接續上回談到env_init，列出相關代碼：注意，這裡的代碼分散在不同的檔案
typedef struct environment_s {
uint32_t crc; /* CRC32 over data bytes */
#ifdef CONFIG_SYS_REDUNDAND_ENVIRONMENT
unsigned char flags; /* active/obsolete flags */
#endif
unsigned char data[ENV_SIZE]; /* Environment data */
} env_t;
#define ENV_SIZE (CONFIG_ENV_SIZE - ENV_HEADER_SIZE)
#ifdef CONFIG_SYS_REDUNDAND_ENVIRONMENT
# define ENV_HEADER_SIZE (sizeof(uint32_t) + 1)
#else
# define ENV_HEADER_SIZE (sizeof(uint32_t))
#endif
#ifdef ENV_IS_EMBEDDED //這個marco未定義
extern uchar environment[];
env_t *env_ptr = (env_t *)(&environment[0]); //因此以上兩行未執行
#else /* ! ENV_IS_EMBEDDED */
env_t *env_ptr = 0;
#endif /* ENV_IS_EMBEDDED */
#define local static
#define ZEXPORT
#ifdef DYNAMIC_CRC_TABLE //在u-boot的code裡並未定義這個MARCO;所以並不會動態產生CRC_TABLE
local int crc_table_empty = 1;
local uint32_t crc_table[256];
local void make_crc_table OF((void));
local void make_crc_table()
{
uint32_t c;
int n, k;
uLong poly; /* polynomial exclusive-or pattern */
/* terms of polynomial defining this crc (except x^32): */
static const Byte p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};
/* make exclusive-or pattern from polynomial (0xedb88320L) */
poly = 0L;
for (n = 0; n < n =" 0;" c =" (uLong)n;" k =" 0;" c =" c">> 1) : c >> 1;
crc_table[n] = c;
}
crc_table_empty = 0;
}
#else
/* ========================================================================
* Table of CRC-32's of all single-byte values (made by make_crc_table)
*/
local const uint32_t crc_table[256] = { //這個crc_table會被配置
0x00000000L, 0x77073096L, 0xee0e612cL, 0x990951baL, 0x076dc419L,
0x706af48fL, 0xe963a535L, 0x9e6495a3L, 0x0edb8832L, 0x79dcb8a4L,
0xe0d5e91eL, 0x97d2d988L, 0x09b64c2bL, 0x7eb17cbdL, 0xe7b82d07L,
0x90bf1d91L, 0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL,
0x1adad47dL, 0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L, 0x136c9856L,
0x646ba8c0L, 0xfd62f97aL, 0x8a65c9ecL, 0x14015c4fL, 0x63066cd9L,
0xfa0f3d63L, 0x8d080df5L, 0x3b6e20c8L, 0x4c69105eL, 0xd56041e4L,
0xa2677172L, 0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL, 0xa50ab56bL,
0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L, 0x32d86ce3L,
0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L, 0x26d930acL, 0x51de003aL,
0xc8d75180L, 0xbfd06116L, 0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L,
0xb8bda50fL, 0x2802b89eL, 0x5f058808L, 0xc60cd9b2L, 0xb10be924L,
0x2f6f7c87L, 0x58684c11L, 0xc1611dabL, 0xb6662d3dL, 0x76dc4190L,
0x01db7106L, 0x98d220bcL, 0xefd5102aL, 0x71b18589L, 0x06b6b51fL,
0x9fbfe4a5L, 0xe8b8d433L, 0x7807c9a2L, 0x0f00f934L, 0x9609a88eL,
0xe10e9818L, 0x7f6a0dbbL, 0x086d3d2dL, 0x91646c97L, 0xe6635c01L,
0x6b6b51f4L, 0x1c6c6162L, 0x856530d8L, 0xf262004eL, 0x6c0695edL,
0x1b01a57bL, 0x8208f4c1L, 0xf50fc457L, 0x65b0d9c6L, 0x12b7e950L,
0x8bbeb8eaL, 0xfcb9887cL, 0x62dd1ddfL, 0x15da2d49L, 0x8cd37cf3L,
0xfbd44c65L, 0x4db26158L, 0x3ab551ceL, 0xa3bc0074L, 0xd4bb30e2L,
