UART: low level interface

A simple UART is just a few registers w/ a few bits
Some bits for baud rate and parity
A few bits for interrupt control
A status register
One RX register and one TX register

Minimal code is as follows (warning: serious style bug!)

    void tx(char c)
    {
        while (STATUSREG & TXBUSY)
            ;
        TXREG = c;
    }

    int rx(void)
    {
        if (!STATUSREG & RXFULL)
            return -1;
        retrun RXREG;
    }