA CHECK (block devices)
2 BUILD BPB (block devices)
3 IOCTL INPUT
4 INPUT (read)
5 NONDESTRUCTIVE INPUT NO WAIT
6 INPUT STATUS
7 INPUT FLUSH
8 OUTPUT (write)
9 OUTPUT WITH VERIFY
10 OUTPUT STATUS
11 OUTPUT FLUSH
12 IOCTL OUTPUT
13 DEVICE OPEN
14 DEVICE CLOSE
15 REMOVABLE MEDIA (block devices)
16 OUTPUT UNTIL BUSY
128 READ LONG (NEW)
129 Reserved
130 READ LONG PREFETCH (NEW)
131 SEEK (NEW)
132 PLAY AUDIO (NEW)
133 STOP AUDIO (NEW)
134 WRITE LONG (NEW)
135 WRITE LONG VERIFY (NEW)
136 RESUME AUDIO (NEW)
Unsupported or illegal commands will set the error bit and return the error code for Unknown
Command. This includes command codes 1, 2, 4, 5, 6, 8, 9, 10, 15, 16, and 129; and 11, 134
and 135 for systems that do not support writing.
If, in the time since the last request to that device driver unit, the media has changed,
the device driver will return the error code for invalid disk change and set the error bit.
MSCDEX.EXE will then decide whether to retry the request or abort it.
The minimal CD-ROM device driver will read cooked Mode 1 data sectors using HSG addressing
mode and return appropriate values for the IOCTL calls. Most other features enhance
performance or add useful capabilities.
INIT
Command code = 0
ES:BX = INIT
INIT DB 13 dup (0); Request header
DB 0 ; Number of units (must be 0)
DD ? ; End address
DD ? ; Ptr to BPB array
DB 0 ; Block device number
This call is made only once, when the device is installed. INIT and a single IOCTL call for
the device header address are the only device driver calls that come directly from MS-DOS.
Because the INIT function is called from MS- DOS, the number of units returned is 0, as for
normal MS-DOS character devices. MSCDEX.EXE will get the number of units supported from the
device header.
The device must return the END ADDRESS, which is a DWORD pointer to the end of the portion
of the device driver to remain resident. Code and data following the pointer is used for
initialization and then discarded. If there are multiple device drivers in a single file,
the ending address returned by the last INIT call will be the one that MS-DOS uses, but it
is recommended that all the device drivers in the file return the same address. The code to
remain resident for all the devices in a single file should be grouped together low in memory
with the initialization code for all devices following it in memory.
The pointer to BPB array points to the character after the "=" on the line in CONFIG.SYS that
caused this device driver to be loaded. This data is read-only and allows the device driver
to scan the invocation line for parameters. This line is terminated by a carriage return or
a line feed.
During initialization, the device driver must set the device name field in the device header
to the argument provided on the invocation line in CONFIG.SYS. The device driver must also
check that the device_name command line argument is a legal 8-character filename and pad it
out to 8 characters with spaces (20H) when copying it to the device name field.
The block device number and number of units are both 0 for character devices.
READ (IOCTL Input)
Command code = 3
ES:BX = IOCTLI
IOCTLI DB 13 dup (0); Request header
DB 0 ; Media descriptor byte from BPB
DD ? ; Transfer address
DW ? ; Number of bytes to transfer
DW 0 ; Starting sector number
DD 0 ; DWORD ptr to requested vol ID if error 0FH
The media descriptor byte, starting sector number, and volume ID fields are all 0.
The transfer address points to a control block that is used to communicate with the device
driver. The first byte of the control block determines the request that is being made. If
the command code is reserved or the function not supported, then the device driver will
return the error code for Unknown Command. If, for some reason, the device driver is not able
to process the request at that time, it will return the error code for Drive Not Ready.
