8051 Full-Featured Calculator

A complete expression calculator built in pure 8051 Assembly Language for the AT89C51 microcontroller. Supports operator precedence, brackets, signed 24-bit arithmetic, memory register, and special math functions.

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📋 Project Overview

This project implements a full expression calculator from scratch in 8051 Assembly (MIDE-51). Unlike simple left-to-right calculators, this one correctly evaluates expressions like 3 + 4 * 2 = 11 respecting operator precedence — using Dijkstra's Shunting-Yard algorithm implemented entirely in assembly.

✨ Features

• ✅ Operator precedence (* / before + -)
• ✅ Bracket support with implicit multiplication (3(4+5) → 27)
• ✅ 24-bit signed arithmetic (range: ±8,388,607)
• ✅ Decimal division (7÷2 = 3.5)
• ✅ Memory register — M+, M−, MR, MC
• ✅ Special functions — x², x³, √ (via MODE layer)
• ✅ Percentage operator %
• ✅ Unary minus (e.g. -5 * 3 = -15)
• ✅ 4 error states — DIV/0, Overflow, 3-digit limit, Bracket mismatch

🛠️ Hardware

Component	Part	Connection
Microcontroller	AT89C51 (8051)	U1 — 12MHz crystal
Display	LM044L 20×4 LCD	Data: P1, RS: P2.1, EN: P2.2
Keypad	4×4 Matrix	P3.0–P3.7
MODE Button	Tactile Switch	P2.3 (active LOW)
M−, MR, MC	Tactile Switches	P2.4–P2.6
Crystal	12 MHz + 33pF ×2	XTAL1 / XTAL2
Reset	10kΩ + 10µF	RST (pin 9)

⌨️ Keypad Layout

[ 7 SQU ][ 8 CUB ][ 9  (  ][ ÷ ]
[ 4  M- ][ 5  %  ][ 6  √  ][ × ]
[ 1  MR ][ 2  MC ][ 3  M+ ][ - ]
[ ON/C  ][ 0  )  ][   =   ][ + ]

Small labels = function in MODE layer. Press MODE first, then the key.

🗂️ Project Structure

8051-calculator/
├── src/
│   ├── calculator.asm        # Full source (1485 lines)
├── simulation/
│   └── calculator8051.pdsprj     # Proteus 8 project file
├── hex/
│   └── calculator.hex        # Compiled Intel HEX output
└── docs/
    ├── circuit.png           # Schematic screenshot
    ├── ram-map.md            # RAM layout documentation
    └── how-it-works.md       # Algorithm explanation

🚀 How to Run

Simulate in Proteus

1. Open simulation/calculator.pdsprj in Proteus 8
2. Load hex/calculator.hex into the AT89C51 component
3. Click Play ▶ and use the virtual keypad

Assemble from source

1. Open src/calculator.asm in MIDE-51
2. Press F9 (Build) to assemble
3. The .hex file is generated in the same folder

📐 How It Works

The calculator uses the Shunting-Yard algorithm to evaluate expressions with correct precedence. Two stacks are maintained in internal RAM:

• Value Stack (40H–5DH) — stores 24-bit operands (10 deep)
• Operator Stack (5EH–67H) — stores operator characters (10 deep)

When an operator is pressed, its precedence is compared to the top of the operator stack. Higher-precedence operators are applied first.

See docs/how-it-works.md for full details.

⚠️ Error Handling

Error Message	Trigger
ERROR: DIV BY 0	Divisor is zero
OVERFLOW!	Result exceeds ±8,388,607 or stack full
ERR:3DIG	Typing a 4th digit (max input = 999)
BRACKET ERR	Mismatched ( ) in expression

🧰 Tools Used

• MIDE-51 — 8051 assembler and IDE
• Proteus 8 — Circuit simulation

📄 License

MIT License — see LICENSE for details.