They don't usually run an OS. But are structured like

  void setup_code() {
    // configure SPI
    // analog to digital converters
    // timers
    // fix internal and external buggy hardware
  }

  void interrupt_handler() {
    TURN_OFF_INTERRUPTS;
    // packet recv
    // time expired
    // etc
    TURN_ON_INTERRUPTS;
  }


  setup_code();
  register_interrupt_handler(interrupt_handler);
  while(1) {
    // application code
  }

Microcontrollers like this only have 2k of working state, the presentable complexity is in the code, not the data.

Freescale provides an RTOS for free called MQX (http://www.freescale.com/webapp/sps/site/homepage.jsp?code=M...) that I have seen used on some projects. Otherwise, bare metal or one of the other standard RTOS options.

There are various FORTHs which run on this and similar chips. Search for M0 FORTH. For example I think this one will run on the Freedom Board:

http://mecrisp.sourceforge.net/

You quite possibly don't run an OS, but there are a number of RTOS stacks available - or some people still brew their own.

an RTOS (uC/OS, Freertos, chibios, ...) or simply a hal such as CMSIS.

Bare metal, and preferably Assembly as you need every byte in those 32KB/2KB.