Levels of Design Abstraction in Computer Architecture

Design abstraction levels in computer architecture – A computer can be defined as a collection of objects, often large and complex, called components that are connected in a predefined manner with a specific function or purpose.

The functions of a computer system are determined by the functions of its components and how the components are connected. The design of a computer system includes the determination of a system structure that exhibits the desired behavior.

Other typical requirements to be considered for the design process are cost minimization and performance maximization. Therefore, the complex design of a computer must be broken down into smaller and easier designs.

The design of a computer is carried out at several levels of abstraction. The following figure shows an example of a full adder circuit for gate-level abstraction.

Such levels are generally recognized in computer design, although they are referred to by various names in the literature.

Design abstraction levels

1. The gate level
2. The register level
3. The processor level

The Gate Level

The gate level design is concerned with processing binary variables whose possible values are restricted to the bits 0 and 1. In such designs, the components are logic gates and flip-flops (one-bit storage).

This design level is based on SSI-IC technology and primarily the concern of the hardware designer. This level is not seen by the programmer.

The Register Level

The register level is also known as register-transfer level (RTL). This is approximately the level of detail seen by a programmer. The components of such designs are registers, counters, combinational circuits, and small sequential circuits. This is based on MSI-IC technology.

A register-transfer level system consists of a set of registers connected by combinational data transfer and a data processing circuit. The figure below shows one of the example structures for register transfer level.

The Processor Level

The processor level corresponds to a user’s view of a computer. The components used in this design level are CPUs, memories, and input/output devices. These components are available as VLSI parts in various IC series.

The components at this level are complex and usually based on sequential circuits. The processor-level design is very much a heuristic process, as there is little design theory at this level of abstraction. The table below shows the basic comparison of various design-level abstractions.

Level	Components	IC Technology	Information Unit	Time Unit
Gate	Registers, counters, combinational circuits, and small sequential circuit	SSI	Bits	10-2 to 10-19 sec
Register	Registers, counters, combinational circuits, and small sequential circuits	MSI	Words	10-9 to 10-9 sec
Processor	CPUs, memories, I/O devices	VLSI	Blocks of words	10-3 to 10+3 sec

However, the above table shows various design-level abstractions, but the boundaries between the levels are far from clear-cut, and it is common to encounter descriptions that mix components from more than one level.