0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL, 0x4369e96aL,
0x346ed9fcL, 0xad678846L, 0xda60b8d0L, 0x44042d73L, 0x33031de5L,
0xaa0a4c5fL, 0xdd0d7cc9L, 0x5005713cL, 0x270241aaL, 0xbe0b1010L,
0xc90c2086L, 0x5768b525L, 0x206f85b3L, 0xb966d409L, 0xce61e49fL,
0x5edef90eL, 0x29d9c998L, 0xb0d09822L, 0xc7d7a8b4L, 0x59b33d17L,
0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL, 0xedb88320L, 0x9abfb3b6L,
0x03b6e20cL, 0x74b1d29aL, 0xead54739L, 0x9dd277afL, 0x04db2615L,
0x73dc1683L, 0xe3630b12L, 0x94643b84L, 0x0d6d6a3eL, 0x7a6a5aa8L,
0xe40ecf0bL, 0x9309ff9dL, 0x0a00ae27L, 0x7d079eb1L, 0xf00f9344L,
0x8708a3d2L, 0x1e01f268L, 0x6906c2feL, 0xf762575dL, 0x806567cbL,
0x196c3671L, 0x6e6b06e7L, 0xfed41b76L, 0x89d32be0L, 0x10da7a5aL,
0x67dd4accL, 0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L, 0x60b08ed5L,
0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L, 0xd1bb67f1L,
0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL, 0xd80d2bdaL, 0xaf0a1b4cL,
0x36034af6L, 0x41047a60L, 0xdf60efc3L, 0xa867df55L, 0x316e8eefL,
0x4669be79L, 0xcb61b38cL, 0xbc66831aL, 0x256fd2a0L, 0x5268e236L,
0xcc0c7795L, 0xbb0b4703L, 0x220216b9L, 0x5505262fL, 0xc5ba3bbeL,
0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L, 0xc2d7ffa7L, 0xb5d0cf31L,
0x2cd99e8bL, 0x5bdeae1dL, 0x9b64c2b0L, 0xec63f226L, 0x756aa39cL,
0x026d930aL, 0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x05005713L,
0x95bf4a82L, 0xe2b87a14L, 0x7bb12baeL, 0x0cb61b38L, 0x92d28e9bL,
0xe5d5be0dL, 0x7cdcefb7L, 0x0bdbdf21L, 0x86d3d2d4L, 0xf1d4e242L,
0x68ddb3f8L, 0x1fda836eL, 0x81be16cdL, 0xf6b9265bL, 0x6fb077e1L,
0x18b74777L, 0x88085ae6L, 0xff0f6a70L, 0x66063bcaL, 0x11010b5cL,
0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L, 0xa00ae278L,
0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L, 0xa7672661L, 0xd06016f7L,
0x4969474dL, 0x3e6e77dbL, 0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L,
0x37d83bf0L, 0xa9bcae53L, 0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L,
0xbdbdf21cL, 0xcabac28aL, 0x53b39330L, 0x24b4a3a6L, 0xbad03605L,
0xcdd70693L, 0x54de5729L, 0x23d967bfL, 0xb3667a2eL, 0xc4614ab8L,
0x5d681b02L, 0x2a6f2b94L, 0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL,
0x2d02ef8dL
};
#endif /*DYNAMIC_CRC_TABLE */
#ifndef OF /* function prototypes */
# ifdef STDC
# define OF(args) args
# else
# define OF(args) ()
# endif
#endif
#define DO1(buf) crc = crc_table[((int)crc ^ (*buf++)) & 0xff] ^ (crc >> 8);
#define DO2(buf) DO1(buf); DO1(buf);
#define DO4(buf) DO2(buf); DO2(buf);
#define DO8(buf) DO4(buf); DO4(buf);
//以下這個routine在lib_generic/crc32.c
uint32_t ZEXPORT crc32 (uint32_t crc, const Bytef *buf, uInt len)
{
#ifdef DYNAMIC_CRC_TABLE
if (crc_table_empty) make_crc_table(); //這行不會被執行
#endif
crc = crc ^ 0xffffffffL;
while (len >= 8)
{
DO8(buf);
len -= 8;
}
if (len) do {
DO1(buf);
} while (--len);
return crc ^ 0xffffffffL;
}
--> 閱讀更多...
