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The button command
if ( GPRM0 & GPRM0 ) { Mov GPRM7, 2 ; LinkTailPGC, button=1 }
demonstrates some handy tricks for use in BOV and other areas.
First is the use of the & compare operator and the same operand on both sides. What this accomplishes is to test the GPRM for non-zero.
Any non-zero value ANDed with itself will be non-zero, and therefore true. This allows us to set up a GPRM to tell us if the BOV subpictures
are being displayed, and that the button command, which is present regardless of which subpicture stream is displaying (if any), should
activate the feature.
If GPRM0 (in this example) is zero neither the Mov or LinkTailPGC will be performed, causing the button to be ignored.
The rest of the command is typical of a BOV command in that it sets a GPRM to a value which post commands can then use to determine at what point in the movie the button was pressed. And the LinkTailPGC is how we cause the post commands to execute.
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