Motherboard CPU ( Central Processing Unit )
The processor,
also called the microprocessor or CPU, is the brain
of the PC. It performs all general computing
tasks and coordinates tasks done by memory,
video, disk storage, and other
system components. The CPU is a very
complex chip that resides directly on the motherboard
of most PCs, but may instead reside on
a daughtercard that connects to the motherboard
via a dedicated specialized slot.
Processor Design
A processor executes programs including the
operating system itself and user applications all 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.
Processor Components
Modern processors
have the following internal components:
- Execution unit
The core of the CPU, the execution
unit processes instructions.
- Branch predictor
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.
- Floating point unit
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.
- Primary cache
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
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.
- 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.
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:
- Wider data busses and registers
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.
- FPUs
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.
- Pipelining
Early CPUs took five ticks to process an instruction—one
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.
- Superscalar architecture
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.
Extract
from "PC Hardware IN A NutShell a desktop quick reference"
3rd edition. written by Robert Bruce Thompson and Barbara
Fritchman Thompson. published by O'REILLY 2003
