Wednesday, December 3, 2025

✅ UNIT 5 — MEMORY & I/O ORGANIZATION

This unit explains how data is stored, accessed, and transferred inside a computer — essential for understanding how modern CPUs achieve high speed.

⭐ 5.1 Memory Hierarchy

Memory is organized in layers based on speed, cost, and capacity.

From Fastest → Slowest

Registers
Cache Memory
Main Memory (RAM)
Secondary Memory (HDD/SSD)
Tertiary (CD/DVD)

Key Idea:

Higher levels → faster, expensive, smaller
Lower levels → slower, cheap, larger

CPU always tries to read from the fastest possible level.

⭐ 5.2 Cache Memory

Cache = small, very fast memory between CPU & RAM.

Why cache is needed?

Because CPU is extremely fast but RAM is slower → cache avoids waiting.

Types of Cache

1. L1 Cache

Fastest, smallest, inside CPU core.

2. L2 Cache

Bigger, slower.

3. L3 Cache

Shared between multiple cores.

Cache Mapping Techniques

1️⃣ Direct Mapping

Each block of main memory maps to exactly one cache line.

Advantages: Simple
Disadvantages: Conflict misses

2️⃣ Associative Mapping

Any memory block → any cache line.

Advantages: Flexible
Disadvantages: Expensive hardware

3️⃣ Set-Associative Mapping

Memory block → a small set of cache lines.
Most commonly used.

⭐ 5.3 Main Memory (RAM + ROM)

RAM Types

SRAM → used in cache
DRAM → used in main memory

ROM Types

⭐ 5.4 Memory Interleaving

Memory speed can be increased by storing data across multiple banks.

Types:

1️⃣ Lower-Order Interleaving

Low-order bits choose memory bank.

2️⃣ Higher-Order Interleaving

High-order bits choose memory bank.

Goal: Increase reading speed by accessing multiple banks simultaneously.

⭐ 5.5 Associative Memory (Content Addressable Memory)

Data is accessed by content, not by address.

Used in:

Cache tag matching
TLB (Translation Lookaside Buffer)

⭐ 5.6 I/O Organization

Two main categories:

1️⃣ Programmed I/O

CPU controls I/O directly.
Simple but slow.

2️⃣ Interrupt-Driven I/O

I/O device interrupts CPU when ready.
Faster than programmed I/O.

3️⃣ Direct Memory Access (DMA)

Fastest I/O method.

DMA Features:

Transfers data directly between memory & I/O
CPU is freed during transfer
Very efficient for large data (disk, graphics)

⭐ 5.7 Memory-Mapped vs I/O-Mapped I/O

Feature	Memory-Mapped I/O	I/O-Mapped I/O
Address Space	Uses normal memory addresses	Separate I/O address space
Instructions	Normal memory instructions	Special I/O instructions (IN, OUT)
Speed	Faster	Moderate
Example	Modern CPUs	8085 (IN/OUT)

⭐ 5.8 Secondary Storage Concepts

HDD

Mechanical
Slower
Large capacity

SSD

No moving parts
Much faster
Expensive

CD/DVD

Optical storage
Used for distribution & backup

✅ UNIT 4 — Microprocessor Basics, Instruction Cycle, Pipelining & Hazards

This unit connects digital electronics to how a CPU actually works internally.

⭐ 4.1 What is a Microprocessor?

A microprocessor is the brain of the computer, an IC that performs:

Arithmetic operations (ALU)
Logical operations
Control (via CU)
Data transfer

Examples:

8085 (8-bit, classic for exams)
8086 (16-bit)
i3, i5, i7 (modern CPUs)

⭐ 4.2 Basic Microprocessor Architecture

Major Components:

1️⃣ ALU (Arithmetic Logic Unit)

Performs:

Add, Subtract
AND, OR, NOT
Compare

2️⃣ Control Unit (CU)

Controls timing
Fetch, Decode, Execute instructions
Sends control signals to memory and I/O

3️⃣ Registers

High-speed small storage inside CPU:

Accumulator (A)
General registers (B, C, D, E, H, L)
Program Counter (PC)
Stack Pointer (SP)
Flag Register (Z, S, CY, P, AC flags)

⭐ 4.3 Instruction Cycle

Every instruction executes in three steps:

1️⃣ Fetch Cycle

PC → Address Bus
Memory → Instruction → CPU
PC = PC + 1

2️⃣ Decode Cycle

Instruction decoded by Control Unit
Identifies required operation

3️⃣ Execute Cycle

ALU performs operation
Result stored in Register/Memory
Flags updated

⭐ 4.4 Machine Cycle & T-States

Machine Cycle:

A part of instruction cycle (memory read, memory write, I/O read, etc.)

