Programmer 7500/MAX_PROGRAMMERS.iso/PASCAL/PAS_ALL.ZIP/TI174.ASC

Text File | 1991-09-11 | 18.1 KB | 595 lines

  PRODUCT  :  Turbo Pascal                           NUMBER  :  174
  VERSION  :  3.01A
       OS  :  PC DOS
     DATE  :  August 1, 1986                           PAGE  :  1/9

    TITLE  :  Files Open Extend




  The following is public domain information that has been uploaded
  to our Forum on CompuServe. As a courtesy to our users that do
  not have immediate access to CompuServe, Technical Support
  distributes these routines free of charge.

  However, because these routines are public domain programs, not
  developed by Borland International, we are unable to provide any
  technical support or assistance using these routines. If you need
  assistance using these routines, or are experiencing
  difficulties, we recommend that you log onto CompuServe and
  request assistance from the Forum members that developed these
  routines.

  Written by:
  Randy Forgaard, CompuServe 70307,521
  Many thanks to Bela Lubkin (CompuServe 76703,3015) for
  masterminding this idea, and Kim Kokkonen (CompuServe 72457,2131)
  for helping me debug it. For more discussion of Handle Tables and
  the implementation of DOS redirection, please see Stan Mitchell,
  "Command Line Redirection," PC Tech Journal, January 1986, Page
  44.

  Due to a limitation of DOS, Turbo Pascal version 3.0 only allows
  up to 15 files at a time to be open. The following method allows
  you to have up to 96 files open simultaneously under DOS 2.0 or
  2.1, or 252 files open simultaneously under DOS 3.0 or greater.

  TO USE THIS TECHNIQUE:

  You need the routines and global declarations below. Everywhere
  in your program that you Reset or Rewrite a file "f" for the
  first time, insert an "OpenExtend(f);" invocation immediately
  after the Reset or Rewrite. Each time your program calls one of
  Turbo's built-in routines (other than Assign) for handling files
  (e.g., Read, Write, Close, Seek, Reset, Rewrite, etc.), put an
  "UnExtend(f);" invocation immediately prior to the call, and a
  "ReExtend(f);" invocation immediately after the call. Do not
  insert UnExtend and ReExtend calls around the very first Reset or
  Rewrite that you use to initially open a file.

  At the top of your program, prior to the "program" statement, put
  the compiler directive {$F252}. (You may use a value smaller than
  252 if you wish. Under DOS 2.0/2.1, values above 96 have













  PRODUCT  :  Turbo Pascal                           NUMBER  :  174
  VERSION  :  3.01A
       OS  :  PC DOS
     DATE  :  August 1, 1986                           PAGE  :  2/9

    TITLE  :  Files Open Extend




  no additional benefit. Each larger value for the {$F} directive
  uses 2 additional bytes in the program's global data space.) The
  value you specify for the {$F} directive is the maximum number of
  files you will be able to have open at the same time in your
  program.

  Edit your CONFIG.SYS file (see the DOS manual for details) so
  that it includes a line that says "FILES=XXX". XXX should be a
  number that is 3 greater than the value you specified for the
  {$F} directive (larger values will provide no additional benefit
  with respect to your individual program), and should not exceed
  99 (for DOS 2.0/2.1) or 255 (for DOS 3.0 and higher). Under any
  version of DOS, the minimum allowable value for XXX is 8. Then,
  reboot your computer so that the FILES=XXX parameter takes hold.
  Running the sample program at the bottom of this file will tell
  you the maximum number of files you can open at once. (Usually 96
  or 252, unless a resident program has opened some files and
  hasn't closed them yet.)

  THE TECHNICAL DETAILS:

  Much of the following information is not documented in the DOS
  Technical Reference manual.

  Under DOS 1.0 and 1.1, all files were accessed via File Control
  Blocks (FCB's). There was no limit to the number of FCB's that a
  program could use, so there was no limit to the number of files
  open simultaneously.

  Under DOS 2.0 and greater, an alternate (and preferable) method
  of accessing files was introduced which uses a 2-byte integer
  called a "handle" to refer to a file. A "handle" file is
  described using a data structure called a Device Control Block
  (DCB). However, DOS provides the storage space for all DCB's,
  rather than having the application program store the DCB's, so
  the number of available DCB's is limited to the amount of space
  that DOS has set aside for them. The maximum number of
  files/devices that can be open simultaneously is the number of
  slots available in the DCB Table created by DOS. The DCB's in the
  DCB Table are consecutively numbered, starting with 0. DCB's 0,
  1, and 2 are predefined by DOS to correspond to the AUX, CON, and
  PRN devices, respectively. All remaining DCB's in the DCB Table
  are available for files or devices used by application programs.













