CPU ARCHITECTURE Primary Functions of Computer and other instruction set processor is to execute the sequence of instructions i.e. programs which are stored in our external Main Memory. Program execution is therefore carried as follows:- (i) CPU transfers instructions & when necessary, their input data(operands) from Main Memory to CPU registers. (ii) CPU executes instructions in their stored sequence except interrupts. (iii)When necessary CPU transfers output data(results) from CPU registers to Main Memory.
CPU communicates with Input/Output devices in much the same way as it communicates with external memory. Input/Output devices are associated with addressable registers called Input/Output ports to which the CPU can store a word (O/P operation) or from it which it can load a word (I/P operation). In some computers there are no Input/Output instructions per second, hence all I/O data transfers are implemented by memory referencing instructions, an approach called ‘ Memory Mapped I/O ’. It requires that memory location & IO ports share the same set of addresses, so to improve efficiency. Other computers employ IO instruction that are different from memory referencing instruction. These instructions produce control signals to I/O ports. It is called ‘IO-mapped IO’. User mode:-This mode deals with primary application programs used to handle specific applications for computer users. Supervisor mode:- It manages routine aspects of a computer system on behalf of its users.(part of Operating System) Ex- Handling Interrupts. (i) Normally, the CPU switches back & forth between user & supervisor programs. (ii) Supervisor mode allows better handling of IO devices & their requests. àBasically, CPU checks continuously for instructions & interrupts & if found any, it fetches and executes them.