ÜBERLEBEN.../UEBERLEBEN.ISO/_treiber/cd_emu/cdemu2.txt

Text File | 1992-11-18 | 7KB | 149 lines

    CDEMU2: an MSCDEX emulator for networked CD-ROM drives
                         User's Guide
                        By J.M.A. Hall

Overview
On MS-DOS based computers, CD-ROM drives that are directly
connected are normally made 'known' to the system using a
program called MSCDEX, the Microsoft CD-ROM Extensions. The
purpose of this program is firstly to make the files of the CD-
ROM appear like an MS-DOS disk drive (which is very large, and
read-only); and secondly to allow access to CD-ROM specific
functions (such as the playing of audio tracks) in a hardware-
independent manner.
Several commercial solutions now exist for sharing CD-ROMS
within a local-area network (LAN); most, however, are
expensive and only cost-effective on large (and therefore
expensive) networks. For small networks, a cheaper option is
to use a peer-to-peer networking package such as NetWare Lite
to share the drive letter assigned to the CD-ROM drive by
MSCDEX across the network. The pitfall with this scheme is
that a number of CD-ROM access packages do not simply access
the CD-ROM as if it were an MS-DOS disk drive; instead they
make special calls to MSCDEX, which fail since MSCDEX is not
loaded on the machines accessing the drive across the network.
Happily, most only make a few calls, principally to determine
that MSCDEX is loaded and to determine which drives on the
system are in fact CD-ROMs
The function of CDEMU2, therefore, is to emulate MSCDEX in a
limited fashion in order to fool CD-ROM applications into
working across a network. It must be noted that not all CD-ROM
applications will work in this fashion: applications that rely
on playing of audio tracks will obviously not be usable in a
network environment. Others, such as the Harrap Multilingual
Dictionary, ignore the disk drive set up by MSCDEX and access
the sectors of the CD-ROM directly; this and other
applications that do this cannot be shared in this fashion. It
should also be noted that using a CD-ROM application across a
network legally requires that it be licensed for use on
multiple machines concurrently.

Installation
The package consists of this document file and two programs,
CDEMU2.COM and CDINFO.EXE. These may be placed in any
convenient directory, though it is suggested that they be
stored in a single directory on a file server which is
accessible to all prospective clients of the networked CD-ROM
drive and that this directory be included in the DOS PATH on
all such machines.

Usage of CDEMU2
The command line format for CDEMU2 is as follows:
          CDEMU2 d[:][=vtoc-file] [...] or
          CDEMU2 /U to unload from memory
where
d[:]      is the MS-DOS disk drive letter which will be used
          to access the CD-ROM drive across the network. It is
          not necessary that this disk drive actually exist or
          have been created at the time CDEMU2 is loaded,
          though of course it must when the CD-ROM application
          is started.
vtoc-file is the name of a file created by the CDINFO utility.
          This is used when it is necessary to emulate the
          "Get VTOC" function in MSCDEX (see below for further
          details).
...       indicates that the above can be repeated as often as
          necessary up to the length limit of an MS-DOS
          command line (127 characters)

Usage of CDINFO
Certain CD-ROM packages, such as "Le Robert Electronique", not
only check that MSCDEX is loaded, but read the "VTOC" or
"Volume Table of Contents" as well (presumably in order to
check that the CDROM is the correct one). As the VTOC is not
accessed as an MS-DOS file, this function cannot be directly
handled by the emulator. There is, however, a solution: the
VTOC can be read from the CD-ROM into an MS-DOS file, and
CDEMU2 can emulate the "Read VTOC" function  by reading this
file instead. This is what the CDINFO utility is for.
The command line format for CDINFO is
          CDINFO d: vtoc-file
where
d:        is the drive letter assigned to the CD-ROM drive.
          Note that you must run CDINFO on the system that the
          physical CD-ROM drive is connected to!
vtoc-file is the name of the file to store the VTOC
          information in. It is suggested that this be stored
          on the system acting as a fileserver and made
          available to the client PCs.


Examples
Suppose that the CDEMU stuff (together with the VTOC emulation
files) live in the directory C:\CDEMU2 on the machine acting
as the CD-ROM server and  the CD-ROM product to be shared is
"Le Robert Electronique" which appears to the server as drive
E:. The steps to be followed are then:
On the server:
1.   Make C:\CDEMU2 and E:\ available to the network (this
     assumes that the network software is already set up).
2    Ensure that the CD-ROM is mounted in its drive. Change
     directory to C:\CDEMU2 and issue the command
               CDINFO E: LEROBERT.CDI
On the client:
1.   Access the CD emulation software and the CD-ROM drive on
     the server using the network software. It will be assumed
     that these will be accessed as drives G: and H: on the
     client (i.e. G:\ on the client corresponds to C:\CDEMU2
     on the server and H:\ on the client corresponds to E:\ on
     the server).
2.   Add the directory G:\ to the PATH statement on the
     client. If the network software provides no other way to
     do this, write a batch file that contains the line
               PATH G:\;%PATH%
     and execute it.
3.   Load CDEMU2 on the client with the following command
     line:
               CDEMU2 H:=G:\LEROBERT.CDI
4.   Run the "Le Robert" access program
5.   Unload the emulator from memory, if required, using the
     command
               CDEMU2 /U

Special notes
1.   Most CD-ROM packages have a piece of software (usually
called a "search engine" for database type applications) which
needs to be installed on a hard disk This software may or may
not be sharable via the network, depending on how it is
written.
2.   If the application makes a call to MSCDEX that is not
supported by the emulator the screen will be cleared and an
error message will be reported, together with the parameters
to the call (i.e. the CPU register values at the time). This
will normally be an indication that the application cannot be
shared over a network using this emulator. The user should
press a key, and then quit the application, since its screen
contents will have been destroyed.
3    It should be noted that an application must be licensed
for multiple users on a network even if this emulator is
capable of making it sharable.

Copyright
This software is copyright (c) 1992 J.M.A. Hall. However, it
may be freely redistributed providing all the files comprising
it are distributed together, and on the understanding that
there is NO WARRANTY attached to this software. The author
would be interested in comments and suggestions (email
J.M.Hall@soton on JANET or [e-mail] on the
Internet)