Bitwise Hardware Control Register

Challenge

Embedded systems communicate with peripherals by reading and modifying individual bits within hardware control registers using bitwise operators (&, |, ^, ~).

Your Task

Given the bitmask definitions:

#define FLAG_POWER      (1u << 0) // 0x01
#define FLAG_TX_READY   (1u << 1) // 0x02
#define FLAG_RX_READY   (1u << 2) // 0x04
#define FLAG_ERROR      (1u << 3) // 0x08
#define FLAG_INTERRUPT  (1u << 4) // 0x10

Implement register manipulation functions:

uint8_t reg_set_flag(uint8_t reg, uint8_t mask);
uint8_t reg_clear_flag(uint8_t reg, uint8_t mask);
uint8_t reg_toggle_flag(uint8_t reg, uint8_t mask);
int reg_has_flag(uint8_t reg, uint8_t mask);

Rules

1. reg_set_flag: sets all bits in mask using |.
2. reg_clear_flag: clears all bits in mask using & ~mask.
3. reg_toggle_flag: toggles all bits in mask using ^.
4. reg_has_flag: returns 1 if all bits in mask are currently set in reg, or 0 otherwise. If mask == 0, return 0.

Example

uint8_t r = 0;
r = reg_set_flag(r, FLAG_POWER | FLAG_TX_READY); // 0x03
reg_has_flag(r, FLAG_POWER); // returns 1
reg_has_flag(r, FLAG_RX_READY); // returns 0
r = reg_clear_flag(r, FLAG_POWER); // 0x02

Input format

reg: 8-bit register state; mask: 8-bit flag mask.

Output format

Returns updated register value or boolean match flag.

Constraints

Bitwise operations only. No branches in set/clear/toggle.

Starter code

#include <stdint.h>

#define FLAG_POWER      (1u << 0)
#define FLAG_TX_READY   (1u << 1)
#define FLAG_RX_READY   (1u << 2)
#define FLAG_ERROR      (1u << 3)
#define FLAG_INTERRUPT  (1u << 4)

uint8_t reg_set_flag(uint8_t reg, uint8_t mask) {
    (void)reg; (void)mask;
    return 0;
}
uint8_t reg_clear_flag(uint8_t reg, uint8_t mask) {
    (void)reg; (void)mask;
    return 0;
}
uint8_t reg_toggle_flag(uint8_t reg, uint8_t mask) {
    (void)reg; (void)mask;
    return 0;
}
int reg_has_flag(uint8_t reg, uint8_t mask) {
    (void)reg; (void)mask;
    return 0;
}

Common mistakes

Using logical || instead of bitwise |; forgetting ~ when clearing bits.

Edge cases to handle

mask == 0 returns 0 in has_flag; multi-bit masks must all be present in has_flag.

Background lessons

• Operators and precedence
• Variables