Number of Bytes
Code to Transfer Function
0 5 Return Address of Device Header
1 6 Location of Head
2 ? Reserved
3 ? Error Statistics
4 9 Audio Channel Info
5 130 Read Drive Bytes
6 5 Device Status
7 4 Return Sector Size
8 5 Return Volume Size
9 2 Media Changed
10 7 Audio Disk Info
11 7 Audio Track Info
12 11 Audio Q-Channel Info
13 13 Audio Sub-Channel Info
14 11 UPC Code
15 11 Audio Status Info
16-255 ? Reserved
Return Address of Device Header
Raddr DB 0 ; Control block code
DD ? ; Address of device header
The device driver will fill the 4-byte field with the address of its device header. This is
used by MSCDEX.EXE to locate the device driver's strategy and interrupt routines.
Location of Head
LocHead DB 1 ; Control block code
DB ? ; Addressing mode
DD ? ; Location of drive head
The device driver will return a 4-byte address that indicates where the head is located. The
value will be interpreted based on the addressing mode. (See function READ LONG for more
information about addressing modes.)
───────────────────────────────────────────────────────────────────────────
NOTE: The drive could provide this information by monitoring the Q-channel on the disk.
───────────────────────────────────────────────────────────────────────────
Error Statistics
ErrStat DB 3 ; Control block code
DB N dup (?) ; Error statistics
The format of the Error Statistics is not yet defined.
Audio Channel Info
AudInfo DB 4 ; Control block code
DB ? ; Input channel (0, 1, 2, or 3) for output channel 0
DB ? ; Volume control (0 - 0xff) for output channel 0
DB ? ; Input channel (0, 1, 2, or 3) for output channel 1
DB ? ; Volume control (0 - 0xff) for output channel 1
DB ? ; Input channel (0, 1, 2, or 3) for output channel 2
DB ? ; Volume control (0 - 0xff) for output channel 2
DB ? ; Input channel (0, 1, 2, or 3) for output channel 3
DB ? ; Volume control (0 - 0xff) for output channel 3
This function returns the present settings of the audio channel control set with the Audio
Channel Control Ioctl Write function. The default settings for the audio channel control are
for each input channel to be assigned to its corresponding output channel (0 to 0, 1 to 1,
etc.) and for the volume control on each channel is set at 0xff.
Read Drive Bytes
DrvBytes DB 5 ; Control block code
DB ? ; Number bytes read
DB 128 dup (?); Read buffer
Data returned from the CD-ROM drive itself can be read using this function. The number-bytes-
read field returns the length of the number of bytes read, which will not exceed 128 per
call. If more than this needs to be returned, the call will be repeated until the number
returned is 0.
The function and content of these bytes are entirely device and device driver dependent. This
function is provided to allow access to device- specific features that are not addressed
under any other portion of the device driver spec.
Device Status
DevStat DB 6 ; Control block code
DD ? ; Device parameters
The device driver will return a 32-bit value. Bit 0 is the least significant bit. The bits
are interpreted as follows:
Bit 0 0 Door closed
1 Door open
Bit 1 0 Door locked
1 Door unlocked
Bit 2 0 Supports only cooked reading
1 Supports cooked and raw reading
Bit 3 0 Read only
1 Read/write
Bit 4 0 Data read only
1 Data read and plays audio/video tracks
Bit 5 0 No interleaving
1 Supports interleaving
Bit 6 0 Reserved
Bit 7 0 No prefetching
1 Supports prefetching requests
Bit 8 0 No audio channel manipulation
1 Supports audio channel manipulation
Bit 9 0 Supports HSG addressing mode
1 Supports HSG and Red Book addressing modes
Bit 10-31 0 Reserved (all 0)
Return Sector Size
SectSize DB 7 ; Control block code
DB ? ; Read mode
DW ? ; Sector size
The device driver will return the sector size of the device given the read mode provided.
In the case of CD-ROM, the value returned for cooked is 2048, and the return value for raw
is 2352.
Return Volume Size
VolSize DB 8 ; Control block code
DD ? ; Volume size
The device driver will return the number of sectors on the device. The size returned is the
address of the lead-out track in the TOC converted to a binary value according to FRAME +
(SEC * 75) + (MIN * 60 * 75). A disc with a lead out track starting at 31:14.63 would return
a volume size of 140613. The address of the lead-out track is assumed to point to the first
sector following the last addressable sector recorded on the disc.