  PRODUCT  :  Turbo Pascal                           NUMBER  :  174
  VERSION  :  3.01A
       OS  :  PC DOS
     DATE  :  August 1, 1986                           PAGE  :  3/9

    TITLE  :  Files Open Extend




  So that I/O redirection can be supported, the DCB numbers are not
  used directly when accessing files. Instead, a file "handle" is
  used. A "handle" is an index into a 20-byte array, called the
  Handle Table, which is located at offset 18H of the Program
  Segment Prefix (PSP) for a program (for a general discussion of
  the PSP, see the DOS Technical Reference manual). Each element of
  the Handle Table is the DCB number of a file or device. The value
  at index "handle" in the Handle Table is the DCB number of the
  file or device that implements that file handle. Thus, if the
  value 8 is in the 6th byte of the Handle Table, the handle "6"
  refers to the file (or device) described by the DCB in slot 8 of
  the DCB Table. If a handle is not currently being used, its entry
  in the Handle Table is FFH. DOS predefines the first 5 handles to
  be primary input, primary output, error, auxiliary, and printer,
  so the first 5 entries in the Handle Table are 1, 1, 1, 0, and 2,
  corresponding to the DCB numbers for the CON (1), AUX (0), and
  PRN (2) devices. This leaves only 15 available handles for
  opening files (or new devices).

  Every time a new handle file is opened, a new handle gets used.
  Since there are only 20 slots available in the Handle Table for a
  program, DOS only allows a "process" to have a maximum of 20 file
  handles in use simultaneously (and the first 5 entries are
  predefined, as just noted, unless those handles get closed and
  reused). Every new handle file requires a unique handle, so only
  20 files/devices can be open at the same time by a single process
  (unless FCB's are used). (A "process" is any program spawned
  using the DOS EXEC function call. A process can be invoked by
  COMMAND.COM, or by another program.) There can be many more than
  20 DCB's in the DCB Table, so the real limitation is in the size
  of the Handle Table in the PSP.

  The size of the DCB Table (i.e., the maximum number of
  files/devices that can be open simultaneously in the whole
  computer) is controlled by the FILES=XXX entry in the CONFIG.SYS
  file. The minimum number of slots is 8. Under DOS 2.0/2.1, the
  maximum number is 99, and under DOS 3.0 and higher, the maximum
  is 255. As previously mentioned, the first three of these DCB
  slots are occupied by the AUX, CON, and PRN devices.

  A single program can use all of the DCB's in the DCB Table
  (except for the 3 reserved by DOS) all on its own, by effectively
  bypassing the Handle Table in the PSP, except on a temporary













  PRODUCT  :  Turbo Pascal                           NUMBER  :  174
  VERSION  :  3.01A
       OS  :  PC DOS
     DATE  :  August 1, 1986                           PAGE  :  4/9

    TITLE  :  Files Open Extend




  basis. The program can accomplish this feat by using, say, only
  one entry in the Handle Table for all of its files. Instead of
  allowing DOS to store the DCB numbers in the Handle Table, the
  program can store these numbers elsewhere. Then, to manipulate a
  file using DOS, the program can temporarily put the DCB number of
  that file into a designated slot in the Handle Table, pass the
  index of that table slot (i.e., that "handle") to DOS, and DOS
  will operate on that handle/DCB number. After the DOS call, the
  program can remove that DCB number from the designated Handle
  Table slot, freeing up that handle for use in another DOS call
  for another file. In this way, DOS can be fooled into accessing
  up to 96 (or 252) different files/devices using a single handle
  entry in the Handle Table.

