Computer Architecture Notes
Subject Overview
Computer Architecture gives BCA students the digital-logic and hardware-organization foundation beneath every layer of software they'll write — logic gates and Boolean algebra, combinational circuits (adders, multiplexers, encoders/
Subject at a glance
4
Units
36
Topics
4
Credits
4
Infographics
Unit-wise Syllabus
4 units — open any unit for detailed topic-wise notes, diagrams and important exam questions
- AND
- OR
- NOT
- NAND
- NOR
- XOR
- XNOR gates
- NAND/NOR as universal gates
- logic gate applications
- Boolean algebra theorems
- simplification of Boolean expressions
- SOP and POS forms
- realization using gates
- K-maps and K-map simplification
Basic: AND, OR, NOT
The building blocks of every circuit
Universal: NAND, NOR
Either one alone can build any circuit
Exclusive: XOR, XNOR
Output depends on whether inputs differ
Simplification
Boolean theorems and K-maps reduce gate count
Study guide
Logic gates are the tiny switches from which every processor is built. You learn the basic gates (AND, OR, NOT), the derived gates (NAND, NOR, XOR, XNOR) and why NAND and NOR are called universal gates, since any circuit can be built from either alone. Boolean algebra lets you simplify expressions, and Karnaugh maps (K-maps) give a visual way to reduce a circuit to its simplest sum-of-products or product-of-sums form.
- Half adder/subtractor
- full adder/subtractor
- parallel binary adder
- binary adder/subtractor
- multiplexers and demultiplexers
- implementing Boolean equations using MUX/DEMUX
- encoders and decoders
Inputs
2 bits (A, B)
3 bits (A, B, carry-in)
Outputs
Sum, Carry
Sum, Carry-out
Sum expression
A ⊕ B
A ⊕ B ⊕ Cin
Use
Adding the least significant bits
Chaining bits in a parallel adder
Study guide
Combinational circuits produce an output that depends only on the current inputs. Here you design half and full adders and subtractors, combine them into parallel binary adders, and study multiplexers (many inputs to one output), demultiplexers, encoders and decoders. A good habit is to write the truth table first, derive the Boolean expression from it, then draw the gate-level circuit.
- Latches
- flip-flops — R-S
- J-K (including race-around condition and its removal)
- Master-Slave J-K
- D flip-flop
- T flip-flop
- applications of flip-flops
- SR flip-flop
- Set/reset; S = R = 1 is not allowed
- JK flip-flop
- Like SR but J = K = 1 toggles the output
- Master-slave JK
- Two stages that remove the race-around problem
- D flip-flop
- Output follows the D input on the clock edge
- T flip-flop
- Toggles when T = 1; used in counters
Study guide
Sequential circuits have memory: their output depends on past inputs as well as current ones. You study latches and the main flip-flops (R-S, J-K, D and T), the race-around problem in the J-K flip-flop and how the Master-Slave design removes it. Flip-flops are the building blocks of registers, counters and memory, so learn each one's characteristic table and behaviour.
- Introduction to computer and CPU organization
- stored program concept
- Von Neumann and Harvard architectures
- RISC and CISC architecture
- register transfer and micro-operations
- instruction formats (memory reference, register reference, I/O instructions)
- common bus system — data
- control and address buses
- 1Fetch
Read the instruction from memory into the instruction register
- 2Decode
The control unit works out what the instruction means
- 3Execute
The ALU or other unit performs the operation
- 4Store
Write the result to a register or memory
Study guide
This unit zooms out from circuits to the whole computer. You learn the stored-program concept, the Von Neumann and Harvard architectures, the difference between RISC and CISC processors, register transfer and micro-operations, instruction formats and the data, address and control buses that connect the CPU, memory and input-output. It explains what actually happens when a program runs.
Why Computer Architecture Matters
Computer Architecture explains the hardware beneath every program you will ever write. Understanding gates, memory and the CPU helps you reason about performance, debugging and how operating systems and compilers work, and it is a standard topic in technical interviews and competitive exams.
How to Prepare for Computer Architecture
- Practise K-map simplification until you can do it quickly for 3 and 4 variables.
- For every flip-flop, memorise the symbol, truth table and characteristic equation together.
- Draw circuit diagrams neatly with labelled inputs and outputs; diagrams carry marks in theory and practical exams.
- Compare Von Neumann and Harvard, RISC and CISC in a short table so you can reproduce it in the exam.
Computer Architecture: Frequently Asked Questions
What is the difference between combinational and sequential circuits?
A combinational circuit's output depends only on its present inputs, as in an adder or multiplexer. A sequential circuit also depends on stored past values, which is why it uses flip-flops.
Why are NAND and NOR called universal gates?
Because any other gate, and therefore any digital circuit, can be built using only NAND gates or only NOR gates.
What is the difference between Von Neumann and Harvard architecture?
In Von Neumann architecture, instructions and data share the same memory and bus. In Harvard architecture they are kept in separate memories with separate buses, which can make access faster.