Media Changed
MedChng DB 9 ; Control block code
DB ? ; Media byte
The normal media check function (command code 1) is not performed on character devices and
contains additional semantics that are not needed for CD-ROM device drivers. This is why
there is an IOCTL request for this function.
When the device driver receives a call to see if the media has changed on that subunit, it
will return one of the following values:
1 Media not changed
0 Don't know if changed
-1 (0FFh) Media changed
If the driver can assure that the media has not been changed (through a door-lock or other
interlock mechanism), performance is enhanced because MSCDEX.EXE does not need to reread the
VTOC and invalidate in-memory buffers for each directory access. For drives that do not
report if the media has changed, CD-ROM device drivers can utilize the same solution that
has been applied to floppy disks. In some floppy-disk device drivers, if the MEDIA CHECK
occurs within 2 seconds of a floppy-disk access, the driver reports "Media not changed." It
is highly recommended though that drives be able to detect and report media changes.
If the drive can enforce a door lock mechanism so that the device driver is notified when
the door lock has been unlocked or the device driver is requested to do so by MSCDEX.EXE,
then to improve performance, the driver could return that the media has not changed without
bothering to communicate with the physical device.
If the media has not been changed, MSCDEX.EXE will proceed with the disk access. If the value
returned is "Don't know," or "Media changed," then MSCDEX.EXE will check to see if the disk
has changed. It will continue if it has not, and reinitialize what it knows about the disk
if it has.
It is not necessary for the device driver to do anything for the volume ID when the media
has changed.
Audio Disk Info
DiskInfo DB 10 ; Control block code
DB ? ; Lowest track number
DB ? ; Highest track number
DD ? ; Starting point of the lead-out track
This function returns TOC (Table of Contents) information from the Q-Channel in the lead-in
track indicating what the first and last track numbers are and the Red Book address for the
lead-out track (PMIN/PSEC/PFRAME when POINT = A2). The first and last track numbers are
binary values and not BCD. It is recommended that the information for Audio Disk Info and
Audio Track Info should be read by the drive when the disc is initialized and made accessible
to the driver so that when these functions are called, the drive or driver do not have to
interrupt audio play to read them from the TOC. If the TOC is not made available to the
driver and the driver must obtain the information itself from the lead-in track, the driver
should read and and attempt to cache the disk and track information during the Audio Disk
Info command and invalidate this information only if the media changes.
Audio Track Info
TnoInfo DB 11 ; Control block code
DB ? ; Track number
DD ? ; Starting point of the track
DB ? ; Track control information
This function takes a binary track number, from within the range specified by the lowest and
highest track number given by the Audio Disk Info command, and returns the Red Book address
for the starting point of the track and the track control information for that track. The
track control information byte corresponds to the byte in the TOC in the lead-in track
containing the two 4-bit fields for CONTROL and ADR in the entry for that track. The CONTROL
information is in the most significant 4 bits and the ADR information is in the lower 4 bits.
The track control information is encoded as follows:
00x00000 - 2 audio channels without pre-emphasis
00x10000 - 2 audio channels with pre-emphasis
10x00000 - 4 audio channels without pre-emphasis
10x10000 - 4 audio channels with pre-emphasis
01x00000 - data track
01x10000 - reserved
11xx0000 - reserved
xx0x0000 - digital copy prohibited
xx1x0000 - digital copy permitted
Audio Q-Channel Info
QInfo DB 12 ; Control block code
DB ? ; CONTROL and ADR byte
DB ? ; Track number (TNO)
DB ? ; (POINT) or Index (X)
; Running time within a track
DB ? ; (MIN)
DB ? ; (SEC)
DB ? ; (FRAME)
DB ? ; (ZERO)
; Running time on the disk
DB ? ; (AMIN) or (PMIN)
DB ? ; (ASEC) or (PSEC)
DB ? ; (AFRAME) or (PFRAME)
This function reads and returns the most up to date Q-channel address presently available.
