OCR - GCSE - Computer Science -
Systems Architecture
Central Processing Unit
Control Unit
The Control Unit (CU) controls all of the other
components of the CPU. It also contains the
decoder.
The decoder interprets
program instructions and
tells the ALU what
operations to carry out.
Arithimetic Logic Unit
The Arithmetic Logic Unit (ALU)
performs all of the logical
operations to carry out program
instructions
The ALU carries out the following
activities: subtraction, addition,
multiplication and division.
Clock
The Clock controls the rate at which program
instructions are carried out, by sending control
electrical signals at regular intervals, called cycles.
Clock Speed
The clock is a vibrating quartz crystal and the faster it
vibrates, the faster the instructions are processed, at least 1
per cycle.
The faster the clock speed the
hotter the processor gets, which
causes it to malfunction.
The processor must have a
heat sink and a fan to
dissipate this heat.
Multi-core Processors
A multi-core
processor contains
more than one CPU.
Multi-core Processors can give
faster processing speeds than
single-core processors and can
work on different tasks at the
same time.
Positives: The cores can
work together on the
same program - parallel
processing. The Cores
can work on different
programs at the same
time - multitasking.
Negatives: Not all
programs will run
at twice the speed
with a dual-core
processor. The
programs may be
sequential so that
one task requires
output from a
previous task and
so the second task
cannot start until
the first has
finished.
Buses
These are electrical conductors that carry electrical
signals between components in the CPU and between
the CPU and other components on the motherboard.
Cache
This is very fast
Random Access
Memory (RAM).
The Cache memory
speeds up processing
by storing recently or
frequently used
instructions so that
they do not have to be
fetched from the main
memory which is much
slower.
As the cache
memory becomes
larger it takes
longer to find the
data and so it
becomes slower.
Therefore, it is split
into different levels
L1, L2 with the
smallest nearest to
the CPU.
Registers
These are memory locations. Some
perform special functions in the
fetch-decode-execute cycle.
The Fetch-Decode-Execute cycle is what the CPU
uses when carrying out the program instructions.
Fetch Stage: The next
instruction to be
executed is transferred
from the RAM to the
CPU.
Decode Stage: The
CU interprets the
instruction.
Execute Stage: The CU then
carries out the instruction. It
instructs the ALU if calculations
need to be performed.
Program Counter is one
of the registers which
holds the address of
the next instruction to
be fetched.
The Memory
Data Register
(MDR)
temporary
store (buffer)
for anything
copied from
memory.
The Memory Address Register (MAR)
holds the address of the memory
location currently being read (fetched)
or written to.
Accumulator is a register
which stores the results of
the calculations carried
out by the ALU.
Embedded Systems
Components within an
embedded system are on a
Single Printed Circuit Board
(PCB) and have a processor,
memory, input and output
interfaces.
Each embedded
system is built for a
small range of specific
tasks.
Embedded Systems are
called real-time systems
because they must ensure
an immediate response in
order for the system to
react to different
situations.