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When interacting with Hardware-components of your computer by writing to specific memory adresses, this is called memory mapped IO. Sending a command to your soundcard (i.e. writing a value in one of its registers) is simply done by writing to a memory location.
volatile char * foo = 0xDEADBEAF; // 1 byte register maped to memory location 0xDEADBEAF
*foo = 0x00;  // initialize the register
do not forget to use volatile pointers for that.