It should not interrupt the present status of the drive as one of its intended purposes is
to monitor the location of the read head while playing audio tracks. This function should
return valid information even when no audio tracks are being played and the head is
stationary. The fields returned correspond to the data that is stored in the Q-channel as
described in the Red Book. The values in MIN-SEC-FRAME, AMIN-ASEC-AFRAME and PMIN-PSEC-PFRAME
are converted by the driver from BCD to binary so that minutes range from 0 to 59+, seconds
from 0 to 59, and frames from 0 to 74. The Control and ADR byte, TNO, and POINT/Index bytes
are always passed through as they appear on the disc and are not converted. If the drive
returns Q-channel information when ADR is not equal to 1, then when ADR is not equal to 1
all ten bytes of information are passed through unmodified to the caller.
Audio Sub-Channel Info
SubChanInfo DB 13 ; Control block code
DD ? ; Starting frame address
DD ? ; Transfer address
DD ? ; Number of sectors to read
This function takes a Red Book address for a particular frame (also known as a block or
frame) and copies 96 bytes of sub-channel information per frame for all the sectors that are
requested sequentially at the transfer address given. Each 96 bytes of information do not
include the two sync patterns (S0 and S1) that head the subcoding block but only the the 96
bytes of subcoding symbols each with one bit of information for the eight different channels
(P-W) that follow them. P is the MSB, W is the LSB of each byte.
The caller is responsible for making sure that 96 * Number_of_sectors_to_read bytes are
available at the transfer address for the device driver to store the results.
Data definition and integrity restrictions for data received with this command are
interpreted according to the CD-ROM standard (Red and Yellow Book).
UPC Code
UPCCode DB 14 ; Control block code
DB ? ; CONTROL and ADR byte
DB 7 dup (?) ; UPC/EAN code
; (last 4 bits are zero; the low-order nibble of ; byte 7)
DB ? ; Zero
DB ? ; Aframe
This function returns the UPC/EAN (Universal Product Code - BAR coding) for the disc. This
information is stored as a mode-2 (ADR=2) Q-channel entry. The UPC code is 13 successive BCD
digits (4 bits each) followed by 12 bits of zero. The last byte is the continuation of FRAME
in mode-1 though in the lead-in track (TNO=0) this byte is zero. If the CONTROL/ADR byte is
zero or if the 13 digits of UPC code are all zero, then either no catalog number was encoded
on the disc or it was missed by the device driver. If the command is not supported, then the
driver will return an error code of Unknown Command. If the command is supported but the disc
does not have a UPC Code recorded, then the driver will return an error code of Sector not
Found.
Audio Status Info
AudStat DB 15 ; Control block code
DW ? ; Audio status bits
; Bit 0 is Audio Paused bit
; Bits 1-15 are reserved
DD ? ; Starting location of last Play or for next Resume
DD ? ; Ending location for last Play or for next Resume
The Audio Paused bit and Starting and Ending locations are those referred to in the RESUME
command.
WRITE (IOCTL OUTPUT)
Command code = 12
ES:BX = IOCTLO
IOCTLO DB 13 dup (0); Request header
DB 0 ; Media descriptor byte from BPB
DD ? ; Transfer address
DW ? ; Number of bytes to transfer
DW 0 ; Starting sector number
DD 0 ; DWORD ptr to requested vol ID if error 0FH
The media descriptor byte, starting sector number, and volume ID fields are all 0.
The transfer address points to a control block that is used to communicate with the device
driver. The first byte of the control block determines the request that is being made. The
Length of Block is the number of bytes to transfer.
Length of Code Block Function
0 1 Eject Disk
1 2 Lock/Unlock Door
2 1 Reset Drive
3 9 Audio Channel Control
4 ? Write Device Control String
5 1 Close Tray
6-255 ? Reserved
Eject Disk
Eject DB 0 ; Control block code
The device driver will unlock the drive and eject the CD-ROM disk from the drive unit. The
door will report as being open until the user has inserted a disk into the drive unit and
closed the door. The status bit for door open can be monitored to determine when a disk has
been reinserted.
