25. Pack Multiple Fields into a 16-bit Control Register
In embedded systems, multiple configuration fields are often packed into a single register using bit-level operations.
You are given the following field specifications to be packed into a 16-bit control register:
| Field | Bits | Position (LSB-first) |
| Mode | 3 | Bits 0–2 |
| Speed | 5 | Bits 3–7 |
| Reserved | 2 | Bits 8–9 (must be 0) |
| Status | 6 | Bits 10–15 |
Your task is to:
- Read mode, speed, and status from input
- Pack them into a uint16_t register following the given bit layout
- Ensure reserved bits (8–9) remain 0
- Print the resulting packed value
Example-1
Input: mode = 3, speed = 10, status = 12
Output: 12371
(Hex: 0x3053, Binary: 0011000001010011)Example-2
Input: mode = 7, speed = 31, status = 63
Output: 64767
(Hex: 0xFCFF, Binary: 1111110011111111)Example-3
Input: mode = 4, speed = 16, status = 8
Output: 8324
(Hex: 0x2084, Binary: 0010000010000100)
Need Help? Refer to the Quick Guide below
In embedded systems, a single register often contains multiple fields, each controlling a different function. These fields are packed into specific bit positions.
Example: A 16-bit Control Register
| Bits | Field Name |
|---|---|
| 15 – 12 | Mode |
| 11 – 8 | Speed |
| 7 – 0 | Flags |
To work with them, we need two core skills:
- Extracting a field (reading the bits at a specific position)
- Replacing or updating that field (without touching other bits)
This is done using bit masking and bit shifting.
To extract Speed field (bits 8 to 11):
uint16_t speed = (reg >> 8) & 0x0F;To update the Mode field (bits 12 to 15):
reg &= ~(0x0F << 12); // Clear Mode bits
reg |= ((new_mode & 0x0F) << 12); // Set new ModeKey Operations
Extract a bit field
value = (reg >> position) & mask;Replace a bit field
/* Where mask is a binary mask for the number of bits in the field.
E.g., For 3-bit field → mask = 0x07 */
reg &= ~(mask << position); // Clear the bits (mask= high bits)
reg |= ((new_value & mask) << position); // Set the new bitsRelevance in Embedded/Firmware
- Most hardware registers (UART, Timer, ADC, SPI) use bit fields to define different parameters.
- Peripheral setup often involves:
- Extracting status bits (e.g., data ready, error)
- Modifying specific bits (e.g., set baud rate, enable TX)
- It’s unsafe to overwrite the entire register — instead, we update only the target bits.
This is why precise extraction and modification using bit masks is a core embedded skill.