An application can change a playlist dynamically to achieve various effects in controlling the data stream in memory. In the case of the Clock Sample program, if the chime is an hour chime, then the program will dynamically modify the loop instruction in the memory playlist structure. This is how the device playing the playlist knows how many times to play the chime. The variable usHour is assigned the valid hour in the UpdateTheClock procedure.
/*
* Set the playlist to loop for the number of hours to
* be played. The hour value will be placed into operand one
* of the loop instruction playlist structure.*/
apltPlayList[ HOUR_PLAYLIST ][ LOOP_LINE ].ulOperandOne =
((ULONG) (usHour));
Manipulations that entail atomic, or uninterruptible, operations are acceptable techniques for achieving special effects with playlists. For example, the modification of the 32-bit operation code of an instruction is considered to be an atomic operation. Changing BRANCH to NOP, NOP to BRANCH, or changing the loop count value of a LOOP instruction are examples of atomic operations that produce special effects.
Because the playlist processor is asynchronously interpreting instructions, care must be taken to avoid a situation where unexpected results can occur. For example, suppose a situation exists that enables the loop count field to be modified by the application at the same time that the LOOP instruction is being executed by the playlist processor. The playlist processor can overwrite the application modification with a loop count value based on the execution of the LOOP instruction.
More extensive modifications (particularly those to pointer and length parameters of DATA instructions) should also be performed with caution. You do not want the playlist processor to gain control of a partially modified instruction.
As a rule, it is not a good idea to modify a DATA instruction unless you are sure the playlist processor cannot interpret the instruction during the modification process; that is, you know the instruction is currently unreachable by the processor. One way to determine the location of the processor is with the MESSAGE instruction. For example, suppose you precede a LOOP instruction with a MESSAGE instruction. When the message is returned to your application, you know the processor is busy with the loop and you can safely modify any DATA instructions outside the loop.
After DATA instructions have been modified, BRANCH or NOP instructions can be changed to direct playlist interpretation to the modified section of the playlist.