y<address>,<value>

<address> (-INT_MAXINT_MAX)
Address to put the value at. The recommended range of this parameter is $0000 to $FFFF

<value> (-INT_MAXINT_MAX)
The value to write. Only the 8 least significant bits of this value will be used.

This somewhat specific command allow to directly write any value to anywhere in the NES memory. You could directly write to sound registers as well as write into NES RAM — this could end up into corrupting your song, making wierd effects, who knows?
Unless you know what you're doing, you might avoid using this command.

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