Conditionally Loading QBHERC Hercules Support from QuickBasic (37307)

SUMMARY

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.

MORE INFORMATION

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

Run a batch file that starts an assembler program that tests if a Hercules card is installed.
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.)
If necessary, invoke QBHERC.COM from the batch file.
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.