# Calculator Program for Multi-Base Arithmetic

## How to Use the Calculator

### Initial Setup
- On power-up, the calculator starts in **DECIMAL mode**
- LCD shows: `Mode: DEC` on line 1, `Inp: 0` on line 2
- 7-segment displays the last digit of your input

### Basic Operations

1. **Entering Numbers**
   - Press keys 0-9 for decimal digits
   - In HEX mode, you can also use A-F (keys 10-15)
   - In OCT mode, only 0-7 are valid
   - In BIN mode, only 0-1 are valid
   - Invalid digits for the current mode are ignored

2. **Changing Base Mode**
   - Press **Key A (10)** to cycle through bases
   - Order: BIN → OCT → DEC → HEX → BIN...
   - Current mode displays on LCD top line

3. **Performing Calculations**
   - Enter first number
   - Press operation key:
     - **Key B (11)**: Addition
     - **Key D (13)**: Subtraction
     - **Key E (14)**: Multiplication
   - Enter second number
   - Press **Key F (15)** for equals/result

4. **Clearing**
   - Press **Key C (12)** to clear all (input, stored number, operation)

### Example Usage

**Example 1: Add 5 + 3 in Decimal**
```
Press: 5 → B → 3 → F
Display: Res: 8
```

**Example 2: Multiply 1010 × 11 in Binary**
```
Press: A (change to BIN mode)
Press: 1 → 0 → 1 → 0 → E → 1 → 1 → F
Display: Res: 11110 (30 in decimal)
```

**Example 3: Subtract F - A in Hexadecimal**
```
Press: A (cycle to HEX mode if not already)
Press: F → D → A (10) → F
Display: Res: 5
```

### Tips
- The 7-segment always shows the last hex digit of your current input
- Results stay in the current base mode
- If result exceeds display capacity, only visible portion shows
- After equals, the result becomes the new input for next operation