T-States:

Each machine cycle is divided into clock cycles.

Example (8085):

Memory Read Cycle = 3 T-states
Memory Write Cycle = 3 T-states
Opcode Fetch Cycle = 4 T-states

⭐ 4.5 Addressing Modes

Defines how data is accessed.

Mode	Example	Meaning
Immediate	MVI A, 05H	Data is inside instruction
Direct	LDA 2050H	Access memory address directly
Register	MOV A, B	Data is in register
Indirect	LDAX B	Address is held in register pair
Implicit	CMA	Operand is fixed

⭐ 4.6 Instruction Format

8085 uses:

1-byte instructions
2-byte instructions
3-byte instructions

Example:
MOV A, B → 1 byte
MVI A, 32H → 2 bytes
LDA 2050H → 3 bytes

⭐ 4.7 Interrupts

Interrupt = CPU’s attention signal.

Types:

Hardware – RST 7.5, RST 6.5, RST 5.5, INTR
Software – RST instruction
Maskable / Non-maskable
Vectored / Non-vectored

Example:

TRAP → Non-maskable, vectored
RST 7.5 → Maskable, vectored

⭐ 4.8 Pipelining

Modern CPUs use pipelines to speed up execution.

Typical stages (RISC pipeline):

IF – Instruction Fetch
ID – Instruction Decode
EX – Execute
MEM – Memory Access
WB – Write Back

Why pipelining is fast?

Because multiple instructions execute in parallel.

⭐ 4.9 Types of Pipelines

Superpipelining
(More pipeline stages)
Superscalar Pipeline
(More than one instruction issued per cycle)

⭐ 4.10 Pipeline Hazards

These are problems that slow down pipelined execution.

1️⃣ Data Hazard

Occurs when one instruction depends on the result of the previous one.

Example:


I1: ADD R1, R2, R3  
I2: SUB R4, R1, R5   ← depends on R1

2️⃣ Control Hazard

Occurs in branching.

Example:


BEQ LABEL

Branch decision unknown → pipeline stalls.

3️⃣ Structural Hazard

Two instructions need the same hardware resource at the same time.

Solutions to hazards

Forwarding / Bypassing
Pipeline Stalling
Branch Prediction
Separate instruction & data memory

✅ UNIT 3 — Combinational & Sequential Circuits

⭐ 3.1 ADDERS

1️⃣ Half Adder

Adds two single-bit numbers (A and B).

Outputs:

Sum = A ⊕ B
Carry = A • B

2️⃣ Full Adder

Adds three bits (A, B, Cin).

Outputs:

Sum = A ⊕ B ⊕ Cin
Cout = AB + BCin + ACin

3️⃣ 4-bit & 8-bit Parallel Adder

Uses four full adders connected so that carry passes from one stage to next (ripple carry).

⭐ 3.2 SUBTRACTORS

Half Subtractor

Inputs: A, B
Outputs:

Difference = A ⊕ B
Borrow = A' • B

Full Subtractor

Inputs: A, B, Bin
Outputs:

Difference = A ⊕ B ⊕ Bin
Borrow = A'B + B Bin + A' Bin

⭐ 3.3 ADDER–SUBTRACTOR COMBINED CIRCUIT

A single circuit performs both add & subtract using XOR-based control.

Control input M:

M = 0 → Add
M = 1 → Subtract

⭐ 3.4 MAGNITUDE COMPARATOR

Compares two numbers A and B and generates:

A > B
A = B
A < B

Used in CPU for decision-making (conditional branches).

⭐ 3.5 MULTIPLEXER (MUX) & DEMULTIPLEXER (DEMUX)

Multiplexer (MUX)

Selects one input from many and sends to output.

Example: 4-to-1 MUX

Select lines: S1, S0
Output:
Y = S1'S0'I0 + S1'S0I1 + S1S0'I2 + S1S0I3

Demultiplexer (DEMUX)

Opposite of MUX — one input → many outputs.

