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					Introduction to Computer                        1                           Handout No: 11

The CPU:
The CPU is the brain of the computer. In term of computing power, the CPU is the most
important element of a computer system.
The CPU is generally located on the motherboard. Since the CPU carries out the large share of
the work in the computer, data pass continually through it. The data come from the RAM and the
units (Keyboard, drives etc.). After processing, the data is send back to RAM and the units.

The CPU continually receives instructions to be executed. Each instruction is a data processing
order. The work itself consists mostly or calculation and data transport:
On large machines, CPUs require one or more printed circuited boards. On personal computers
and small workstations, the CPU is housed in a single chip called a microprocessor. Two typical
components of CPU are:

The arithmetic logic unit (ALU):
The ALU is the part of the computer that performs all arithmetic computations, such as addition
and multiplication and all comparison operations. The ALU is one component of the CPU.

The Control unit:
Which extracts instruction from memory and decodes and executes them, calling on the ALU
when necessary.

Clock frequencies
If we look at a CPU, the first thing we notice is the clock frequency. All CPU’s have a working
speed, which is regulated by a tiny crystal.
The crystal is constantly vibrating at a very large number of “beats” per second. For each clock
tick, an impulse is sent to the CPU, and each pulse can, in principle, cause the CPU to perform
one (or more) actions.
The number of clock ticks per second is measured in Hertz. Since the CPU’s crystal vibrates
millions of times each second, the clock speed is measured in millions of oscillations (megahertz
or MHz). Modern CPU’s actually have clock speeds running into billions of ticks per second, so
we have started having to use gigahertz (GHz).

                                                                       Prof. Gulzar Ahmad
Introduction to Computer                          2                            Handout No: 11

More transistors
New types of processors are constantly being developed, for which the clock frequency keeps
getting pushed up a notch. The original PC from 1981 worked at a modest 4.77 MHz, whereas
the clock frequency 20 years later was up to 2 GHz.
The latest version of Pentium 4 is known under the code name Prescott.
   2     80286                  1982       6-12.5 MHz            134,000
   3     80386                  1985       16-33 MHz             275,000
   4     80486                  1989       25-100 MHz            1,200,000
   5     Pentium                1993       60-200 MHz            3,100,000
         Pentium MMX            1997       166-300 MHz           4,500,000
   6     Pentium Pro            1995       150-200 MHz           5,500,000
         Pentium II             1997       233-450 MHz           7,500,000
         Pentium III            1999       450-1200 MHz          28,000,000
   7     Pentium 4              2000       1400-2200             42,000,000
                                2002       2200-2800             55,000,000
                                2003       2600-3200             55,000,000
         “Prescott“             2004       2800-3600             125,000,000
Figure: Seven generations of CPU’s from Intel. The number of transistors in the Pentium III and
4 includes the L2 cache.

CPU Cache
A CPU cache is a cache used by the central processing unit of a computer to reduce the average
time to access memory. The cache is a smaller, faster memory which stores copies of the data
from the most frequently used main memory locations. As long as most memory accesses are to
cached memory locations, the average latency of memory accesses will be closer to the cache
latency than to the latency of main memory.

Two levels of cache
In practice there are always at least two close stores. They are called Level 1, Level 2, and (if
applicable) Level 3 cache. Some processors (like the Intel Itanium) have three levels of cache,
but these are only used for very special server applications. In standard PC’s we find processors
with L1 and L2 cache.

Figure: The cache system tries to ensure that relevant data is constantly being fetched from
RAM, so that the CPU (ideally) never has to wait for data.

L1 cache
Level 1 cache is built into the actual processor core. It is a piece of RAM, typically 8, 16, 20, 32,
64 or 128 Kbytes, which operates at the same clock frequency as the rest of the CPU. Thus you
could say the L1 cache is part of the processor.
                                                                          Prof. Gulzar Ahmad
Introduction to Computer                         3                             Handout No: 11
L1 cache is normally divided into two sections, one for data and one for instructions. If the cache
is common for both data and instructions, it is called a unified cache.

L2 cache
The level 2 cache is normally much bigger (and unified), such as 256, 512 or 1024 KB. The
purpose of the L2 cache is to constantly read in slightly larger quantities of data from RAM, so
that these are available to the L1 cache.
In the earlier processor generations, the L2 cache was placed outside the chip: either on the
motherboard (as in the original Pentium processors), or on a special module together with the
CPU (as in the first Pentium II’s).

Figure: The CPU is mounted on a rectangular printed circuit board, together with the L2 cache,

As process technology has developed, it has become possible to make room for the L2 cache
inside the actual processor chip. Thus the L2 cache has been integrated and that makes it
function much better in relation to the L1 cache and the processor core.
The L2 cache is not as fast as the L1 cache, but it is still much faster than normal RAM.

                         CPU                         L2 cache
                         Pentium                     External
                         Pentium Pro                 Internal, in the CPU
                         Pentium II                  External
                         Celeron (1st generation)    None
                         Celeron (later gen.),       Internal, in the CPU
                         Pentium III, Pentium 4

Traditionally the L2 cache is connected to the front side bus. Through it, it connects to the
chipset’s north bridge and RAM:

                      Pentium 4 (III, “Prescott”)     28 KB       1024 KB
                      CPU                             L1 cache    L2 cache
                      Pentium III                     16KB        256KB
                      Pentium 4 (I)                   20 KB       256 KB
                      Pentium 4 (II, “Northwood”)     20 KB       512 KB

                                                                            Prof. Gulzar Ahmad
Introduction to Computer                          4                            Handout No: 11

                        Figure: The way the processor uses the L1 and L2.

The north bridge
The north bridge is a controller which controls the flow of data between the CPU and RAM, and
to the AGP port.
The AGP is actually an I/O port. It is used for the video card. In contrast to the other I/O devices,
the AGP port is connected directly to the north bridge, because it has to be as close to the RAM
as possible. The same goes for the PCI Express x16 port, which is the replacement of AGP in
new motherboards. But more on that later.

Fig: The south bridge connects a large number of different devices with the CPU and RAM.

The south bridge
The south bridge incorporates a number of different controller functions. It looks after the
transfer of data to and from the hard disk and all the other I/O devices, and passes this data into
the link channel which connects to the north bridge.

                                                                          Prof. Gulzar Ahmad
Introduction to Computer                          5                             Handout No: 11

                The ATA interface works like a bus in relation to the south bridge.

How stored data is transferred to the CPU

Historical overview
I will close off this review with a graphical summary of a number of different CPU’s from the
last 25 years. The division into generations is not always crystal clear, but I have tried to present
things in a straightforward and reasonably

                                                                          Prof. Gulzar Ahmad
Introduction to Computer   6      Handout No: 11

                               Prof. Gulzar Ahmad

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