typedef struct environment_s {
uint32_t crc; /* CRC32 over data bytes */
#ifdef CONFIG_SYS_REDUNDAND_ENVIRONMENT
unsigned char flags; /* active/obsolete flags */
#endif
unsigned char data[ENV_SIZE]; /* Environment data */
} env_t;
#define ENV_SIZE (CONFIG_ENV_SIZE - ENV_HEADER_SIZE)
#ifdef CONFIG_SYS_REDUNDAND_ENVIRONMENT
# define ENV_HEADER_SIZE (sizeof(uint32_t) + 1)
#else
# define ENV_HEADER_SIZE (sizeof(uint32_t))
#endif
#ifdef ENV_IS_EMBEDDED //這個marco未定義
extern uchar environment[];
env_t *env_ptr = (env_t *)(&environment[0]); //因此以上兩行未執行
#else /* ! ENV_IS_EMBEDDED */
env_t *env_ptr = 0;
#endif /* ENV_IS_EMBEDDED */
#define local static
#define ZEXPORT
#ifdef DYNAMIC_CRC_TABLE //在u-boot的code裡並未定義這個MARCO;所以並不會動態產生CRC_TABLE
local int crc_table_empty = 1;
local uint32_t crc_table[256];
local void make_crc_table OF((void));
local void make_crc_table()
{
uint32_t c;
int n, k;
uLong poly; /* polynomial exclusive-or pattern */
/* terms of polynomial defining this crc (except x^32): */
static const Byte p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};
/* make exclusive-or pattern from polynomial (0xedb88320L) */
poly = 0L;
for (n = 0; n < n =" 0;" c =" (uLong)n;" k =" 0;" c =" c">> 1) : c >> 1;
crc_table[n] = c;
}
crc_table_empty = 0;
}
#else
/* ========================================================================
* Table of CRC-32's of all single-byte values (made by make_crc_table)
*/
local const uint32_t crc_table[256] = { //這個crc_table會被配置
0x00000000L, 0x77073096L, 0xee0e612cL, 0x990951baL, 0x076dc419L,
0x706af48fL, 0xe963a535L, 0x9e6495a3L, 0x0edb8832L, 0x79dcb8a4L,
0xe0d5e91eL, 0x97d2d988L, 0x09b64c2bL, 0x7eb17cbdL, 0xe7b82d07L,
0x90bf1d91L, 0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL,
0x1adad47dL, 0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L, 0x136c9856L,
0x646ba8c0L, 0xfd62f97aL, 0x8a65c9ecL, 0x14015c4fL, 0x63066cd9L,
0xfa0f3d63L, 0x8d080df5L, 0x3b6e20c8L, 0x4c69105eL, 0xd56041e4L,
0xa2677172L, 0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL, 0xa50ab56bL,
0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L, 0x32d86ce3L,
0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L, 0x26d930acL, 0x51de003aL,
0xc8d75180L, 0xbfd06116L, 0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L,
0xb8bda50fL, 0x2802b89eL, 0x5f058808L, 0xc60cd9b2L, 0xb10be924L,
0x2f6f7c87L, 0x58684c11L, 0xc1611dabL, 0xb6662d3dL, 0x76dc4190L,
0x01db7106L, 0x98d220bcL, 0xefd5102aL, 0x71b18589L, 0x06b6b51fL,
0x9fbfe4a5L, 0xe8b8d433L, 0x7807c9a2L, 0x0f00f934L, 0x9609a88eL,
0xe10e9818L, 0x7f6a0dbbL, 0x086d3d2dL, 0x91646c97L, 0xe6635c01L,
0x6b6b51f4L, 0x1c6c6162L, 0x856530d8L, 0xf262004eL, 0x6c0695edL,
0x1b01a57bL, 0x8208f4c1L, 0xf50fc457L, 0x65b0d9c6L, 0x12b7e950L,
0x8bbeb8eaL, 0xfcb9887cL, 0x62dd1ddfL, 0x15da2d49L, 0x8cd37cf3L,
0xfbd44c65L, 0x4db26158L, 0x3ab551ceL, 0xa3bc0074L, 0xd4bb30e2L,
0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL, 0x4369e96aL,
0x346ed9fcL, 0xad678846L, 0xda60b8d0L, 0x44042d73L, 0x33031de5L,
0xaa0a4c5fL, 0xdd0d7cc9L, 0x5005713cL, 0x270241aaL, 0xbe0b1010L,
0xc90c2086L, 0x5768b525L, 0x206f85b3L, 0xb966d409L, 0xce61e49fL,
0x5edef90eL, 0x29d9c998L, 0xb0d09822L, 0xc7d7a8b4L, 0x59b33d17L,
0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL, 0xedb88320L, 0x9abfb3b6L,
