The document discusses the basics of computer systems including memory, the central processing unit (CPU), and how they work together. It explains that memory is divided into cells that store data and are accessed via addresses. The CPU fetches and executes instructions from memory in a continuous cycle, using the program counter to track the next instruction address. It can perform operations on data from registers and memory and branch execution conditionally based on the results.

1. DR ATIF SHAHZAD
Fundamentals of
Computer Systems
IE-321
LECTURE #04

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RECAP
Logic
Logical variables
Conditional, Negation, Contrapositive,Biconditional
AND, OR,NOT,XOR
Logic gates
TruthTables
Boolean Algebra
Examples
Q&A
Input

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Memory
Memory Cell
CPU
Register
Program Counter
Fetch-Execute Cycle
Q&A

4. Fundamentals of
Computer Systems
IE321

5. Computers
COUNTLESS DIFFERENT MACHINES
were invented to solve problems.
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6. Computers
Many types of computers
embedded in robots roaming Mars
powering the navigational systems of nuclear
submarines
But all computers have the same
working principle as the first
computing model invented by
Von-Neumann in 1945.
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7. Main Components
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Memory
where we write
the instructions
It also stores the
data to operate
on
Processor
gets instructions
and data from
the memory
performs
calculations
accordingly

8. Memory
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divided into many cells.
Each cell stores a tiny amount of data, and
has a numerical address.

9. Memory
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Reading or writing data in memory is done
through operations that affect one cell at a
time.

10. Memory
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To read or write a specific memory cell,
we must communicate its numerical
address.
Informing the RAM to operate on cell # 210

11. Memory Cell
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Each memory cell stores an 8-digit binary
number, which is called a byte .
Informing the RAM to operate on cell # 210

12. What Memory does to a Cell address?
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get its value,
or store a new
value.

13. What Memory does to a Cell address?
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The memory has a special input wire for
setting its operational mode:

14. Read Mode of Memory
In “read” mode, the memory retrieves
the byte stored in a cell, and outputs it
in eight data-transmitting wires:
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15. Read Mode of Memory
Reading the number
32 from memory
address 211
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16. Write Mode of Memory
When the memory is in “write” mode,
it gets a byte from these wires, and
writes it to the informed cell.
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17. Write Mode of Memory
Writing the number
33 in memory
address 212
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20. What is a Bus?
A group of wires used for transmitting
the same data is a bus .
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21. Address bus & Data bus
While the address bus is unidirectional
(only used to receive data), the data bus is
bidirectional (used to send and receive data).
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The eight wires used to transmit addresses
form the address bus .
The other eight wires used to transmit data to
and from memory cells form the data bus .

22. CPU & RAM interaction
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CPU and RAM are constantly exchanging data

23. Central Processing Unit
CPU has some internal memory cells
called REGISTERS.
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24. What CPU can do?
simple mathematical operations (with
numbers stored in registers).
move data between the RAM and
registers.
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Copy data from memory position #220 into register #3,
Add the number in register #3 to the number in register #1.

25. Instruction set
The collection of all operations a CPU
can do is called its instruction set.
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Each operation in the instruction set is
assigned a number.
Computer code is essentially a sequence of
numbers representing CPU operations.
These operations are stored as numbers in the
RAM.

26. Fetch & Execute
The CPU works in a never-ending
cycle, always fetching and executing an
instruction from memory.
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27. Program Counter
At the core of this cycle is the PC
register, or Program Counter.
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It’s a special register that stores
the memory address of the next
instruction to be executed.

28. Program Counter
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Go back to step 1.
Execute the instruction,
Increment the PC by 1,
Fetch the instruction at the memory address given by the PC,

29. CPU Operation
When the CPU is powered up, the PC
is reset to its default value which is the
address of the first instruction to be
executed by the machine.
That’s usually an immutable built-in program
responsible for loading the computer’s basic
functionalities.
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30. Fetch-Execute
After being powered up, the CPU keeps
following fetch execute cycle until
the computer shuts down.
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31. Branching
But if the CPU could only follow an ordered,
sequential list of operations, it would be just a fancy
calculator.
it can be instructed to write a new value to the PC,
causing the execution to branch, or “jump” to
somewhere else in the memory.And this branching can
be conditional.
“set PC to address #200 if register #1 equals zero” .
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32. SupplementaryVideo
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33. SupplementaryVideo
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NEVER hesitate to
contact should you
have any question
Dr.Atif Shahzad