Example: 1-to-4 DEMUX.

⭐ 3.6 DECODER & ENCODER

Decoder

Converts n inputs → 2ⁿ outputs.

Example:
2-to-4 decoder
3-to-8 decoder

Used in memory address decoding.

Encoder

Converts 2ⁿ inputs → n outputs.

Example:
8-to-3 encoder
Priority encoder (ignores lower priority inputs)

⭐ 3.7 LATCHES & FLIP-FLOPS

1️⃣ LATCH (Level Triggered)

SR Latch

Basic memory element.

S	R	Q(next)
0	0	Q (no change)
0	1	0
1	0	1
1	1	Invalid

2️⃣ Flip-Flop (Edge Triggered)

D Flip-Flop

Stores 1-bit of data.

Q(next) = D

JK Flip-Flop

J	K	Qnext
0	0	Q
0	1	0
1	0	1
1	1	Toggle

T Flip-Flop

T = 1 → Toggle
T = 0 → Hold

⭐ 3.8 COUNTERS

1️⃣ Ripple Counter (Asynchronous)

Every flip-flop triggers the next one.

Example: 4-bit ripple counter (counts 0–15).

2️⃣ Ring Counter

A single 1 rotates through flip-flops.

3️⃣ Johnson Counter

Complement of last flip-flop output is fed to first.

⭐ 3.9 SEQUENTIAL CIRCUITS

A sequential circuit =
combinational circuit + memory (flip-flops).

Used in:

Registers
Counters
Finite State Machines
Control units of CPUs

✅ UNIT 2 — Logic Gates, Boolean Algebra, K-Maps, Codes

🌟 2.1 LOGIC GATES

Logic gates are electronic circuits that perform logical operations on binary inputs (0 or 1).

1️⃣ Basic Gates

AND Gate

A	B	Output = A • B
0	0	0
0	1	0
1	0	0
1	1	1

OR Gate

A	B	A + B
0	0	0
0	1	1
1	0	1
1	1	1

NOT Gate

A	A'
0	1
1	0

2️⃣ Universal Gates (NAND, NOR)

Any circuit can be built using only NAND or only NOR.

NAND Gate

Output = (A • B)’
Truth Table → Only 1 when both inputs are NOT 1.

NOR Gate

Output = (A + B)’
Truth Table → Only 1 when both inputs are 0.

3️⃣ Exclusive Gates

XOR Gate

Output = 1 when inputs are different.
Equation: A ⊕ B = A’B + AB’

XNOR Gate

Output = 1 when inputs are same.

🌟 2.2 BOOLEAN ALGEBRA

Important Laws

1. Identity Laws

A + 0 = A
A • 1 = A

2. Null Laws

A + 1 = 1
A • 0 = 0

3. Idempotent Laws

A + A = A
A • A = A

4. Complement Laws

A + A’ = 1
A • A’ = 0

5. De Morgan’s Laws

(AB)’ = A’ + B’
(A + B)’ = A’B’

🌟 2.3 Minterms & Maxterms

Minterm (SOP Form)

Each minterm = product term (AND)
Example for 2 variables:
m0 = A’B’
m1 = A’B
m2 = AB’
m3 = AB

Sum of Minterms = SOP

Maxterm (POS Form)

Each maxterm = sum term (OR)
M0 = A + B
M1 = A + B’ etc.

🌟 2.4 Simplification Using Karnaugh Map (K-MAP)

Example:

Simplify F(A, B, C) = Σ(1, 3, 5, 7)

K-MAP:

Fill 1’s in cells 1,3,5,7.

Groups formed:

Group of four (1,3,5,7)

Final simplified function:
👉 F = B

K-map makes long Boolean expressions very small.

🌟 2.5 NUMBER CODES

✔ 1. Weighted Codes

Each digit has a weight.
Examples:

8421 (BCD)
2421
5211

✔ 2. Non-Weighted Codes

Weights do NOT apply.
Examples:

Gray Code
Excess-3
ASCII

Gray Code

Only one bit changes between consecutive numbers (used in error-free encoding).

Binary to Gray:

G1 = B1
G2 = B1 ⊕ B2
G3 = B2 ⊕ B3

Example:
Binary 101 → Gray = 111

Excess-3 Code

Add 3 to each BCD digit.

