what is Central Processing Unit

The heart of any computer is the central processing unit (CPU). The CPU communicates with the memory over a bidirectional data bus. In memory reside program instructions, data constants, and variables, all placed in an ordered sequence. The CPU reaches out to this sequence by controlling and manipulating the address bus. Special memory locations called input/output (I/O) ports pass binary information to or from the real world in the form of parallel or serial data bytes. The system clock oversees the whole network of gates, latches, and registers, ensuring that all bits arrive on time at the right place and that no data trains collide. Of the four parts of a computer (CPU, memory, I/O, and clock), the most important part is the CPU. The CPU consists of several subgroups, including the arithmetic and logic unit (ALU), the instruction decoder, the program counter, and a bank of internal memory cells called registers. In a typical CPU sequence, the program counter reaches out to the memory via the address bus to retrieve the next instruction. This instruction is passed over the data bus to the internal registers. The first part of the instruction is passed to the instruction decoder. It decides which data paths must be opened and closed to execute the instruction. In some cases, all the information needed to complete the operation is embedded within the instruction. An example of this type of instruction is “clear the accumulator.” In other cases, the instruction needs additional information, and it returns to memory for the added data. An example of this type of instruction might be “load Register 2 with the data constant 5.” Once all the information is in place, the instruction is executed by opening and closing various gates to allow execution of the instruction. Typical instructions available to all CPUs include simple instructions with data already inside the CPU, such as clear, complement, or increment the accumulator. More complex instructions use two internal registers or data coming from memory. This lab illustrates how the CPU executes simple and a few complex operations using basic logic functions.