Page 123 - A Practical Guide from Design Planning to Manufacturing

96 Chapter Four

advance of the microprocessor, the processor now determines the archi-
tecture of a computer.
Every microprocessor is designed to support a finite number of specific
instructions. These instructions must be encoded as binary numbers to
be read by the processor. This list of instructions, their behavior, and their
encoding define the processors’ architecture. All any processor can do is
run programs, but any program it runs must first be converted to the
instructions and encoding specific to that processor architecture. If two
processors share the same architecture, any program written for one
will run on the other and vice versa. Some example architectures and the
processors that support them are shown in Table 4-1.
The VAX architecture was introduced by Digital Equipment Corporation
(DEC) in 1977 and was so popular that new machines were still being sold
through 1999. Although no longer being supported, the VAX architecture
remains perhaps the most thoroughly studied computer architecture ever
created.
The most common desktop PC architecture is often called simply x86
after the numbering of the early Intel processors, which first defined this
architecture. This is the oldest computer architecture for which new proces-
sors are still being designed. Intel, AMD, and others carefully design new
processors to be compatible with all the software written for this archi-
tecture. Companies also often add new instructions while still supporting
all the old instructions. These architectural extensions mean that the new
processors are not identical in architecture but are backward compatible.
Programs written for older processors will run on the newer implemen-
tations, but the reverse may not be true. Intel’s Multi-Media Extension
TM
(MMX ) and AMD’s 3DNow! TM are examples of “x86” architectural exten-
sions. Older programs still run on processors supporting these extensions,
but new software is required to take advantage of the new instructions.
In the early 1980s, research began into improving the performance of
microprocessors by simplifying their architectures. Early implementa-
tion efforts were led at IBM by John Cocke, at Stanford by John
Hennessy, and at Berkeley by Dave Patterson. These three teams pro-
duced the IBM 801, MIPS, and RISC-I processors. None of these were

TABLE 4-1 Computer Architectures
Architecture Processors Manufacturer
VAX MicroVax 78032 DEC
x86 Pentium 4, Athlon XP Intel, AMD
SPARC UltraSPARC IV Sun
PA-RISC PA 8800 Hewlett Packard
PowerPC PPC 970 (G5) IBM
JVM PicoJava Sun
EPIC Itanium 2 Intel