記錄計算前導零C語言實作方法，
前導零：即一個16進制數從最高位開始數，直到遇到第一個1，前面幾有個零，如0x00FFFFFF前導零為8；0xF0000000前導零為0；0x08010002前導零為4.
計算前導零有些硬體可以用匯編指令支持，如STM32F4系列，可以用匯編指令快速得到前導零，
現在用C語言采用查表方式實作，代碼如下：摘自UCOSIII3.03版本cpu_core.c
static const CPU_INT08U CPU_CntLeadZerosTbl[256] = { /* Data vals : */
/* 0 1 2 3 4 5 6 7 8 9 A B C D E F */
8u, 7u, 6u, 6u, 5u, 5u, 5u, 5u, 4u, 4u, 4u, 4u, 4u, 4u, 4u, 4u, /* 0x00 to 0x0F */
3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, 3u, /* 0x10 to 0x1F */
2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, /* 0x20 to 0x2F */
2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, 2u, /* 0x30 to 0x3F */
1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, /* 0x40 to 0x4F */
1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, /* 0x50 to 0x5F */
1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, /* 0x60 to 0x6F */
1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, 1u, /* 0x70 to 0x7F */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0x80 to 0x8F */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0x90 to 0x9F */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0xA0 to 0xAF */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0xB0 to 0xBF */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0xC0 to 0xCF */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0xD0 to 0xDF */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, /* 0xE0 to 0xEF */
0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u, 0u /* 0xF0 to 0xFF */
};
/*8bit型別資料計算前導零*/
CPU_DATA CPU_CntLeadZeros08 (CPU_INT08U val)
{
CPU_DATA ix;
CPU_DATA nbr_lead_zeros;
/* ----------- C-OPTIMIZED ------------ */
/* Chk bits [07:00] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 0u); /* .. lookup tbl ix = 'val' >> 0 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 0u); /* .. plus nbr msb lead zeros = 0 bits.*/
return (nbr_lead_zeros);
}
/*16bit型別資料計算前導零*/
CPU_DATA CPU_CntLeadZeros16 (CPU_INT16U val)
{
CPU_DATA ix;
CPU_DATA nbr_lead_zeros;
if (val > 0x00FFu) { /* Chk bits [15:08] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 8u); /* .. lookup tbl ix = 'val' >> 8 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 0u); /* .. plus nbr msb lead zeros = 0 bits.*/
} else { /* Chk bits [07:00] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 0u); /* .. lookup tbl ix = 'val' >> 0 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 8u); /* .. plus nbr msb lead zeros = 8 bits.*/
}
return (nbr_lead_zeros);
}
/*32bit型別資料計算前導零*/
CPU_DATA CPU_CntLeadZeros32 (CPU_INT32U val)
{
CPU_DATA ix;
CPU_DATA nbr_lead_zeros;
/* ---------- ASM-OPTIMIZED ----------- */
if (val > 0x0000FFFFu) {
if (val > 0x00FFFFFFu) { /* Chk bits [31:24] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 24u); /* .. lookup tbl ix = 'val' >> 24 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 0u); /* .. plus nbr msb lead zeros = 0 bits.*/
} else { /* Chk bits [23:16] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 16u); /* .. lookup tbl ix = 'val' >> 16 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 8u); /* .. plus nbr msb lead zeros = 8 bits.*/
}
} else {
if (val > 0x000000FFu) { /* Chk bits [15:08] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 8u); /* .. lookup tbl ix = 'val' >> 8 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 16u); /* .. plus nbr msb lead zeros = 16 bits.*/
} else { /* Chk bits [07:00] : */
/* .. Nbr lead zeros = .. */
ix = (CPU_DATA)(val >> 0u); /* .. lookup tbl ix = 'val' >> 0 bits */
nbr_lead_zeros = (CPU_DATA)(CPU_CntLeadZerosTbl[ix] + 24u); /* .. plus nbr msb lead zeros = 24 bits.*/
}
}
return (nbr_lead_zeros);
}
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