Baud to bit mapping for the UART?
The UART used in a PC COM port doesn't carry out modulation as such, that would only happen when you send the serial data through something like a modem or radio link where it has to be converted into an audio signal in the range carried by a telephone line or used to modulate a carrier wave.

The "mapping" is defined by the interface standard; in the case of RS-232, anything less than -3V is considered a logic 1 and anything greater than +3V a logic 0.