Bit Fields in 32-bit Registers: Extract, Clear, Replace (Safely)

Working with control/status registers means precise bit slicing. Here are three tiny, safe helpers for 32-bit registers (0-based indexing; LSB=0) with a mask utility that avoids undefined shifts.

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✅ Extract Field

Pull len bits starting at pos.

#include <stdint.h>
static inline uint32_t u32_mask(uint8_t len){ return len>=32 ? 0xFFFFFFFFu : ((1u<<len)-1u); }
static inline uint32_t extract_field(uint32_t reg, uint8_t pos, uint8_t len){
    return (reg >> pos) & u32_mask(len);
}

Examples:
reg=…1011xxxx, pos=28, len=4 → 0b1011 · reg=0x000000FF, pos=0, len=8 → 0xFF

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❌ Clear Bits

Zero len bits starting at pos (others untouched).

static inline uint32_t clear_bits(uint32_t reg, uint8_t pos, uint8_t len){
    return reg & ~(u32_mask(len) << pos);
}

Examples:
0xFF, pos=4, len=4 → 0x0F · 0x0F, pos=0, len=2 → 0x0C

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🔄 Replace Field

Overwrite len bits at pos with val (masked-in).

static inline uint32_t replace_field(uint32_t reg, uint32_t val, uint8_t pos, uint8_t len){
    uint32_t m = u32_mask(len) << pos;
    return (reg & ~m) | ((val & u32_mask(len)) << pos);
}

Examples:
reg=0xFF, val=0, pos=4, len=4 → 0x0F · reg=0x0F, val=0b10, pos=1, len=2 → 0x0D

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⚡ Pro Tips

• Guard inputs: pos < 32 and len > 0 && pos + len <= 32.

• Always shift with 1u (unsigned) and use uint32_t.

• For HW registers, use volatile and consider atomic sections if ISRs/DMA also touch the register.

• Keep these in a header (static inline) for zero-overhead reuse.

🎯 Conclusion

These one-liners let you slice, zero, and patch register fields without collateral damage—portable, predictable, and ISR-friendly.

Written By: Musaab Taha

This article was improved with the assistance of AI.