Lock/Unlock Door
LockDoor DB 1 ; Control block code
DB ? ; Lock function
When this function is received, the device driver will ask the CD-ROM drive to unlock or lock
the door. If lock function is 0, the device driver will unlock the door. If lock function
is 1, it will lock the door.
Reset Drive
ResetDrv DB 2 ; Control block code
This function directs the device driver to reset and reinitialize the drive.
Audio Channel Control
AudInfo DB 3 ; Control block code
DB ? ; Input channel (0, 1, 2, or 3) for output channel 0 DB ? ;
Volume control (0 - 0xff) for output channel 0
DB ? ; Input channel (0, 1, 2, or 3) for output channel 1 DB ? ;
Volume control (0 - 0xff) for output channel 1
DB ? ; Input channel (0, 1, 2, or 3) for output channel 2 DB ? ;
Volume control (0 - 0xff) for output channel 2
DB ? ; Input channel (0, 1, 2, or 3) for output channel 3 DB ?
; Volume control (0 - 0xff) for output channel 3
This function is intended to provide playback control of audio information on the disk. It
allows input channels on the CD-ROM to be assigned to specific output speaker connections.
The purpose of this function is to allow two independent channels to be recorded──in
different languages for example──and to play back only one of them at a time or to be able
to manipulate an audio signal so that the source appears to move──to make a sound seem to
move from left to right for example.
Output channel 0 is the left channel, 1 is right, 2 is left prime, and 3 is right prime. The
Red Book specification allows for 4 audio channels. The two "prime" channels (2 and 3) extend
stereo to quadrophonic stereo.
An audio volume setting of 0 means off. Drives that don't support 4 output audio channels
may ignore output to channels 2 and 3. Assignment of input channels 2 and 3 to output
channels 0 and 1 may be treated as though the volume control for that channel is 0.
Drives that do not support variable audio control will treat a setting of 0 as off and 1-
0xff as on. Drives that support less than 256 volume settings will do their best to break
up the 256 settings among the settings they can support. E.g. if there are 16 settings
supported, then the first setting will cover 0x01-0x10, the second 0x11-0x20...the sixteenth
0xf1-0xff. Drives that can't play a single channel in both must play only that one channel
and try to suppress the other if possible. Drives that can't swap channels should play the
channel that was moved in its normal channel.
Write Device Control String
DrvBytes DB 4 ; Control block code
DB N dup (?) ; Write buffer
This function is provided to allow programs to talk directly to the CD-ROM drive. All
remaining bytes are sent uninterpreted to the drive unit.
The function and content of these bytes are entirely device and device driver dependent. This
function is provided to allow access to device- specific features that are not addressed
under any other portion of the device driver spec.
Close Tray
CloseTray DB 5 ; Control block code
This command is the logical complement to the Eject Disk command. This command will instructs
drives that can do so to close the door or tray.
READ LONG
Command code = 128
ES:BX = ReadL
ReadL DB 13 dup (0); Request header
DB ? ; Addressing mode
DD ? ; Transfer address
DW ? ; Number of sectors to read
DD ? ; Starting sector number
DB ? ; Data read mode
DB ? ; Interleave size
DB ? ; Interleave skip factor
The request block is different from a normal character device READ to accommodate the larger
size and different characteristics of CD-ROM devices.
The media descriptor byte, which has no meaning for character devices, is now the addressing
mode field. The following values are recognized addressing modes:
0 HSG addressing mode
1 Red Book addressing mode
2-255 Reserved
The default addressing mode is the HSG addressing mode. Long (DWORD) address values are
treated as logical block numbers, as defined by the High Sierra proposal. When Red Book
addressing mode is on, all disk addresses are interpreted as Minute/Second/Frame addresses,
according to the Philips/Sony Red Book standard. Each of these fields is 1 byte. The frame
byte is the least significant byte of the address field, the "second" byte the next most
significant, the minute byte the next, and the most significant byte of the 4-byte field is
unused. These values are represented in binary rather than
in BCD format. For example, if we are referencing the sector addressed by minute 36, second
24, frame 12, the hex long value for this would be 0x0024180C. The relationship between High
Sierra sectors and Red Book frames is described by the equation:
Sector = Minute * 60 * 75 + Second * 75 + Frame - 150
The byte/sector count field becomes the number of sectors to read and the starting sector
number expands from one word to two, which means we can address up to 4 giga-sectors (over
8 terabytes). The DWORD ptr for requested volume ID is eliminated and MSCDEX.EXE will keep
track of what volume is needed.
