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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.
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 version 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.
So that I/O redirection can be supported, the DCB numbers are not
used directly when accessing files. Instead, a file "handle" is
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
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
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 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.
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
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}
{Internal routine. Returns the address of the Handle Table,
