AT&T 6300 Programmer's Manual page 371

Syntax
n
linenum
ON PLAY(n) GOSUB
Statement
Specifies the first line number of a subroutine
to be executed when the music buffer contains
fewer than "n" notes. This permits continuous
background music during program execution.
The ON PLAY(n) GOSUB statement is only
used in a program.
ON PLAY( n) GOSUB linenum
is an integer expression in the range 1 to 32. Values outside
this range result in an "Illegal function call" error.
is the first line number of the associated trap routine. A line
number of 0 disables play trapping.
To Enable or Disable PLAY(n) Trapping
PLAY ON
PLAY OFF
PLAY(n) trapping is enabled
PLAY(n) trapping is disabled
Remarks
an error trap occurs
all event trapping is disabled
PLAY STOP PLAY(n) trapping is
suspended, i.e., the GOSUB is
not performed, but it is
performed as soon as a PLAY
ON is executed.
If PLAY(n) trapping is enabled, and the
background music buffer has gone from 'n' to
'n-l' notes, then the ON PLAY(n) GOSUB line
is executed, and the corresponding routine
activated. To avoid recursive traps, a PLAY
STOP is automatically executed when the trap
occurs
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