  The OpenExtend, UnExtend, and ReExtend routines below use this
  technique. OpenExtend(f) is used on a previously-opened file,
  "f." It removes f's DCB number from the Handle Table, and stores
  that DCB number in place of the file handle in Turbo's data
  structure for "f." UnExtend(f) copies the current DCB number (if
  any) in the last slot of the Handle Table to a safe place, copies
  the DCB number of "f" to that slot, and then puts the handle of
  that slot into the "handle" position of the data structure for
  "f," in preparation for its use by Turbo/DOS. ReExtend(f)
  replaces the file handle of "f" with the DCB number for "f" in
  the data structure for "f," and restores the previous value (if
  any) of the last slot in the Handle Table. Note that the last
  slot of the Handle Table was chosen totally arbitrarily. Any  H
  Table entry can be used for fooling DOS, and (since we are saving
  the previous DCB number in that entry) it does not preclude also
  using that designated slot as a legitimate handle.

  To obtain the address of the Handle Table, which is at offset 18H
  in the PSP, the program needs to find the address of its PSP.
  Normally, this is very easy: when DOS loads a .COM file, the
  address of the PSP is just CS:0000. Using CS:0000 in this manner
  would be viable as long as we compile the program to a .COM file
  and execute the .COM file. However, if we run the program in
  DOS, it still thinks of Turbo's PSP as being the "official" PSP
  for  the running program, since DOS did not "see" Turbo invoke
  the program. Hence, CS:0000 is not the valid PSP address when the
  program is running in memory, since that is the address of the
  program's "fake" PSP rather than Turbo's PSP.














  PRODUCT  :  Turbo Pascal                           NUMBER  :  174
  VERSION  :  3.01A
       OS  :  PC DOS
     DATE  :  August 1, 1986                           PAGE  :  5/9

    TITLE  :  Files Open Extend




  To allow the program to work correctly both when running in
  memory and when run as a .COM file, we use the DOS function call
  62H, "Get Program Segment Prefix Address (PSP)." This function
  call is available in DOS 3.0 and higher. There is an identical
  function call in DOS 2.0/2.1, but its function number is 51H, and
  it is not documented. Function 51H is also available in DOS
  3.0/3.1. However, for upward-compatibility reasons with future
  versions of DOS, we will use the undocumented 51H function with
  DOS 2.0/2.1 (since we know 51H is available in those versions of
  DOS), and use 62H for DOS 3.0 and higher (since 62H is a
  documented function). There is no such function call in DOS
  1.0/1.1, but the technique below will not work with those early
  versions of DOS anyway, since they did not provide file handles.
  To decide whether to use function 51H or 62H, we call DOS
  function 30H, "Get DOS Version Number," to determine which
  version of DOS is running. This strategy for obtaining the Handle
  Table address is implemented in the GetHandleTableAddr function,
  below, which gets called only once (the first time that
  OpenExtend is called).

  Note: This technique will not interfere with overlays in your
  program (since it only uses the Handle Table slot temporarily),
  provided that your program leaves at least one DCB available for
  use by the Turbo run-time library to read in overlay files.

  {$F252}

  const
    LastHandle = 19;    {Highest-numbered handle}
    UnusedHandle = $FF; {DcbTable entry that denotes an unused
  handle}
  type
    HandleTable = array[0..LastHandle] of Byte;
    HandleTablePtr = ^HandleTable;
  const
    TablePtrOk: Boolean = false; {"True" iff TablePtr is
  initialized}
  var
    TablePtr: HandleTablePtr; {Points to Handle Table for this
  process}
    SaveDcb:  Byte;  {Temporary variable for a DCB number during a
  function
                                  call}













  PRODUCT  :  Turbo Pascal                           NUMBER  :  174
  VERSION  :  3.01A
       OS  :  PC DOS
     DATE  :  August 1, 1986                           PAGE  :  6/9

    TITLE  :  Files Open Extend




  {Internal  routine.   Returns  the address of the  Handle  Table,
  which is at offset 18H in the PSP.}

  function GetHandleTableAddr: HandleTablePtr;
  var
    regs: record
                  case Integer of
                    1: (AX, BX, CX, DX, BP, SI, DI, DS, ES, Flags:
  Integer);
                    2: (AL, AH, BL, BH, CL, CH, DL, DH: Byte)
                  end;

  begin
    regs.AH := $30;
    MsDos(regs);      {Get DOS version number}
    case regs.AL of
      0: begin
                  writeln('This program only works with DOS 2.0 and
  higher');
                  Halt
            end;
      2: regs.AH := $51; {Undocumented, but works with DOS 2.0/2.1
  (and 3.X)}
    else regs.AH := $62  {Works with DOS 3.0 and higher}
    end;
    MsDos(regs);      {Get PSP address}
    GetHandleTableAddr := Ptr(regs.BX, $18)
  end {GetHandleTableAddr};


