16. Bitwise Operations-III
Question.2
A DMA controller's 8-bit status register has the following layout:
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|
| ERR | DONE | BUSY | CH | DIR | EN | ||
The register currently reads 0xB5. What is the value of the CH (channel) field?
Need Help? Refer to the Quick Guide below
In C, bitwise operations allow direct manipulation of individual bits within a byte, word, or register. These operations are performed using the following operators:
- | → Bitwise OR — typically used to set a bit
- & → Bitwise AND — used to clear or check a bit
- ^ → Bitwise XOR — used to toggle (invert) a bit
- ~ → Bitwise NOT — used to flip all bits (1 to 0, 0 to 1)
- <<, >> → Bit shift left/right — used to move bit positions
Common bit-masking patterns:
reg |= (1 << n); // Set bit n
reg &= ~(1 << n); // Clear bit n
reg ^= (1 << n); // Toggle bit n
if (reg & (1 << n)) // Check if bit n is setThese operations are used to target and modify only specific bits, without disturbing others.
8-bit Visualization Example
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|
| Value | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
E.g.
- To set bit 3 →
reg |= (1 << 3) - To clear bit 6 →
reg &= ~(1 << 6) - To check bit 0 →
reg & (1 << 0) - To toggle but 3 →
reg ^= (1 << 3)
32-bit Register Usage
Bitwise techniques apply the same way for uint32_t types — often used in 32-bit MCUs for status/configuration registers.
Example:
ctrl_reg |= (1U << 23); // Set bit 23 in a 32-bit control register
Common Pitfalls (Practical Tips)
- Forgetting parentheses:
1 << nmust be inside parentheses during masking:
✅reg |= (1 << 4)vs ❌reg |= 1 << 4 & 0xF - Using signed types: Never use int for register work — use
uint8_t, uint16_t, or uint32_t.
Signed values may cause undefined behavior when shifting, as MSB (Most Significant Bit) used for sign. - Wrong shift width: Shifting more than the data type width (e.g. 1 << 32 on a 32-bit int) is undefined.
- Accidental overwrite:
reg = (1 << 2); // WRONG: overwrites entire register
reg |= (1 << 2); // RIGHT: sets only the 2nd bit- Check before toggle: Be careful when toggling flags — you may unintentionally clear a set flag.
Relevance in Embedded/Firmware
- All MCU peripherals are controlled via register bits — timers, GPIOs, UART, ADC, interrupts, etc.
- Bit masking ensures safe modification without overwriting unrelated fields.
- Used in flags, control masks, interrupt status checks, and power/clock configuration.
Restart quiz!