MSCDEX.EXE handles buffering requests, but performance may be improved if the device driver
reads ahead or uses a sector caching scheme, given the slow seek times of CD-ROM drives. The
operating system will use the prefetch function when it can to give hints to the driver.
The data read mode field will be one of the following:
0 Cooked mode
1 Raw mode
2-255 Reserved
Cooked mode is the default mode in which the hardware typically handles the EDC/ECC and the
device driver returns 2048 bytes of data per sector read. When raw mode is set, the driver
will return all 2352 bytes of user data, including any EDC/ECC present independent of the
actual sector mode (Mode 2 Form 1 vs. Mode 2 Form 2). User programs will have to consider
this and allow enough room for buffer space when reading in raw mode as each sector returned
will take up 2352 bytes of space. Drives that cannot return all 2352 bytes will return what
they can and leave blank what they cannot. For example, drives that can return all 2336 bytes
except the 16 byte header will leave a space in the first 16 bytes where the header would
go so that the sectors align on 2352 byte boundaries. Drivers should do what they can to
return as much of the user data per sector as possible.
The two interleave parameters are for drivers that support interleaved reading. If the driver
does not support interleaving, these fields are both ignored. If it does, interleave size
is the number of consecutive logical blocks or sectors that are stored sequentially, and the
interleave skip factor is the number of consecutive logical blocks or sectors that separate
portions of the interleaved file.
READ LONG PREFETCH
Command code = 130
ES:BX = ReadLPre
ReadLPre DB 13 dup (0); Request header
DB ? ; Addressing mode
DD 0 ; Transfer address
DW ? ; Number of sectors to read
DD ? ; Starting sector number
DB ? ; Read mode
DB ? ; Interleave size
DB ? ; Interleave skip factor
This function is similar in form to READ LONG, but control returns immediately to the
requesting process. The device driver is not obligated to read in the requested sectors but
can instead consider the request for these sectors as hints from the operating system that
they are likely to be needed. It is recommended that at a minimum, the driver seek to the
location provided. The attribute in the device status for prefetching is used to distinguish
drivers that do more than just seek to the given location. The requests are low priority and
preemptible by other requests for service. A READ LONG PREFETCH with 0 number of sectors to
read should be treated as an advisory seek, and the driver can, if it is not busy, move the
head to the starting sector. Since prefetching requests are advisory, there will be no
functional difference between a device driver that supports prefetching from one that does
not, except in terms of performance. The transfer address is not applicable for this call
as the driver is not meant to transfer any data into the user address space.
SEEK
Command code = 131
ES:BX = SeekReq
SeekReq DB 13 dup (0); Request header
DB ? ; Addressing mode
DD 0 ; Transfer address
DW 0 ; Number of sectors to read
DD ? ; Starting sector number
Control returns immediately to the caller without blocking and waiting for the seek to be
completed. The number of sectors to be read and the transfer address are ignored. SEEK is
used to relocate the head in order to begin playing audio or video tracks, or in anticipation
of reading in a particular region on the disk. Further requests for disk activity will wait
until the given SEEK is completed. This seek is not advisory and the head must move to the
desired location.
PLAY AUDIO
Command code = 132
ES:BX = PlayReq
PlayReq DB 13 dup (0); Request header
DB ? ; Addressing mode
DD ? ; Starting sector number
DD ? ; Number of sectors to read
This function will cause the driver to play the selected audio tracks until the requested
sectors have been exhausted or until play is interrupted with a AUDIO STOP request. Control
returns immediately to the caller. Monitoring the busy bit in the status word will determine
if the drive is presently playing audio and also when the play request is completed.
