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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:
- use the program counter to fetch the next instruction.
- decode the instruction and fetch data from memory into internal
registers as required,
- perform the instruction and put the result in another internal
- 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
Tue Jul 13 16:55:15 EDT 1999