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238 CHAPTER 6 / NONARITHMETIC COMBINATIONAL LOGIC DEVICES

(a) (b)
Non-Arithmetic Combinational Logic Arithmetic-Type Combinational Logic
Devices Circuits
Multiplexers (Data Selectors)
Decoders/Demultiplexers Adders
Priority Encoders Subtractors
Code Converters Arithmetic and Logic Units
Comparators Multipliers
Parity Detectors Dividers
Combinational Shifters
FIGURE 6.1
Partial lists of available nonarithmetic 1C devices and arithmetic 1C devices.

6.1.2 Classification of Chips

1C chips for the devices of the type listed in Fig. 6.1 can be classified as small-scale integrated
(SSI) circuits, medium-scale integrated (MSI) circuits, large-scale integrated (LSI) circuits,
very-large-scale integrated (VLSI) circuits, and wafer-scale integrated (WSI) circuits. It has
become customary to assign one of the preceding acronyms to a given 1C circuit on the
basis of the number of equivalent fundamental gates (meaning AND, OR, Inverter or NAND,
NOR, Inverter) that are required to implement it. By one convention, these acronyms may
be assigned the following gate count ranges:

SSI circuits: up to 20 gates
MSI circuits: 20 to about 200 gates
LSI circuits: 200 to thousands of gates
VLSI circuits: thousands to millions of gates

WSI chips might contain tens to hundreds of VLSI circuits. This classification scheme is
obviously ineffective in revealing the true complexity of a given 1C relative to the digital
system in which it operates. For example, an LSI chip might be a 64-bit adder or it might
be a moderately complex microprocessor. Thus, the reader should exercise caution when
evaluating the complexity of a chip based on some count system. Finally, it is now common
practice for logic designers to design chips for a limited, specific application. Such chips are
called application-specific ICs, or ASICs, and may differ greatly from the usual commercial
chips. ASICs can reduce total manufacturing costs and can often provide higher performance
than is possible by combining commercially available devices.

6.1.3 Performance Characteristics and Other Practical Matters
The most desirable features a designer would want in a switching device, say, for integrated
circuit applications are as follows:

• Fast switching speed
• Low power dissipation
• Wide noise margins