Example:
BCD 0101 (5)
Excess-3 = 0101 + 0011 = 1000

Code Conversions Covered

✔ Binary ↔ Gray
✔ Binary ↔ BCD
✔ BCD ↔ Excess-3
✔ Hex ↔ Binary
✔ Decimal ↔ Binary

🧩 UNIT 1 — Computer Basics and Number Systems

🌍 1.1 Generation of Computers

Computers have evolved drastically — from vacuum tubes to AI-powered processors. Each generation is marked by hardware technology, processing speed, and language level.

Generation	Years	Technology Used	Language Used	Example Computers	Features
1st Generation	1940–1956	Vacuum Tubes	Machine Language	ENIAC, UNIVAC	Large, slow, expensive, consumed a lot of power
2nd Generation	1956–1963	Transistors	Assembly Language	IBM 1401, CDC 1604	Faster, smaller, less heat, more reliable
3rd Generation	1964–1971	Integrated Circuits (ICs)	High-Level Languages (FORTRAN, COBOL)	IBM 360, PDP-8	Smaller, faster, multitasking possible
4th Generation	1971–1980s	Microprocessors (VLSI)	High-Level Languages (C, BASIC)	Intel 4004, Apple II	Personal computers introduced
5th Generation	1980s–Present	ULSI, AI, Parallel Processing	AI languages (Prolog, Python, ML)	IBM Watson, Quantum computers	AI, machine learning, natural language processing

🧠 Key Idea:
Each generation increased speed, reliability, miniaturization, and user-friendliness while reducing cost and power consumption.

💻 1.2 Functional Block Diagram of a Computer

Every computer follows a simple structure known as the Von Neumann Architecture.


+--------------------------------------------+
|                USER INTERFACE              |
+--------------------------------------------+
| INPUT → CPU (ALU + CU + Registers) → OUTPUT |
|                 ↑          ↓                |
|              MEMORY (Primary + Secondary)  |
+--------------------------------------------+

1️⃣ Input Unit:
Takes data and instructions (e.g., keyboard, mouse, scanner).

2️⃣ Output Unit:
Displays results to the user (e.g., monitor, printer).

3️⃣ Memory Unit:
Stores data temporarily or permanently.

Primary Memory: RAM, ROM
Secondary Memory: Hard Disk, SSD

4️⃣ CPU (Central Processing Unit):

ALU (Arithmetic Logic Unit): Performs calculations and logical operations.
CU (Control Unit): Controls the flow of data and instruction execution.
Registers: High-speed storage inside CPU for quick data access.

🧩 Flow:


Input → Process (CPU) → Output
          ↑
        Memory

🧠 1.3 Hardware vs Software

Aspect	Hardware	Software
Definition	Physical components of a computer	Set of programs/instructions
Examples	Keyboard, CPU, Hard disk	OS, MS Word, Browser
Tangibility	Can be touched	Intangible
Failure	Hardware wears out	Software bugs
Dependency	Needs software to function	Runs on hardware

💬 1.4 Types of Programming Languages

Type	Description	Example
Machine Language	Binary (0s and 1s), directly understood by computer	10110100 01100001
Assembly Language	Uses mnemonics for machine instructions	MOV A, B
High-Level Language	Human-readable, needs compiler/interpreter	C, Java, Python

Conversion Tools:

Assembler: Assembly → Machine
Compiler: High-Level → Machine (all at once)
Interpreter: High-Level → Machine (line by line)

🔢 1.5 Number Systems

System	Base	Digits Used	Example
Binary	2	0, 1	1011
Octal	8	0–7	27
Decimal	10	0–9	45
Hexadecimal	16	0–9, A–F	2F

Conversions:

Binary → Decimal: Multiply each bit by 2ⁿ and sum.
Example: 1011₂ = 1×8 + 0×4 + 1×2 + 1×1 = 11₁₀
Decimal → Binary: Divide by 2 repeatedly and write remainders in reverse.
Example: 13₁₀ → 1101₂

➕ 1.6 Binary Arithmetic

Operation	Example	Result
Addition	101 + 10	111
Subtraction	101 – 10	11
Multiplication	101 × 10	1010
Division	1010 ÷ 10	101

Binary Addition Rules:

+	0	1
0	0	1
1	1	10 (carry 1)