Page 124 - A Practical Guide from Design Planning to Manufacturing

Computer Architecture 97

ever sold commercially, but they inspired a new wave of architectures
referred to by the name of the Berkeley project as Reduce Instruction
Set Computers (RISC).
Sun (with direct help from Patterson) created Scalable Processor
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Architecture (SPARC ), and Hewlett Packard created the Precision
Architecture RISC (PA-RISC). IBM created the POWER TM architecture,
which was later slightly modified to become the PowerPC architecture
now used in Macintosh computers. The fundamental difference between
Macintosh and PC software is that programs written for the Macintosh
are written in the PowerPC architecture and PC programs are written
in the x86 architecture. SPARC, PA-RISC, and PowerPC are all con-
sidered RISC architectures. Computer architects still debate their merits
compared to earlier architectures like VAX and x86, which are called
Complex Instruction Set Computers (CISC) in comparison.
Java is a high-level programming language created by Sun in 1995.
To make it easier to run programs written in Java on any computer, Sun
defined the Java Virtual Machine (JVM) architecture. This was a vir-
tual architecture because there was not any processor that actually
could run JVM code directly. However, translating Java code that had
already been compiled for a “virtual” processor was far simpler and
faster than translating directly from a high-level programming lan-
guage like Java. This allows JVM code to be used by Web sites accessed
by machines with many different architectures, as long as each machine
has its own translation program. Sun created the first physical imple-
mentation of a JVM processor in 1997.
In 2001, Intel began shipping the Itanium processor, which supported
a new architecture called Explicitly Parallel Instruction Computing
(EPIC). This architecture was designed to allow software to make more
performance optimizations and to use 64-bit addresses to allow access
to more memory. Since then, both AMD and Intel have added architectural
extensions to their x86 processors to support 64-bit memory addressing.
It is not really possible to compare the performance of different archi-
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tectures independent of their implementations. The Pentium and
Pentium 4 processors support the same architecture, but have dramat-
ically different performance. Ultimately processor microarchitecture
and fabrication technologies will have the largest impact on perform-
ance, but the architecture can make it easier or harder to achieve high
performance for different applications. In creating a new architecture
or adding an extension to an existing architecture, designers must bal-
ance the impact to software and hardware. As a bridge from software
to hardware, a good architecture will allow efficient bug-free creation
of software while also being easily implemented in high-performance
hardware. In the end, because software applications and hardware
implementations are always changing, there is no “perfect” architecture.