A shell is a program that presents an interface to various operating system functions and services. The shell is so called because it is an outer layer of interface between the user and the innards of the operating system (the kernel). Kernel is the one which manages all the resources of the Operating System.
DOS is not a RTOS (real time Operating system), however MS DOS can be used with certain APIs to achieve the RTOS functionality. For example, the RT Kernel (Real Time Kernel) which can be used with MS DOs to achieve the RTOS functionality. Other example is of Tics. Tics is delivered as a hardware support file for MS DOS.
Real-time operating systems are: QNX, LynxOS, OS-9, Intergrity, Katix, Fusion, IRIX, DeltaOS, eCos, AMX, CMX-RTX, INTime, PDOS and many others.
A real time system is a computer system that updates the information at the same rate it receives it. It is of two types, hard real time system and a soft real time system.
Because it has got seperate program and data memory(highly pipelined architecture)