Nand Flash设计中针对坏块处理的理解

发布时间：2020-12-15 18:44:13 所属栏目：百科来源：网络整理

导读：引自：http://www.cnmaizi.com/tech/driver/nand-flash-bad-block-handling-understanding/#comment-342 研究了几天的nand datasheet后，发现要真正做好Nand Flash的控制还是一件比较麻烦的事情。首先就是坏块（Invalid Block）的处理，初始情况下（出厂时）

引自：http://www.cnmaizi.com/tech/driver/nand-flash-bad-block-handling-understanding/#comment-342

研究了几天的nand datasheet后，发现要真正做好Nand Flash的控制还是一件比较麻烦的事情。首先就是坏块（Invalid Block）的处理，初始情况下（出厂时），坏块被标记在每一个Block的Page0或1的2048列上，如果该地址的数据是0xff，那么可以证明该Block在出厂时（注意仅仅是出厂时，Flash的坏块会随着使用而慢慢增多）不是坏块，反之则是一个坏块，而且坏块的判断不能在芯片擦除后进行，因为坏块标记可能会被擦除（Datasheet上说的非常明确），在记录下来整个Flash的坏块信息后，就需要建立坏块映射表。

收集到的处理办法如下：将Flash划分为三个区，第一个区使用信息（包括坏块表，使用Block数目等，预留10个Block），第二个区为数据记录区，第三个区为坏块映射区（专用于替换坏块，为保留区，不能直接往里面写数据，为128个Block）。第一次使用时，读取Block0中Page0的资料（一次全部读出），如果Page0的0、1Byte不是’O'、’K'（自己建立的”使用头”标志）的话，那么就运行初始化坏块扫描程序，记录坏块的数目和坏块地址，并且为他们分配坏块映射区的地址（依次分配），并且将坏块信息以及被替换块信息分别保存到两个数组中，检查完后，得到了所有坏块的数目和坏块的映射信息以及最后一个被替换块（用以决定下次替换的地址），将他们格式化后，分别写入到使用信息记录区（Block0-9）中，如果发现Block0编程失败，那么擦除该块直接跳至下一个信息记录Block，直到成功写进去，或者Block0-9全部失败（那么标记整个Flash失效，因为连续10个Block在生命周期内全部失效的几率非常小，所以基本可以忽略~）至此，已经记录下了所有坏块信息、替换信息以及使用状况。初始信息写进去后，下次重新启动后，系统从Page0至Page9检查是否有头标志，检测到头标志后，继而将所有坏块信息以及实使用信息分别读取到Ram中去，在正常使用中，将要写的Block号在坏块信息数组中查询，如果有，则直接写入被替换的块中，如果发现此时被替换块也是坏块，那么直接读出最后被替换块的值，将此值写到替换表中，并将最后被替换块+1，直到找到合适的替换块，此时将之前坏块的有用信息全部写入到被替换块中，然后继续写入新的数据，最后更新坏块表及坏块信息，并写至使用信息Flash块中，至此，Flash坏块替换完毕。

其次就是位交换问题了，Flash靠电荷储存来存储信息，当由于工艺问题造成某个位写入失败时，位交换发生，需要采用专用算法去校正。比较典型的算法就是ECC，专门用来校正Flash的单个位交换。校验方式涉及到众多的移位以及异或运算。

以下是三星Nand Flash芯片的参考资料:
http://www.samsung.com/global/business/semiconductor/products/flash/downloads/applicationnote/eccalgo_040624.pdf?

http://www.samsung.com/global/business/semiconductor/products/flash/downloads/applicationnote/samsung_new_ecc_for_512byte_256word.txt

