Big Picture: Where This Fits in COA

In Computer Organization & Architecture (COA), we study how a computer really works inside—from tiny logic gates up to the CPU, memory, buses, and I/O.

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CPU Design

Inside it is the Microprocessor Architecture.

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Classic Examples

The 8085 (8-bit) and 8086 (16-bit) Intel chips show how real processors work.

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Learning Model

Understanding the 8085 is like looking at a small but complete model of how today's powerful CPUs still operate.

The Story of Microprocessors

Imagine the early 1970s: computers were huge, slow, and built from many separate parts. Then came a revolution that changed everything.

📅Evolution Timeline

1971 – Intel 4004 (4-bit)

The first microprocessor, tiny but limited in capability.

1972 – 8008 (8-bit)

More powerful but still not enough for complex tasks.

1974 – 8080 (8-bit)

First truly usable general-purpose microprocessor chip.

1977 – 8085 (8-bit)

Simpler design, needed only +5V power, became a learning standard.

1978 – 8086 (16-bit)

Start of the x86 family that would power personal computers.

🚀The Impact

Each step made processors smaller, faster, and more capable, which allowed the birth of the personal computer and transformed how we work and live. This evolution continues today with modern processors that can perform billions of operations per second.

What Is a Microprocessor?

Think of it as the brain on a single chip—a complete computation engine that processes data and controls other parts of a computer system.

🧠Key Characteristics

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Binary Processing

It processes binary numbers (0s and 1s), the fundamental language of computers.

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Instruction Set

Has its own "language" of instructions it can understand and execute.

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Clock Speed

Operates at a specific clock speed that determines how many instructions it can process per second.

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Bit-width

The number of bits it can process at once (8-bit, 16-bit, etc.), indicating its data-handling capacity.

Meet the 8085: The Learning Model

Intel's 8085 is an 8-bit microprocessor that became a standard for learning computer architecture due to its simple yet powerful design.

📋Key Specifications

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8-bit Data Bus

Handles 8 bits of data at a time

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16-bit Address Bus

Can address 64 KB of memory

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3 MHz Clock

Operates at 3 million cycles per second

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Single Power Supply

Requires only +5V to operate

🗃️Registers

The 8085 has six general-purpose registers (B, C, D, E, H, L) that can be paired as BC, DE, HL for 16-bit operations. These registers act as small, fast storage locations within the CPU.

Inside the 8085 – Main Components

Picture a small city of tiny units, each with a specific job, working together to process information:

🧩Component 📝Role
Accumulator A special 8-bit register where numbers come to be added, subtracted, etc. It's the primary workhorse for arithmetic operations.
ALU (Arithmetic Logic Unit) The "math & logic factory" that adds, subtracts, AND/ORs data from the Accumulator and temporary register.
General Purpose Registers B, C, D, E, H, L — small storage cupboards. HL pair can point to a memory location, acting as a memory pointer.
Program Counter (PC) Like a bookmark — always holds the address of the next instruction to be executed.
Stack Pointer (SP) Points to the top of a stack in RAM, used for temporary storage during subroutines or interrupts.
Flags Tiny indicators (Carry, Zero, Sign, Parity, Aux-Carry) that tell if an operation caused a carry, zero result, etc.
Instruction Register & Decoder Receives the instruction fetched from memory and decodes what to do next.
Timing & Control Unit The conductor of the orchestra — synchronizes data flow, sends control signals (RD, WR, ALE).
Buses (Address & Data) The roads for carrying addresses and data between CPU, memory, and I/O.

How the 8085 Executes Instructions

Every program is a list of instructions in memory. The CPU follows a continuous Fetch–Decode–Execute cycle, like breathing for the microprocessor:

🔄The Cycle

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1. Fetch

PC puts the instruction's address on the address bus. Memory places the opcode (instruction code) on the data bus. Instruction Register stores it.

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2. Decode

Decoder reads the opcode and decides the operation and required data.

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3. Execute

ALU or other units perform the action (add, move data, jump, etc.). Flags update if needed. PC updates to next instruction.