  {Causes "f" to become an "extended" file; i.e., to remain open
  without using up any file handles. The parameter "f" must be any
  Turbo file;  e.g., a File, a File of Byte, a File of Foo, Text,
  etc. This routine should be called immediately after the Reset or
  Rewrite that is initially used to open "f." After "f" has become
  extended, it cannot be used by Turbo's built-in file routines
  until it has been unextended using UnExtend.}

  procedure OpenExtend (var f);
  var
    handle: Integer absolute f;
  begin
    if not TablePtrOk then













  PRODUCT  :  Turbo Pascal                           NUMBER  :  174
  VERSION  :  3.01A
       OS  :  PC DOS
     DATE  :  August 1, 1986                           PAGE  :  7/9

    TITLE  :  Files Open Extend




      begin
           TablePtr := GetHandleTableAddr;
           TablePtrOk := true
      end;
    SaveDcb := TablePtr^[handle];
    TablePtr^[handle] := UnusedHandle;
    handle := SaveDcb
  end {OpenExtend};


  {Unextends the extended file "f," so that it can be used by any
  of Turbo's built-in file routines. Note that "f" must have been
  converted  to an extended file by OpenExtend before invoking
  UnExtend(f). After calling UnExtend, and then invoking the Turbo
  file function on "f," ReExtend(f) should be invoked immediately
  to re-extend "f" and restore the DOS file state information.}

  procedure UnExtend (var f);
  var
    handle: Integer absolute f;
  begin
    SaveDcb := TablePtr^[LastHandle];
    TablePtr^[LastHandle] := Lo(handle);
    handle := LastHandle
  end {UnExtend};


  {Re-extends "f" into an extended file. Note that "f" must have
  been converted to an extended file by OpenExtend, and then
  unextended  using UnExtend, before "f" can re-extended using
  ReExtend.  ReExtend(f) should be invoked immediately after any
  normal Turbo file function call using "f."}

  procedure ReExtend (var f);
  var
    handle: Integer absolute f;
  begin
    handle := TablePtr^[LastHandle];
    TablePtr^[LastHandle] := SaveDcb
  end {ReExtend};

  {Example program -- This program opens as many Text files as it
  can,  until DOS runs out of room in its DCB Table. It then













  PRODUCT  :  Turbo Pascal                           NUMBER  :  174
  VERSION  :  3.01A
       OS  :  PC DOS
     DATE  :  August 1, 1986                           PAGE  :  8/9

    TITLE  :  Files Open Extend




  reports how many files were successfully opened, writes a line to
  each of them, then closes and erases each of them. Note: The
  value of the FILES=XXX parameter in the CONFIG.SYS file must be
  set to an appropriate value (see above). If you change the
  FILES=XXX value, be sure to reboot before running this program.

  This program takes a while to run, due to the heavy disk I/O, so
  running it on a hard disk (or, even better, a RAM disk) is
  recommended.  Make sure that you are running the program in a
  subdirectory, so that you don't run up against the DOS limit on
  the number of allowable files in the root directory of a drive.}

  const
    MaxCount = 255;
  var
    num: string[6];
    f: array[1..MaxCount] of Text;
    i, count: Integer;
    result: Byte;
  begin
    writeln('Opening files...');
    i := 0;
    repeat
      i := i + 1;
      Str(i, num);
      Assign(f[i], 'junk' + num + '.txt');
      {$I-} Rewrite(f[i]); {$I+}
      result := IOResult;
      if result = 0 then OpenExtend(f[i])
    until result <> 0;
    count := i - 1;
    writeln('Successfully opened ', count, ' files at the same
  time.  ',
                  'Writing to each file...');
    for i := 1 to count do
      begin
           UnExtend(f[i]);
           writeln(f[i], 'This is a test');
           ReExtend(f[i])
      end;
    writeln('Closing and erasing each file...');
    for i := 1 to count do
      begin













  PRODUCT  :  Turbo Pascal                           NUMBER  :  174
  VERSION  :  3.01A
       OS  :  PC DOS
     DATE  :  August 1, 1986                           PAGE  :  9/9

    TITLE  :  Files Open Extend




           UnExtend(f[i]);
           Close(f[i]);
           Erase(f[i]);
           ReExtend(f[i])
      end;
    writeln('Done.')
  end.
  (**)