Bitmask và trường bit

#ifndef QUYEN_H
#define QUYEN_H

#define BIT(n) (1u << (n))

/* Moi co la MOT bit. Dat ten ro rang, va dung enum
   thay vi define de trinh go loi thay duoc ten. */
enum Quyen {
    Q_DOC      = BIT(0),        /* 0x01 */
    Q_GHI      = BIT(1),        /* 0x02 */
    Q_THUC_THI = BIT(2),        /* 0x04 */
    Q_AN       = BIT(3),        /* 0x08 */
    Q_HE_THONG = BIT(4),        /* 0x10 */

    Q_KHONG    = 0,
    Q_DOC_GHI  = Q_DOC | Q_GHI,
    Q_TAT_CA   = Q_DOC | Q_GHI | Q_THUC_THI | Q_AN | Q_HE_THONG,
};

#endif /* QUYEN_H */

typedef struct { unsigned co; const char *ten; } MucCo;

static const MucCo BANG_CO[] = {
    { Q_DOC,      "DOC"      },
    { Q_GHI,      "GHI"      },
    { Q_THUC_THI, "THUC_THI" },
    { Q_AN,       "AN"       },
    { Q_HE_THONG, "HE_THONG" },
};

void in_quyen(unsigned q) {
    int dau = 1;

    if (q == 0) { printf("KHONG\n"); return; }

    for (size_t i = 0; i < sizeof BANG_CO / sizeof BANG_CO[0]; ++i)
        if (q & BANG_CO[i].co) {
            printf("%s%s", dau ? "" : " | ", BANG_CO[i].ten);
            dau = 0;
            q &= ~BANG_CO[i].co;
        }

    if (q) printf("%s0x%X", dau ? "" : " | ", q);   /* bit la con sot */

    printf("\n");
}

/* Thanh ghi CR1 cua mot bo dinh thoi:
     bit 31..16  chu ky
     bit 15..12  bo chia truoc
     bit 11..8   che do dem
     bit 7       bat ngat
     bit 6..4    nguon xung
     bit 3..1    du tru
     bit 0       bat  */

#define CR1_CHU_KY_HI     31
#define CR1_CHU_KY_LO     16
#define CR1_CHIA_HI       15
#define CR1_CHIA_LO       12
#define CR1_NGUON_HI       6
#define CR1_NGUON_LO       4
#define CR1_NGAT           7
#define CR1_BAT            0

#define MAT_NA(hi, lo)  ((~0u >> (31 - (hi))) & (~0u << (lo)))

static inline unsigned doc_truong(unsigned x, unsigned hi, unsigned lo) {
    return (x & MAT_NA(hi, lo)) >> lo;
}

static inline unsigned ghi_truong(unsigned x, unsigned hi, unsigned lo,
                                  unsigned v) {
    return (x & ~MAT_NA(hi, lo)) | ((v << lo) & MAT_NA(hi, lo));
}

/* Dung: */
unsigned cr1 = 0;

cr1 = ghi_truong(cr1, CR1_CHU_KY_HI, CR1_CHU_KY_LO, 1000);
cr1 = ghi_truong(cr1, CR1_CHIA_HI,   CR1_CHIA_LO,   3);
cr1 = ghi_truong(cr1, CR1_NGUON_HI,  CR1_NGUON_LO,  2);
cr1 |= BIT(CR1_NGAT) | BIT(CR1_BAT);

unsigned chu_ky = doc_truong(cr1, CR1_CHU_KY_HI, CR1_CHU_KY_LO);

struct Co1 {
    unsigned bat    : 1;
    unsigned che_do : 3;
    unsigned ma     : 8;
};

struct Co1 a;
memset(&a, 0, sizeof a);

a.bat    = 1;
a.che_do = 5;
a.ma     = 0xAB;

unsigned m = a.ma;          /* trinh bien dich tu sinh che va dich */

/* Bitmask thu cong */
#define CH_BAT        BIT(0)
#define CH_CHE_DO_HI  3
#define CH_CHE_DO_LO  1
#define CH_MA_HI      11
#define CH_MA_LO      4

static inline unsigned ch_che_do(uint32_t x) {
    return doc_truong(x, CH_CHE_DO_HI, CH_CHE_DO_LO);
}

static inline uint32_t ch_dat_che_do(uint32_t x, unsigned v) {
    return ghi_truong(x, CH_CHE_DO_HI, CH_CHE_DO_LO, v);
}

/* Truong bit */
typedef struct {
    unsigned bat    : 1;
    unsigned che_do : 3;
    unsigned ma     : 8;
} CauHinh;

/* Ca hai deu sinh ra khoang cung mot ma may.
   Khac biet nam o KHA CHUYEN va o CACH DOC. */