ECC算法的代码如下：

C语言: 来源于Samsung代码

 
 /*****************************************************************************/ 
 
/*?????????????????????????????????????????????????????????????????????????? */ 
 
/* PROJECT : SAMSUNG ECC???????????????????????????????????????????????????? */ 
 
/* FILE??? : SAMSUNG_ECC.c?????????????????????????????????????????????????? */ 
 
/* PURPOSE : This file implements core ECC algorithms adopted???????????????? */ 
 
/*?????????? Hamming Error Correction and Detection Algorithm??????????????? */ 
 
/*?????????????????????????????????????????????????????????????????????????? */ 
 
/*—————————————————————————*/ 
 
/*?????????????????????????????????????????????????????????????????????????? */ 
 
/*??????? COPYRIGHT 2000-2004,SAMSUNG ELECTRONICS CO.,LTD.???????????????? */ 
 
/*????????????????????? ALL RIGHTS RESERVED????????????????????????????????? */ 
 
/*?????????????????????????????????????????????????????????????????????????? */ 
 
/*?? Permission is hereby granted to licensees of Samsung Electronics??????? */ 
 
/*?? Co.,Ltd. products to use or abstract this computer program for the???? */ 
 
/*?? sole purpose of implementing a product based on Samsung???????????????? */ 
 
/*?? Electronics Co.,Ltd. products. No other rights to reproduce,use,????? */ 
 
/*?? or disseminate this computer program,whether in part or in whole,????? */ 
 
/*?? are granted.??????????????????????????????????????????????????????????? */ 
 
/*?????????????????????????????????????????????????????????????????????????? */ 
 
/*?? Samsung Electronics Co.,Ltd. makes no representation or warranties???? */ 
 
/*?? with respect to the performance of this computer program,and?????????? */ 
 
/*?? specifically disclaims any responsibility for any damages,????????????? */ 
 
/*?? special or consequential,connected with the use of this program.?????? */ 
 
/*?????????????????????????????????????????????????????????????????????????? */ 
 
/*—————————————————————————*/ 
 
/*?????????????????????????????????????????????????????????????????????????? */ 
 
/* REVISION HISTORY????????????????????????????????????????????????????????? */ 
 
/*?????????????????????????????????????????????????????????????????????????? */ 
 
/*? 13-NOV-2003 [Chang JongBaek] :???? first writing??????????????????????????? */ 
 
/*? 03-MAR-2004 [ Kim YoungGon ] :? Second writing?????????????????????????? */ 
 
/*? 03-MAR-2004 [? Lee JaeBum? ] :? Third writing??????????????????????????? */ 
 
/*—————————————————————————*/ 
 
/*?????????????????????????????????????????????????????????????????????????? */ 
 
/* NOTES???????????????????????????????????????????????????????????????????? */ 
 
/*?????????????????????????????????????????????????????????????????????????? */ 
 
/* – Make ECC parity code of 512bytes(256words) and 3 bytes are represented? */ 
 
/*?? And ECC compare & Correction code is also represented?????????????????? */ 
 
/*?????????????????????????????????????????????????????????????????????????? */ 
 
/*****************************************************************************/ 
 
#include <stdio.h> 
 
#include <conio.h> 
 
#include <stdlib.h> 
 
#include <string.h> 
 
#include “ecc.h”

#define??? XMODE??? 8

/*****************************************************************************/
/* Address Types???????????????????????????????????????????????????????????? */
/*****************************************************************************/

typedef unsigned char *??????? address_t;??????????? /* address (pointer) */
typedef unsigned long??????? address_value_t;??? /* address (for calculation) */

/*****************************************************************************/
/* Integer Types???????????????????????????????????????????????????????????? */
/*****************************************************************************/

typedef unsigned long??????? uint32_t;??????????? /* unsigned 4 byte integer */
typedef signed long??????????? int32_t;??????????? /* signed 4 byte integer */
typedef unsigned short??????? uint16_t;??????????? /* unsigned 2 byte integer */
typedef signed short??????? int16_t;??????????? /* signed 2 byte integer */
typedef unsigned char??????? uint8_t;??????????? /* unsigned 1 byte integer */
typedef signed char??????????? int8_t;??????????????? /* signed 1 byte integer */

typedef enum {
??? ECC_NO_ERROR??????????? = 0,??????? /* no error */
??? ECC_CORRECTABLE_ERROR??? = 1,??????? /* one bit data error */
??? ECC_ECC_ERROR??????????? = 2,??????? /* one bit ECC error */
??? ECC_UNCORRECTABLE_ERROR??? = 3??????????? /* uncorrectable error */
} eccdiff_t;

/*****************************************************************************/
/*?????????????????????????????????????????????????????????????????????????? */
/* NAME????????????????????????????????????????????????????????????????????? */
/*??????? make_ecc_512?????????????????????????????????????????????????????? */
/* DESCRIPTION?????????????????????????????????????????????????????????????? */
/*??????? This function generates 3 byte ECC for 512 byte data.????????????? */
/*????? (Software ECC)?????????????????????????????????????????????????????? */
/* PARAMETERS??????????????????????????????????????????????????????????????? */
/*??????? ecc_buf?????? the location where ECC should be stored????????????? */
/*??????? data_buf????? given data?????????????????????????????????????????? */
/* RETURN VALUES???????????????????????????????????????????????????????????? */
/*??????? none?????????????????????????????????????????????????????????????? */
/*?????????????????????????????????????????????????????????????????????????? */
/*****************************************************************************/
#if (XMODE == 8)
void make_ecc_512(uint8_t * ecc_buf, uint8_t * data_buf)
#else
void make_ecc_512(uint16_t * ecc_buf, uint16_t * data_buf)
#endif
{
???
??? uint32_t??? i, ALIGN_FACTOR;
??? uint32_t??? tmp;
??? uint32_t??? uiparity = 0;
??? uint32_t??? parityCol, ecc = 0;
??? uint32_t??? parityCol4321 = 0, parityCol4343 = 0, parityCol4242 = 0, parityColTot = 0;
??? uint32_t??? *Data;
??? uint32_t??? Xorbit=0;

