28 COMPUTER ENGINEERING
The section on memory is followed by a section containing some general observations about technology evolution: how technology is measured, why it evolves (or does not), cases of it being overthrown, and a general model for how its use in computers operates and is man aged.
SEMICONDUCTOR LOGIC TECHNOLOGY
A single transistor circuit performing a primitive logic function within an integrated circuit is among the smallest and most complex of man made objects. Alone, such a circuit is intrinsically trivial, but the fabrication process required for a set of structures to form a complete integrated circuit is complex. For users of digital integrated circuits there are several relevant parameters:
1. The function of an individual circuit in the integrated circuit, the aggregate function of the integrated circuit, and the functions of a complete integrated circuit family such as the 7400-series.
2. The number of switching circuit functions per integrated circuit. This quantity and density is a measure of the capability of the integrated circuit and the ingenuity of the designers.
4. The speed of each circuit and the speed of the integrated circuit and set of integrated circuits within a family. The semiconductor device family (transistor transistor logic = TTL, Schottky TTL = TTL/S, emitter-coupled logic = ECL, metal oxide semiconductor = MOS, complementary MOS = CMOS, silicon on saphire = SOS, integrated injection logic = 12L) usually determines this performance.
5. The number of interconnections (pins) to communicate outside the integrated circuit.
6. The reliability. This is a function of the circuit technology, the density, the number of pins, the operating temperature, the use (or misuse), and the maturity (experience) of the manufacturing process.
7. Power consumption and speed-power product. A frequently used metric is the speed-power product, where the delay through a typical gate is multiplied by the power consumption of the gate. For a particular technology, the speed-power product tends to be constant because short gate delays usually are accompanied by high power consumption. A technical advance that lowers the speed- power product is considered note worthy.
Figure 1 shows a family tree (taxonomy) of the most common digital integrated circuits. The least complex functions are in the upper portion of the figure, and the most complex are at the bottom. In addition, the circuits are ordered by generation, starting with the second generation on the left side of the figure and progressing to the fifth generation on the right side. The circuits are clustered roughly by the regularity of the function and whether memory is associated with the function. Circuit regularity is important in large-scale integrated circuits because it is desirable to implement regular structures to minimize area-consuming interconnections and, thus, to simplify layout and understanding and to aid testing.
As indicated in Figure 1, the branching of the integrated circuit family tree began in earnest at the beginning of the third generation. At that time, advances in integrated-circuit technology permitted collections of basic logic primitives (AND, NAND, etc.) and sequential circuit components (flip-flops, registers, etc.) to occupy a single integrated circuit rather than an entire module. This had the benefit of providing a drastic reduction in size between the second and third generation computer designs, as