Q31509: More Than 15 Files Open at Once in DOS Versions 3.30 and Later

Article: Q31509
Product(s): See article
Version(s): m4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 4-SEP-1990

To open more than 15 files at once in a program under MS-DOS, you must
do the following:

1. Run under MS-DOS version 3.30 or later. (You can double-check the
   DOS version number with the VER command at the DOS prompt.)

2. Add the statement FILES=n in the DOS CONFIG.SYS file.

3. Call interrupt 21 Hex with function 67 Hex from the BASIC program,
   as shown in the example further below.

4. If you are using the SHARE.EXE utility, you must also invoke
   SHARE/F:nnnnn to increase the area for file-sharing information
   above the default of 2048 bytes. Please refer to your MS-DOS manual
   for more information about the SHARE utility.

   The following is an example of using SHARE/F:

      SHARE/F:16384

This information applies to QuickBASIC versions 4.00, 4.00b, and 4.50
for MS-DOS, to Microsoft BASIC Compiler versions 6.00 and 6.00b for
MS-DOS, and to Microsoft BASIC Professional Development System (PDS)
versions 7.00 and 7.10 for MS-DOS.

This technique will retain the file handle table size across a CHAIN
for programs compiled without the BC /O option. Programs that are
compiled with the BC /O (stand-alone .EXE) option and CHAINed will
revert to the original file handle table size.

Please note that even if you follow the above steps and specify
FILES=255 in the DOS CONFIG.SYS file, you may not be able to access
that many files at once in your program because there may not be
enough memory available inside the DGROUP data segment to allocate
file buffers. Also note that five file handles are taken up by the DOS
standard input/output devices.

We recommend that you use the BASIC SETMEM function to reduce the size
of memory available to BASIC as it loads. This method ensures the
provision of more memory available to the operating system; therefore,
more files can be opened at the same time.

To call the "Set handle count" interrupt, load 67 Hex into the AX
register and load the number of desired handles in the BX register.
Under MS-DOS 3.30, you must use an odd number ranging from 21 to 255
for the number of desired handles, since even numbers may make the
interrupt fail under MS-DOS version 3.30. This problem is corrected in
MS-DOS version 3.30a, where you can use even or odd numbers for the
number of desired handles when doing interrupt 67 Hex.

The following is a BASIC code example of the DOS interrupt necessary
to access more than 20 DOS file handles:

' $INCLUDE: 'qb.bi'
' For BASIC PDS 7.00 include QBX.BI
DIM InRegs AS RegType, OutRegs AS RegType

InRegs.ax = &H6700         'SetFileHandles function
' Value in BX register must be odd in DOS 3.30; odd or even in later
' DOS versions; ranging from 21 to 255:
InRegs.bx = x              'x is the number of files to open
CALL INTERRUPT(&H21, InRegs, OutRegs)

FOR I% = 1 TO x - 5   '5 file handles are reserved for DOS standard I/O.
  File$ = "Junk" + STR$(I%)
  OPEN File$ FOR OUTPUT AS I%
  PRINT I%
NEXT
END

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