Basic 7.0 UEVENT Example Causes Subsequent Programs to Hang (59399)

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Page 311 of "Microsoft Basic 7.0: Programmer's Guide" contains a misprint in the MASM programming example for trapping a user-defined event. If the code is not corrected, an "A2009: Symbol not defined" error will result when the code is assembled.

The last section of code for this example reads as follows:

    RestInt  proc  uses ds
       lds dx, cs:OldVector
       mov  x, 251CH          ; <== this line contains the misprint
				

To correct the code, make the following change:

    RestInt  proc  uses ds
       lds dx, cs:OldVector
       mov ax, 251CH          ; <== change "x" to "ax"
				

However, even when this misprint is corrected, if the Basic program is compiled so that it requires a run-time module or if the assembly code is put into a Quick library, running the program may cause the computer to hang. The problem does not occur if the Basic program is compiled with the BC /O option.

The following compiling and linking steps will reproduce the problem:

1. Assemble the MASM code as follows:

         MASM MASMPROG.ASM;

2. Compile the Basic code so that it requires a run-time module (no /O) and enables event trapping (/V). If the program is compiled as a stand-alone program (with the /O option), the problem does not exist.

         BC BASPROG.BAS /V;
   						

3. LINK the program using the Basic PDS 7.00 or 7.10 linker, as follows:

         LINK BASPROG.OBJ + MASMPROG.OBJ;
   						

4. Run the program.
5. The computer may hang instantly or may hang after attempting to run another program, such as QBX.EXE.