??? ALIGN_FACTOR = (uint32_t)data_buf % 4 ;
??? Data = (uint32_t *)(data_buf + ALIGN_FACTOR);

??? for( i = 0; i < 16; i++)
??? {
??????? parityCol = *Data++;
??????? tmp = *Data++; parityCol ^= tmp; parityCol4242 ^= tmp;
??????? tmp = *Data++; parityCol ^= tmp; parityCol4343 ^= tmp;

??????? tmp = * Data ++; parityCol ^= tmp;

??????? parityCol4343 ^= tmp; parityCol4242 ^= tmp;
???????

??????? tmp = * Data ++; parityCol ^= tmp; parityCol4321 ^= tmp;
??????? tmp = * Data ++; parityCol ^= tmp; parityCol4242 ^= tmp;

??????? parityCol4321 ^= tmp;
???????

??????? parityCol4343 ^= tmp; parityCol4321 ^= tmp;
??????? tmp = * Data ++; parityCol ^= tmp; parityCol4242 ^= tmp;

??????? parityCol4343 ^= tmp; parityCol4321 ^= tmp;

??????? parityColTot ^= parityCol;

??????? tmp = (parityCol >> 16) ^ parityCol;
??????? tmp = (tmp >> 8) ^ tmp;
??????? tmp = (tmp >> 4) ^ tmp;
??????? tmp = ((tmp >> 2) ^ tmp) & 0×03;
??????? if ((tmp == 0×01) || (tmp == 0×02))
??????? {
??????????? uiparity ^= i;
??????????? Xorbit ^= 0×01;
??????? }
??? }

#if (XMODE == 8)
??? tmp = (parityCol4321 >> 16) ^ parityCol4321;
??? tmp = (tmp << 8) ^ tmp;
??? tmp = (tmp >> 4) ^ tmp;
??? tmp = (tmp >> 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×200;??? // p128
#else
??? tmp = (parityCol4321 >> 16) ^ parityCol4321;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp << 4) ^ tmp;
??? tmp = (tmp << 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×80;??? // p128
#endif
#if (XMODE == 8)
??? tmp = (parityCol4343 >> 16) ^ parityCol4343;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp << 4) ^ tmp;
??? tmp = (tmp << 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×80;??? // p64
#else
??? tmp = (parityCol4343 >> 16) ^ parityCol4343;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp << 4) ^ tmp;
??? tmp = (tmp >> 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×20;??? // p64
#endif
#if (XMODE == 8)
??? tmp = (parityCol4242 >> 16) ^ parityCol4242;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp << 4) ^ tmp;
??? tmp = (tmp >> 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×20;??? // p32
#else
??? tmp = (parityCol4242 >> 16) ^ parityCol4242;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp >> 4) ^ tmp;
??? tmp = (tmp << 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×08;??? // p32
#endif
#if (XMODE == 8)
??? tmp = parityColTot & 0xFFFF0000;
??? tmp = tmp >> 16;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp >> 4) ^ tmp;
??? tmp = (tmp << 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×08;??? // p16
#else
??? tmp = parityColTot & 0xFFFF0000;
??? tmp = tmp >> 16;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp >> 4) ^ tmp;
??? tmp = (tmp >> 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×02;??? // p16
#endif
#if (XMODE == 8)
??? tmp = parityColTot & 0xFF00FF00;
??? tmp = (tmp >> 16) ^ tmp;
??? tmp = (tmp >> 8);
??? tmp = (tmp >> 4) ^ tmp;
??? tmp = (tmp >> 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×02;??? // p8
#else
??? tmp = parityColTot & 0xFF00FF00;
??? tmp = (tmp << 16) ^ tmp;
??? tmp = (tmp >> 8);
??? tmp = (tmp << 4) ^ tmp;
??? tmp = (tmp << 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×800000;??? // p8
#endif
#if (XMODE == 8)
??? tmp = parityColTot & 0xF0F0F0F0 ;
??? tmp = (tmp << 16) ^ tmp;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp << 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×800000;??? // p4
#else
??? tmp = parityColTot & 0xF0F0F0F0 ;
??? tmp = (tmp << 16) ^ tmp;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp >> 2) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×200000;??? // p4
#endif
#if (XMODE == 8)
??? tmp = parityColTot & 0xCCCCCCCC ;
??? tmp = (tmp << 16) ^ tmp;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp << 4) ^ tmp;
??? tmp = (tmp >> 2);
??? ecc |= ((tmp << 1) ^ tmp) & 0×200000;??? // p2
#else
??? tmp = parityColTot & 0xCCCCCCCC ;
??? tmp = (tmp << 16) ^ tmp;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp >> 4) ^ tmp;
??? ecc |= ((tmp << 1) ^ tmp) & 0×80000;??? // p2
#endif
#if (XMODE == 8)
??? tmp = parityColTot & 0xAAAAAAAA ;
??? tmp = (tmp << 16) ^ tmp;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp >> 4) ^ tmp;
??? tmp = (tmp << 2) ^ tmp;
??? ecc |= (tmp & 0×80000);??? // p1
#else
??? tmp = parityColTot & 0xAAAAAAAA ;
??? tmp = (tmp << 16) ^ tmp;
??? tmp = (tmp >> 8) ^ tmp;
??? tmp = (tmp >> 4) ^ tmp;
??? tmp = (tmp >> 2) ^ tmp;
??? ecc |= (tmp & 0×20000);??? // p1
#endif
#if (XMODE == 8)
??? ecc |= (uiparity & 0×01) <<11;???
??? ecc |= (uiparity & 0×02) <<12;???
??? ecc |= (uiparity & 0×04) <<13;
??? ecc |= (uiparity & 0×08) <<14;
#else
??? ecc |= (uiparity & 0×01) <<9;
??? ecc |= (uiparity & 0×02) <<10;
??? ecc |= (uiparity & 0×04) <<11;
??? ecc |= (uiparity & 0×08) <<12;
#endif

