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Publisher4.1 Processor Design
A processor executes
programsincluding the operating system itself and user
applicationsall of which perform useful work. From the
processor's point of view, a program is simply a
group of low-level instructions that the
processor executes more or less in sequence as it receives them. How
efficiently and effectively the processor executes instructions is
determined by its internal design, also called its
architecture. The CPU architecture, in
conjunction with CPU speed, determines how fast the CPU executes
instructions of various types. The external design of the processor,
specifically its external interfaces, determines how fast it
communicates information back and forth with external cache, main
memory, the chipset, and other system components.
4.1.1 Processor Components
Modern
processors have the following internal components:
The core of
the CPU, the execution unit processes
instructions.
The
branch predictor attempts to guess where the
program will jump (or branch) next, allowing the prefetch
and decode unit to retrieve instructions and data in
advance so that they will already be available when the CPU requests
them.
The
floating-point unit (FPU)
is a specialized logic unit optimized to perform noninteger
calculations much faster than the general-purpose logic unit can
perform them.
Also called Level
1 or L1 cache,
primary cache is a small amount of very fast
memory that allows the CPU to retrieve data immediately, rather than
waiting for slower main memory to respond. See Chapter 5 for more
information about cache memory.
Bus interfaces
are the pathways that connect the
processor to memory and other components. For example, modern
processors connect to the chipset Northbridge via a dedicated bus
called the frontside bus
(FSB) or host bus.
4.1.2 Processor Speed
The processor
clock coordinates all CPU and memory operations
by periodically generating a time reference signal called a
clock cycle or tick. Clock
frequency is specified in megahertz
(MHz), which specifies millions of ticks per
second, or gigahertz (GHz),
which specifies billions of ticks per second. Clock speed determines
how fast instructions execute. Some instructions require one tick,
others multiple ticks, and some processors execute multiple
instructions during one tick. The number of ticks per instruction
varies according to processor architecture, its instruction
set, and the specific instruction. Complex
Instruction Set Computer (CISC)
processors use complex instructions. Each requires many clock cycles
to execute, but accomplishes a lot of work. Reduced
Instruction Set Computer (RISC)
processors use fewer, simpler instructions. Each takes few ticks but
accomplishes relatively little work.These differences in efficiency mean that one CPU cannot be directly
compared to another purely on the basis of clock speed. For example,
an AMD Athlon XP 3000+, which actually runs at 2.167 GHz, may be
faster than an Intel Pentium 4 running at 3.06 GHz, depending on the
application. The comparison is complicated because different CPUs
have different strengths and weaknesses. For example, the Athlon is
generally faster than the Pentium 4 clock for clock on both integer
and floating-point operations (that is, it does more work per CPU
tick), but the Pentium 4 has an extended instruction set that may
allow it to run optimized software literally twice as fast as the
Athlon. The only safe use of direct clock speed comparisons is within
a single family. A 1.2 GHz Tualatin-core Pentium III, for example, is
roughly 20% faster than a 1.0 GHz Tualatin-core Pentium III, but even
there the relationship is not absolutely linear. And a 1.2 GHz
Tualatin-core Pentium III is more than 20% faster than a 1.0 GHz
Pentium III that uses the older Coppermine core. Also, even within a
family, processors with similar names may differ substantially
internally.
4.1.3 Processor Architecture
Clock speeds increase every year, but the laws of physics limit how
fast CPUs can run. If designers depended only on faster clock speeds
for better performance, CPU performance would have hit the wall years
ago. Instead, designers have improved internal architectures while
also increasing clock speeds. Recent CPUs run at more than 650 times
the clock speed of the PC/XT's 8088 processor, but
provide 6,500 or more times the performance. Here are some major
architectural improvements that have allowed CPUs to continue to get
faster every year:
For a given clock speed, the amount of work done depends on the
amount of data processed in one operation. Early CPUs processed data
in 4-bit (nibble) or 8-bit
(byte) chunks, whereas current CPUs process 32
or 64 bits per operation.
All CPUs work well with integers, but
processing floating-point numbers to high precision on a
general-purpose CPU requires a huge number of operations. All modern
CPUs include a dedicated FPU that handles floating-point operations
efficiently.
Early CPUs took five ticks to
process an instructionone each to load the instruction, decode
it, retrieve the data, execute the instruction, and write the result.
Modern CPUs use pipelining, which dedicates a
separate stage to each process and allows one full instruction to be
executed per clock cycle.
If
one pipeline is good, more are better. Using multiple pipelines
allows multiple instructions to be processed in parallel, an
architecture called superscalar. A superscalar
processor processes multiple instructions per tick.
