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Q33488: QuickBASIC 4.00b UPDATE.DOC: Adds ON UEVENT to Trap Events

Article: Q33488
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: 16-DEC-1989

The following information was taken from the QuickBASIC Version 4.00b
UPDATE.DOC file. It describes features that have been added since
Microsoft QuickBASIC Version 4.00 was released.

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

New Statement: ON UEVENT
------------------------

Action: Defines an event trap for a user-defined event

Syntax: ON UEVENT GOSUB {<linenumber>|<linelabel>}

Remarks:

In this syntax, <linenumber> or <linelabel> specifies the number or
label of the first line in the event-trapping subroutine. The ON
UEVENT statement allows your program to branch to an event-trapping
subroutine when a user-defined event occurs. ON UEVENT is unlike other
event-trapping statements in that it allows your program, not an
external agent, to trigger the event that you expect to trap. For
example, assume that your system contains a data acquisition device
that generates an interrupt when data arrives. Most of your
application is written in QuickBASIC. The main module contains an
event-trapping subroutine that responds to a user-defined event (in
this case, the arrival of data from the acquisition device). The
following three assembly-language procedures are also linked into the
program:

1. An interrupt handler that responds to the interrupt generated by
   the acquisition device

2. A routine that installs the interrupt handler in the chain of
   interrupts

3. A routine that removes the interrupt handler from the chain of
   interrupts

When the program begins, it calls the assembly-language routine that
installs the interrupt handler. Then it executes an ON UEVENT
statement to identify the QuickBASIC subroutine that will be executed
when a user-defined event occurs. The final step in setting up the
user-defined event trap is to execute a UEVENT ON statement. Once this
is done, the BASIC program enters its main execution loop.

When data arrives, the acquisition device generates an interrupt. The
assembly-language interrupt handler, in turn, calls the routine
SetUEvent. SetUEvent is a BASIC run-time-library routine that causes a
user-defined event to occur in BASIC. This special routine can be
called from any Microsoft language. References to SetUEvent are
resolved when your program is linked with the run-time library. The
user-defined event causes the program to branch to the QuickBASIC
subroutine identified by the previous ON UEVENT statement. At this
point, the event-trapping subroutine performs whatever processing is
desired. Just before the application terminates, it calls the
assembly-language routine that removes the interrupt handler from the
chain of interrupts.

New Statements: UEVENT ON, OFF, STOP
------------------------------------

Action: Enable, disable, or suspend event trapping for a user-defined
        event

Syntax: UEVENT ON
        UEVENT OFF
        UEVENT STOP

Remarks:

The effect of UEVENT parallels that of other event-trapping
statements. For example, UEVENT ON enables a user-defined event trap
that you previously set up with an ON UEVENT GOSUB statement.

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