Central Processing Unit

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...