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Q37307: Conditionally Loading QBHERC Hercules Support from QuickBASIC

Article: Q37307
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 28-DEC-1989

You will encounter some limitations if you wish to conditionally load
QBHERC.COM or MSHERC.COM from within a QuickBASIC program.

The only way to invoke QBHERC.COM or MSHERC.COM from within a
QuickBASIC program is with the SHELL statement. QBHERC.COM and
MSHERC.COM are terminate-and-stay resident (TSR) graphics drivers. TSR
programs will fragment MS-DOS memory when loaded with the SHELL
statement. You can only unfragment the memory by rebooting the
computer. Fragmentation can drastically reduce the memory available to
subsequent programs run in MS-DOS. This fragmentation problem can be
serious, and it is best to avoid SHELLing to QBHERC.COM or MSHERC.COM
or any TSR program.

Note that the BASIC run-time routines must know at initialization time
(that is, when the program is started) whether or not QBHERC.COM or
MSHERC.COM has been loaded. This means a program (or batch file) that
loads QBHERC.COM or MSHERC.COM must RUN or CHAIN (or invoke) the .EXE
program that will use Hercules graphics.

This information applies to Microsoft QuickBASIC 4.00, 4.00b, and
4.50, to Microsoft BASIC Compiler Versions 6.00 and 6.00b, and to
Microsoft BASIC PDS Version 7.00 for MS-DOS and MS OS/2.

The following is a possible scenario for conditionally executing
QBHERC.COM or MSHERC.COM:

1. Run a batch file that starts an assembler program that tests if a
   Hercules card is installed.

2. Test to see if a Hercules graphics card is installed, and pass this
   information back to the batch file by using batch ERRORLEVEL (or
   information written in a text file). (Note that a QuickBASIC
   program does not let you return ERRORLEVELs to batch files, because
   it always resets the ERRORLEVEL to zero for normal termination.)

3. If necessary, invoke QBHERC.COM from the batch file.

4. Invoke the QuickBASIC QB.EXE program. The compiler's run-time
   system now reinitializes to take advantage of QBHERC.COM or
   MSHERC.COM, which is now resident in memory.

Normally you would run QBHERC.COM or MSHERC.COM manually or from a
batch file before invoking a graphics program that takes advantage of
SCREEN 3 on a computer with a Hercules graphics adapter installed.

Hercules support was introduced in QuickBASIC Version 4.00. For more
information, please see the README.DOC file for QuickBASIC Versions
4.00 and 4.00b.

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