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8085 Microprocessor Architecture Explained Simply

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The Intel 8085, released in 1976, is an 8-bit microprocessor that taught generations of engineers. It is far simpler than a modern CPU, yet holds every essential idea: registers, an ALU, buses, a control unit, and the fetch-decode-execute cycle. Learning the 8085 is like learning to drive on a simple car before a race car.

You will not find an 8085 in a new laptop, but its ideas apply to every processor ever made. Its small instruction set can be memorised, its timing traced by hand, and tiny programs like adding two numbers reveal exactly what machine code does inside silicon.

Key Concepts

  • 8-bit ALU: operates on 8 bits at a time; 16-bit work needs extra steps.
  • Accumulator (A): the special register where most arithmetic happens; results land here.
  • General registers: B, C, D, E, H, L, usable singly or as pairs BC, DE, HL; HL doubles as a memory pointer.
  • Program counter (PC): 16-bit register with the address of the next instruction.
  • Stack pointer (SP): points to the stack top for subroutine calls and interrupts.
  • Flags: Sign (S), Zero (Z), Auxiliary Carry (AC), Parity (P), Carry (CY); they record what the last operation did.
  • Address bus (16-bit): reaches 216 = 65,536 locations, i.e. 64 KB.
  • Data bus (8-bit): carries one byte at a time.
  • Multiplexed AD0–AD7: lower address lines share pins with the data bus; the ALE signal marks when address is present.
  • Fetch-decode-execute: the endless cycle: fetch the instruction, decode it, execute it.
  • T-state: one clock period; at 3 MHz about 0.33 µs. Instructions take several T-states.

Formulas

  • Memory: 216 = 65,536 = 64 KB.
  • T-state: T = 1/fclock; at 3 MHz, T ≈ 0.333 µs. A 4 T-state instruction takes about 1.33 µs.
  • Flag rules for addition: CY = 1 on overflow past 255; Z = 1 if result is zero; S copies bit 7; P = 1 for even parity.

Worked Example

Trace: MVI A, 05H; MVI B, 03H; ADD B. Find final registers and flags.

  1. After the MVIs: A = 05H (000001012), B = 03H (000000112).
  2. ADD B: 00000101 + 00000011 = 00001000 = 08H into the accumulator.
  3. Flags: nonzero so Z = 0; bit 7 is 0 so S = 0; no overflow so CY = 0; one 1-bit (odd) so P = 0.
  4. Final: A = 08H, B = 03H, S = Z = CY = P = 0, traced by hand exactly as silicon does it.

Common Misconceptions

  • The 8085 is 16-bit because of its address bus. It is 8-bit: ALU and data bus handle bytes; the 16-bit address bus just reaches 64 KB.
  • All 16 address lines have their own pins. The lower 8 share pins with data as AD0–AD7, demultiplexed by ALE.
  • 8085 assembly is a waste of time. Its tiny instruction set makes fetch-decode-execute, addressing modes, and flags visible as modern chips hide them.

Real-World Applications

  • Engineering courses worldwide teach microprocessor fundamentals on the 8085.
  • Legacy industrial controllers, traffic sequencers, and instruments ran on it for decades.
  • Companion chips like the 8255 PPI were designed for its bus.
  • Training kits pair it with LEDs, keypads, and seven-segment displays for hands-on labs.

To recap: the 8085 is an 8-bit CPU with a 16-bit address bus reaching 64 KB, six general registers plus an accumulator, five flags, and a fetch-decode-execute heartbeat of T-states. Tracing tiny programs by hand turns the abstract processor into something you can watch think.

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