Sound generation is carried out through another OS call, OSWORD, which handles a variety of tasks enumerated via a task code placed into the accumulator. All OSWORD calls bear a parameter block used to send and receive multiple data; the address of this block is passed in the X and Y registers, with the low byte in X and the high byte in Y. There are four buffered sound channels three melodic and one noise-based on the sound chip found in the BBC Micro. There is only one waveform for melodic channels; the supported note parameters are pitch, duration, amplitude, envelope selection and various control options. For the amplitude parameter, a zero or negative sets a static amplitude, and a positive value select an amplitude and pitch envelope (a predefined temporal variation) to apply to the note.