Chapter 2:
The system unit is the main body of a computer and is called a motherboard.
Assorted computer components like power supplies, cooling fans, disk drives,
primary memory, secondary memory, CPUs, expansion cards, and other
components are plugged into the motherboard. The system unit includes the
inner workings of a computer: essentially everything inside the box or chassis.
A microprocessor’s speed is measured by clock rate, which is defined as the
number of cycles per second that a computer can perform its most basic tasks.
This is measured in gigahertz.
There are two types of CPU chips: RISC and CISC. RISC (Reduced Instruction Set
Computer) architecture chips are designed for efficiency and are used in
embedded devices such as smartphones, tablets, smart watches and
microwaves. CISC (Complex Instruction Set Computer) chips are designed for
more workload-based machines such as PCs, Macs, servers, and workstations.
Intel Core i5, i7, and i9 processors are examples of CISC chipsets.
Bus lines are the pathways that transfer data and power between components
inside of a computer and sometimes between computers themselves. Bus lines
are controlled by software that enables them to connect peripheral devices like
computer cards and printers. The relationship between the components and the
bus lines is of great importance. If a bus is only a two-lane road, its ability to
handle traffic is greatly reduced. Adding lanes or widening the roads leads to
more efficient bus lines, that are then able to handle larger volumes of traffic.
This is why bus lines are measured in bus width.
All computer components run on DC current, so standard AC current must be
converted to DC current for the PC to operate. A PSU (shown below) typically
converts 100–120 volts of alternating current (AC) to a lower voltage direct
current (DC) that can be used by the internal components of the system unit.
Some PSUs have the ability to sense a wide range of AC current and convert it to
usable DC current automatically. A business looking to purchase a computer
would have to be cognizant of PSU specification.
System units also need to store, retain, and process memory. There are two
types of memory: primary storage (volatile) and secondary storage (non-
volatile). Memory holds the information that is electronically stored on a
computer, like data files and program code. In a computing system, primary
storage can be thought of as what a user is currently working on, while
secondary storage can be described as the location where users store work
when it is not presently needed. An application program’s code will not be
in primary storage unless the program is currently running. Data and
information pre-loaded on a computer that requires no user interaction is
stored in secondary storage.
In the system unit, primary storage is random access memory (RAM) in the
form of RAM chipsets that are mounted on the motherboard. RAM allows
data and information to be accessed in any order (hence the word random),
as opposed to a secondary storage device that typically spins and requires
a moving read/write head.
Each time a computer is started, it requires the same critical programming
information. Computers need to know how much RAM is plugged into the
motherboard, what sort of disk drives are on board, and other critical
information for the system to work properly. The programming information to
perform this startup task resides in a chip called a complimentary metal-oxide
semiconductor, or CMOS chip. CMOS does not lose its content; it is non-volatile
memory. This process is called booting the computer. Once the CMOS program
determines the hardware configuration, it starts the loading process of the
system software, either Windows or Mac OS. The system software is loaded from
the secondary storage into RAM, where it needs to reside so that it can be
executed as described above.
Motherboards contain other forms of memory in the form of ROM chips (read-
only memory). ROM chips (shown above) are preprogrammed, require no user
intervention, and serve to perform specialized internal tasks, like starting the
computer. Since the programming on ROM chips is unchangeable, they are
sometimes called firmware.
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