A microprocessor generally does not have Ram, ROM and IO pins. It usually uses its pins as a bus to interface to peripherals such as RAM, ROM, Serial ports, Digital and Analog IO. It is expandable at the board level due to this. ============================================================== A microcontroller is 'all in one', the processor, ram, IO all on the one chip, as such you cannot (say) increase the amount of RAM available or the number of IO ports. The controlling bus is internal and not available to the board designer. =============================================================== RTOS : an OS that processes must be executed and ends at exactly the deadline without buffering delays. =============================================================== To check a mirrored array i.e. 123321, compare the (n-1) element (n = 1 , 2 , ... ) with the (size - n) element , the loop will take size / 2 times =============================================================== isr function void isr_handler () { Interrupt_enable = 0; Interrupt_flag = 0; do some code ... interrupt_enable =1; }