STOP AUDIO
Command code = 133
ES:BX = StopPlayReq
StopPlayReq DB 13 dup (0) ; Request header
This function is included to interrupt the drive unit when it is currently in play mode. At
the next stopping point it reaches, the drive will discontinue playing and process the next
request. If the drive is not currently playing or does not support playing, this request is
ignored.
RESUME AUDIO
Command code = 136
ES:BX = ResumeReq
ResumeReq DB 13 dup (0) ; Request header
This function is used to resume playing audio tracks when play has been interrupted with the
STOP AUDIO command. Its behavior should correspond to the following:
RESET, NEW DISC, PLAY/RESUME COMPLETED
playing = FALSE;
paused = FALSE;
last_startloc = 0;
last_endloc = 0;
PLAY_AUDIO(startloc, endloc) {
if (play(startloc, endloc) != SUCCESSFUL) {
return error;
playing = TRUE;
paused = FALSE;
last_startloc = startloc
last_endloc = endloc
return no error;
}
STOP_AUDIO() {
if (playing) {
last_startloc = present q-channel location
playing = FALSE;
paused = TRUE;
if (stop() == SUCCESSFUL)
return no error;
return error;
}
else {
playing = FALSE;
paused = FALSE;
last_startloc = 0;
last_endloc = 0;
return no error;
}
}
RESUME_AUDIO() {
if (paused) {
if (play(last_startloc, last_endloc) != SUCCESSFUL)
return error;
playing = TRUE;
paused = FALSE;
return no error;
}
else
return error;
Note that the playing flag corresponds to the state that should be reported by the busy bit
in the status word in the request header when the drive is in audio play mode. The paused
flag corresponds to the Audio Paused bit and last_startloc and last_endloc correspond to the
starting and ending location in the Audio Status Info IOCTL.
WRITE LONG
Command code = 134
ES:BX = WriteL
WriteL DB (dup 13 0); Request header
DB ? ; Addressing mode
DD ? ; Transfer address
DW ? ; Number of sectors to write
DD ? ; Starting sector number
DB ? ; Write mode
DB ? ; Interleave size
DB ? ; Interleave skip factor
The device will copy the data at the transfer address to the CD RAM device at the sector
indicated. The media must be writable for this function to work. Data is written sector by
sector, depending on the current write mode and the interleave parameters. The following
values are recognized as valid write modes:
0 Mode 0
1 Mode 1
2 Mode 2 Form 1
3 Mode 2 Form 2
4-255 Reserved
Writing in Mode 1 is the default and must be supported. If the device driver supports the
other modes, then they can be used. If Mode 0 is used, the transfer address is ignored and
all sectors are written with zeroes. If the current write mode is Mode 1 or Mode 2 Form 1,
each sector will consist of 2048 bytes of data located sequentially at the transfer address.
If the write mode is Mode 2 Form 2, the device driver will expect 2336 bytes of data per
sector at the transfer address.
WRITE LONG VERIFY
Command code = 136
ES:BX = WriteLV
WriteLV DB (dup 13 0); Request header
DB ? ; Addressing mode
DD ? ; Transfer address
DW ? ; Number of sectors to write
DD ? ; Starting sector number
DB ? ; Write mode
DB ? ; Interleave size
DB ? ; Interleave skip factor
This function is identical to WRITE LONG, with the addition that the device driver is
responsible for verifying the data written to the device.
INPUT FLUSH
Command code = 7
ES:BX = FlushI
FlushI DB 13 dup (0) ; Request header
Requests that the device driver free all input buffers and clear any pending requests.
OUTPUT FLUSH
Command code = 11
ES:BX = FlushO
FlushO DB (dup 13 0) ; Request header
Requests that the device driver write all unwritten buffers to the disk.
DEVICE OPEN
DEVICE CLOSE
Command code = 13,14
ES:BX = DevOpen, DevClose
DevOpen DB 13 dup (0) ; Request header
Used by the device driver to monitor how many different callers are currently using the CD-
ROM device driver. All new device drivers should support these calls even if nothing is done
with the information.