🌐Real-World Example

Think of this cycle like following a recipe. First, you read the next step (fetch). Then you understand what it means (decode). Finally, you perform the action (execute). The 8085 does this millions of times per second, allowing it to complete complex tasks by breaking them into simple steps.

Signals and Timing

📡Control Signals

The 8085 uses specific control signals to coordinate with memory and I/O devices:

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RD (Read)

Indicates that the CPU wants to read data from memory or I/O.

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WR (Write)

Indicates that the CPU wants to write data to memory or I/O.

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ALE (Address Latch Enable)

Helps demultiplex the lower address bus from the data bus.

⏱️Timing

The 8085's operations are broken down into:

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T-states

The smallest unit of time, equal to one clock period.

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Machine Cycles

Groups of T-states that perform a basic operation (opcode fetch, memory read/write, I/O read/write).

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Timing Diagrams

Show exactly when each signal goes high or low during operations, helping engineers understand and debug the system.

Instruction Set & Addressing

📚Instruction Types

The 8085's instructions are grouped into five categories:

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Data Transfer

Instructions like MOV (move data between registers) and MVI (move immediate data).

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Arithmetic

Instructions like ADD (add to accumulator) and SUB (subtract from accumulator).

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Logical

Instructions like AND, OR, and CMP (compare) for logical operations.

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Branching

Instructions like JMP (jump) and CALL (call subroutine) that change program flow.

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Machine Control

Instructions like NOP (no operation) and HLT (halt) for controlling the CPU.

🎯Addressing Modes

Addressing modes describe where the data for an instruction is located:

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Immediate

Data is included in the instruction itself (e.g., MVI A, 05H).

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Direct

Instruction contains the exact memory address (e.g., LDA 2050H).

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Register

Data is in a register (e.g., MOV A, B).

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Register Indirect

Register contains the memory address (e.g., MOV A, M).

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Implied

Data location is implied by the instruction (e.g., CMA).

Why It Matters in COA

Studying the 8085 provides fundamental insights into computer organization and architecture that remain relevant today:

🏗️CPU Organization

The 8085 clearly demonstrates how registers, ALU, buses, and control units work together to process information. This organization forms the blueprint for modern processors, even as they've become more complex.

🔄Instruction Execution

The fetch-decode-execute cycle of the 8085 is the same fundamental process used by modern CPUs. Understanding this cycle is essential to grasping how computers perform tasks at the most basic level.

🔌Memory & I/O Interfacing

The 8085 shows how a processor communicates with memory and input/output devices, a concept that remains central to computer design even as interfaces have become more sophisticated.

🌐Real-World Connection

Though today's chips are far more advanced with billions of transistors, multiple cores, and complex instruction sets, their core principles are almost identical to those of the 8085. Learning the 8085 provides a foundation for understanding these modern systems.

Summary

The 8085 microprocessor represents a pivotal moment in computing history and serves as an excellent model for understanding computer organization and architecture. Its simple yet powerful design demonstrates the fundamental principles that continue to guide processor design today.

📋Topic 📝Key Points
Evolution 4004 → 8008 → 8080 → 8085 (8-bit) → 8086 (16-bit)
8085 Specs 8-bit CPU, 16-bit address bus (64 KB memory), 3 MHz clock
Main Units Accumulator, ALU, General Registers, PC, SP, Flags, Control/Timing
Operation Cycle Fetch → Decode → Execute
Signals & Timing RD, WR, ALE, machine cycles (opcode fetch, read, write)
Instruction Types Data transfer, Arithmetic, Logical, Branching, Machine control
Addressing Modes Immediate, Direct, Register, Register Indirect, Implied
COA Link Demonstrates CPU architecture, instruction execution, memory/I-O interface

🧠Key Takeaway

Think of the 8085 as a miniature, easy-to-observe model of the computer brain. By following how it fetches, decodes, and executes instructions, you gain a clear mental picture of what every modern CPU—no matter how powerful—still does at its core. This understanding forms the foundation for studying more advanced computer architectures and designing the computing systems of the future.