Functional Elements of the Computer

The functional elements of a computer are the

• central processing unit (CPU),
• random access storage (memory), and
• input/output to external devices (I/O).

The sub-units of the CPU are

• instruction decode and CPU control,
• control of addressing for memory and I/O ports,
• data transfer control,
• data and address registers and
• arithmetic logic unit.

To keep track of the CPU steps, the processor maintains a special register, known as the program counter. The program counter points to (contains the address of) the next instruction to be executed. The instruction itself specifies

• the operation to be performed,
• the processor registers to be used and
• possibly data (or a pointer to data in memory).

The CPU will typically perform the following execution cycle:

1. use the program counter to fetch the next instruction.
2. decode the instruction and fetch data from memory into internal registers as required,
3. perform the instruction and put the result in another internal register and
4. set status bits in the status register as required.

The various functional units of the computer are connected by one or more multi-wire digital buses which pass data, addresses, and control information between the units as shown in figure 9.1.

 
Figure 9.1:  A typical computer design showing two multi-wire buses, an internal bus connecting functional units within the CPU and an external bus for connecting additional computer subsystems.