Constant VOLUME_UP_SIGNAL 

pub const VOLUME_UP_SIGNAL: u8 = 0x81; // 129u8

Console-stream signals for the audio shortcuts (docs/audio.md, “Keyboard shortcuts”): Ctrl+Alt+Up, Ctrl+Alt+Down and Ctrl+Alt+M. The Ctrl+Alt+Del pattern exactly: the keyboard driver only SAYS the chord happened, on the console stream, and the SHELL - which asks the audio driver - acts. The keyboard drivers gain no capability and never speak to the audio driver. Outside ASCII, so no typed key produces them, and read from here by both ends.