0x03b6e20cL, 0x74b1d29aL, 0xead54739L, 0x9dd277afL, 0x04db2615L,
0x73dc1683L, 0xe3630b12L, 0x94643b84L, 0x0d6d6a3eL, 0x7a6a5aa8L,
0xe40ecf0bL, 0x9309ff9dL, 0x0a00ae27L, 0x7d079eb1L, 0xf00f9344L,
0x8708a3d2L, 0x1e01f268L, 0x6906c2feL, 0xf762575dL, 0x806567cbL,
0x196c3671L, 0x6e6b06e7L, 0xfed41b76L, 0x89d32be0L, 0x10da7a5aL,
0x67dd4accL, 0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L, 0x60b08ed5L,
0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L, 0xd1bb67f1L,
0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL, 0xd80d2bdaL, 0xaf0a1b4cL,
0x36034af6L, 0x41047a60L, 0xdf60efc3L, 0xa867df55L, 0x316e8eefL,
0x4669be79L, 0xcb61b38cL, 0xbc66831aL, 0x256fd2a0L, 0x5268e236L,
0xcc0c7795L, 0xbb0b4703L, 0x220216b9L, 0x5505262fL, 0xc5ba3bbeL,
0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L, 0xc2d7ffa7L, 0xb5d0cf31L,
0x2cd99e8bL, 0x5bdeae1dL, 0x9b64c2b0L, 0xec63f226L, 0x756aa39cL,
0x026d930aL, 0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x05005713L,
0x95bf4a82L, 0xe2b87a14L, 0x7bb12baeL, 0x0cb61b38L, 0x92d28e9bL,
0xe5d5be0dL, 0x7cdcefb7L, 0x0bdbdf21L, 0x86d3d2d4L, 0xf1d4e242L,
0x68ddb3f8L, 0x1fda836eL, 0x81be16cdL, 0xf6b9265bL, 0x6fb077e1L,
0x18b74777L, 0x88085ae6L, 0xff0f6a70L, 0x66063bcaL, 0x11010b5cL,
0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L, 0xa00ae278L,
0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L, 0xa7672661L, 0xd06016f7L,
0x4969474dL, 0x3e6e77dbL, 0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L,
0x37d83bf0L, 0xa9bcae53L, 0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L,
0xbdbdf21cL, 0xcabac28aL, 0x53b39330L, 0x24b4a3a6L, 0xbad03605L,
0xcdd70693L, 0x54de5729L, 0x23d967bfL, 0xb3667a2eL, 0xc4614ab8L,
0x5d681b02L, 0x2a6f2b94L, 0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL,
0x2d02ef8dL
};
#endif /*DYNAMIC_CRC_TABLE */
#ifndef OF /* function prototypes */
# ifdef STDC
# define OF(args) args
# else
# define OF(args) ()
# endif
#endif
#define DO1(buf) crc = crc_table[((int)crc ^ (*buf++)) & 0xff] ^ (crc >> 8);
#define DO2(buf) DO1(buf); DO1(buf);
#define DO4(buf) DO2(buf); DO2(buf);
#define DO8(buf) DO4(buf); DO4(buf);
//以下這個routine在lib_generic/crc32.c
uint32_t ZEXPORT crc32 (uint32_t crc, const Bytef *buf, uInt len)
{
#ifdef DYNAMIC_CRC_TABLE
if (crc_table_empty) make_crc_table(); //這行不會被執行
#endif
crc = crc ^ 0xffffffffL;
while (len >= 8)
{
DO8(buf);
len -= 8;
}
if (len) do {
DO1(buf);
} while (--len);
return crc ^ 0xffffffffL;
}
先說明關於CRC，以CRC-16為例來說明其生成過程：
CRC-16碼由兩個位元組構成，在開始時CRC寄存器的每一位都預置為1，然後把CRC寄存器與8-bit的資料進行異或(異或：二進位運算相同為0，不同為1；0^0=0;0^1=1;1^0=1;1^1=0)，之後對CRC寄存器從高到低進行移位，在最高位（MSB）的位置補零，而最低位（LSB，移位後已經被移出CRC寄存器）如果為1，則把寄存器與預定義的多項式碼進行異或，否則如果LSB為零，則無需進行異或。重複上述的由高至低的移位8次，第一個8-bit資料處理完畢，用此時CRC寄存器的值與下一個8-bit資料異或並進行如前一個資料似的8次移位元。所有的字元處理完成後CRC寄存器內的值即為最終的CRC值。
下面為CRC的計算過程：
来源：(http://blog.sina.com.cn/s/blog_639260ff0100g1ms.html) - 循环冗余校验CRC_Striker_新 1．設置CRC寄存器，並給其賦值FFFF(hex)。
2．將資料的第一個8-bit字元與16位元CRC寄存器的低8位進行異或，並把結果存入CRC寄存器。
3．CRC寄存器向右移一位，MSB補零，移出並檢查LSB。
4．如果LSB為0，重複第三步；若LSB為1，CRC寄存器與多項式碼相異或。
5．重複第3與第4步直到8次移位元全部完成。此時一個8-bit資料處理完畢。
6．重複第2至第5步直到所有資料全部處理完成。
7．最終CRC寄存器的內容即為CRC值。
常用的CRC迴圈冗餘校驗標準多項式如下：
CRC(16位) = X16+X15+X2+1
CRC(CCITT) = X16+X12 +X5+1
CRC(32位) = X32+X26+X23+X16+X12+X11+X10+ X8+X7+X5+X4+X2+X+1
