What is the main task of watch dog timer ?

Watch dog timer is a timer is used to reset the processor in an embedded system.
A timer is set to a large value and is counted down.
When the timer value reaches to zero the processor is reset.

What is mean by kernel ?

The 'heart' of an operating system is called kernel.
Kernel is responsible for task sheduling,inter-task communication,interrupt handling,providing timer service,device management and memory management.

What is Dynamic Memory Access (DMA) ?

A mechanism to transfer data directly between the memory and an input/output device.
A DMA controller shares the memory bus with the prosessor

What is cache memory ?

A very high speed memory between the RAM/ROM and the CPU.
Cache memory is used to supply the most recently used instructions/data to the CPU.
Cache memory is much faster to access than the RAM/ROM but very costly.

What is Application Programming Interface (API) ?

It is defined by a set of function calls using which applications can be developed over an operating system or a software development tool

What is adress mode and address space ?

Adress mode is defined by the way by which an operand is specified for an instruction.
For example : imediate address mode,direct adressing mode,register addressing etc..

Address space is the maximum memory size that can be addressed by a processor
CPU with 16 address lines can address 2^16 bytes of address space

What is a address bus ?

Address bus is a set of wires using which a processor or a microcontroller communicates with the peripheral to sellect a memory location or a register of a peripheral.