??? if (Xorbit)
??? {
??????? ecc |= (ecc ^ 0x00AAAAAA)>>1;
??? }
??? else
??? {
??????? ecc |= (ecc >> 1);
??? }
#if (XMODE == 8)
??? ecc = ~ecc;
??? *(ecc_buf + 2) = (uint8_t) (ecc >> 16);
??? *(ecc_buf + 1) = (uint8_t) (ecc >> 8);
??? *(ecc_buf + 0) = (uint8_t) (ecc);
#else??? // X16
??? ecc = ( ~ecc ) | 0xFF000000;
??? *(ecc_buf + 1) = (uint16_t) (ecc >> 16);
??? *(ecc_buf + 0) = (uint16_t) (ecc);
#endif
}

/*??????? offset??????????? where the error ccurred????????????????????????? */
/*??????? corrected??????? correct data????????????????????????????????????? */
/* RETURN VALUES???????????????????????????????????????????????????????????? */
/*??????? Upon successful completion,compare_ecc returns SSR_SUCCESS.?????? */
/*????? Otherwise,corresponding error code is returned.???????????????????? */
/*?????????????????????????????????????????????????????????????????????????? */
/*****************************************************************************/
#if (XMODE == 8)
eccdiff_t compare_ecc_512( uint8_t * iEccdata1 , uint8_t * iEccdata2 ,
????????? uint8_t * pPagedata , int32_t pOffset , uint8_t pCorrected)
#else??? // X16
eccdiff_t compare_ecc_512( uint16_t * iEccdata1 , uint16_t * iEccdata2 ,
????????? uint16_t * pPagedata , uint16_t pCorrected)
#endif
{

??? uint32_t? iCompecc = 0, iEccsum = 0;
??? uint32_t? iFindbyte?? = 0;
??? uint32_t? iIndex;
??? uint32_t? nT1 = 0, nT2 =0;

#if (XMODE == 8)
??? uint8_t?? iNewvalue;
??? uint8_t?? iFindbit??? = 0;

??? uint8_t?? *pEcc1 = (uint8_t *)iEccdata1;
??? uint8_t?? *pEcc2 = (uint8_t *)iEccdata2;

??? for ( iIndex = 0; iIndex <2; iIndex++)
??? {
??????? nT1 ^= (((*pEcc1) >> iIndex) & 0×01);
??????? nT2 ^= (((*pEcc2) >> iIndex) & 0×01);
??? }

??? for (iIndex = 0; iIndex < 3; iIndex++)
??????? iCompecc |= ((~(*pEcc1++) ^ ~(*pEcc2++)) << iIndex * 8);
???
??? for(iIndex = 0; iIndex < 24; iIndex++) {
??????? iEccsum += ((iCompecc >> iIndex) & 0×01);
??? }

