U. Glaeser

The improvements in line widths, chip sizes, and speed should continue for several years. The International
Roadmap for Semiconductors [36] outlines the expected improvements in technology. Samsung
has already demonstrated a 4-GB DRAM chip [37]. Current fast production CMOS integrated circuit
processes use line widths of 0.18 µ m and are moving to 0.13 µ m [38].
4.3 Processors
Early processors were based on mechanical devices. Later electro-mechanical relays were used to build
computing devices [9]. Electronic digital computers were developed using vacuum tubes in the mid 1940s
[10,11]. Transistors took over in early 1960s because of higher reliability, smaller size, and lower power
consumption [17]. In the late 1960s, standard integrated circuits started to become available and were
used in place of discrete transistors [19]. Integrated circuits allowed a high density of transistors, resulting
in faster computers with lower price, lower power consumption, and higher reliability due to improved
interconnections. The increasing complexity of integrated circuits, as outlined by Moore’s Law, allowed
building ever increasingly complex microprocessors since the early 1970s [29].
Processors have become categorized by application and particularly by cost and speed. In the 1970s,
the main categories were: microcomputers, minicomputers, mainframes, and supercomputers. About
1990, the fastest microprocessors started to overtake the fastest processors in speed. This has meant that
new categories are often used, as almost all computer systems are now microprocessors or collections of
microprocessors.
The performance of processors is based mainly on the clock speed and the internal architecture. The
clock speed depends primarily on the integrated circuit process technology. Internal pipelining [39],
superscalar operation [40], and multiprocessor operation [41] are the main architectural improvements.
RISC processors simplified the internal processor structure to allow faster clock operation [42,43].
Internal code translation has allowed complex instruction sets to execute as sets of micro-operations with
RISC characteristics. The goal of architectural improvements has been to improve performance, measured
by the execution time of specific programs. The execution time equals the instruction count times the
number of clock cycles per instruction (CPI) divided by the clock speed. RISC processors increased the
instruction count, but improved the CPI, and allowed building simpler processors with faster clock
speeds. Fast processors became much faster than the memory speed. Cache memory is used to help
match the slower main memory with the faster CPU speed. Increased processor performance comes with
increased complexity, which is seen as increased number of transistors on the processor chip. In current
processor chips, the on-chip cache memory is sometimes larger than the processor core. Future processor
chips are likely to have multiple processor cores and share large on-chip caches.
Processors have been designed to operate on various word sizes. Some early computers worked with
decimal numbers or variable-sized operands. Most computers used different numbers of binary digits;
4, 8, 12, 18, and 36 bits were used in some computers. Most current computers use either 32 bits or 64 bits
word-lengths. The trend is toward 64 bits, to allow a larger addressing range. Comparing the performance
of different word-length computers may be difficult. Smaller word-lengths allow faster operation in a
cheap processor with a small number of transistors and minimal internal wiring. But, processors with
more address bits allow building larger and more complex programs, and allow easy access to large
amounts of data. A common pitfall of designing general purpose computers has been to provide too
small an address space [43]. Generally, word size and addressing space of processors have been increasing
with time, driven roughly by the increasing complexity of integrated circuits.
Early microprocessors had small 4 or 8 bit word-lengths (see Table 4.4). The size of the early integrated
circuits limited the number of transistors and thus word-length. The very cheapest current microprocessors
use small word-lengths to minimize costs. The most powerful current general purpose microprocessors
have 64 bit word-lengths, although the mass market PCs still use 32 bit processors. Table 4.4
shows some of the features of selected Intel microprocessors [32,34,35,44,45].
The fastest processors are now microprocessors. The latest Pentium 4 processor (in March 2001) has a
clock speed of 1.5 GHz. The distance that light travels in one clock cycle at 1.5 GHz (667 ps) is about 20 cm.
© 2002 by CRC Press LLC