#else??? // X16
??? uint16_t?? iNewvalue;
??? uint16_t?? iFindbit??? = 0;
???
??? uint16_t?? *pEcc1 = (uint16_t *)iEccdata1;
??? uint16_t?? *pEcc2 = (uint16_t *)iEccdata2;

??? for ( iIndex = 0; iIndex <2; iIndex++)
??? {
??????? nT1 ^= (((*pEcc1) >> iIndex) & 0×01);
??????? nT2 ^= (((*pEcc2) >> iIndex) & 0×01);
??? }

??? for (iIndex = 0; iIndex < 2; iIndex++)??????? // 2 word of ECC data
??????? iCompecc |= (((~*pEcc1++) ^ (~*pEcc2++)) << iIndex * 16);
???
??? for(iIndex = 0; iIndex < 24; iIndex++) {
??????? iEccsum += ((iCompecc >> iIndex) & 0×01);
??? }
#endif
???
??? switch (iEccsum) {
??? case 0 :
??????? printf(“RESULT : no errorn“);
??????? return ECC_NO_ERROR;

??? case 1 :
??????? printf(“RESULT : ECC code 1 bit failn“);
??????? return ECC_ECC_ERROR;

??? case 12 :
??????? if (nT1 != nT2)
??????? {

#if (XMODE == 8)
??????????? iFindbyte = (( iCompecc >> 17 & 1) << 8) + (( iCompecc >> 15 & 1) << 7) + (( iCompecc >> 13 & 1) << 6)

????????????????????? + (( iCompecc >> 11 & 1) << 5) + (( iCompecc >> 9 & 1) << 4) + (( iCompecc >> 7 & 1) << 3)
????????????????????? + (( iCompecc >> 5 & 1) << 2) + (( iCompecc >> 3 & 1) << 1) + ( iCompecc >> 1 & 1);

??????????? iFindbit =? ( uint8_t)((( iCompecc >> 23 & 1) << 2) + (( iCompecc >> 21 & 1) << 1) + ( iCompecc >> 19 & 1));

??????????? iNewvalue = ( uint8_t)( pPagedata [ iFindbyte ] ^ ( 1 << iFindbit));

#else // CASE_X16

??????????? iFindbyte = (( iCompecc >> 15 & 1) << 7) + (( iCompecc >> 13 & 1) << 6) + (( iCompecc >> 11 & 1) << 5) + (( iCompecc >> 9 & 1) << 4) + (( iCompecc >> 7 & 1) << 3) + (( iCompecc >> 5 & 1) << 2) + (( iCompecc >> 3 & 1) << 1) + ( iCompecc >> 1 & 1) ;

??????????? iFindbit =? ( uint16_t)((( iCompecc >> 23 & 1) << 3) + (( iCompecc >> 21 & 1) << 2) + (( iCompecc >> 19 & 1) << 1) + ( iCompecc >> 17 & 1) );
??????????? iNewvalue = ( uint16_t)( pPagedata [ iFindbyte ] ^ ( 1 << iFindbit));
#endif

??????????? printf( “iCompecc = %d n “ , iCompecc);
??????????? printf( “RESULT : one bit error rn “);
??????????? printf( “byte = %d,bit = %d rn “ , iFindbyte , iFindbit);
??????????? printf( “corrupted = %x,corrected = %x rn “ ,

??????????????? pPagedata [ iFindbyte ], iNewvalue);

??????????? if (pOffset != NULL) {
??????????????? pOffset = iFindbyte;
??????????? }
??????????? if (pCorrected != NULL) {
??????????????? pCorrected = iNewvalue;
??????????? }
???????????
??????????? return ECC_CORRECTABLE_ERROR;
??????? }
??????? else
??????????? return ECC_UNCORRECTABLE_ERROR;

??? default :
??????? printf(“RESULT : unrecoverable errorn“);
??????? return ECC_UNCORRECTABLE_ERROR;
??? }??
}

one? Comment

　　对于nand最大的问题就是会有bad block，由于bad block的不确定性，所以进一步加大了对nand编程访问的难度。所以只有解决了bad block的问题才可能使用nand，将bad block处理的好才会最大的提升nand的access效率。

　　什么是bad block呢？就是在这一个block里有1个或多个bit的状态不能稳定的编程，所以就没法使用它，但是如果一个block(128KByte)有一个Bit是坏的，那么整个block放弃使用。听起来有点浪费，可能是根据物理原理使整个block的稳定性不能保证吧，或者是其他考虑。不过既然三星要求我们这么做，那么为了系统的稳定，也不要计较那几百KB的容量了。

　　bad block有2种，一种是initial bad block，另一种是runtime bad block。所谓initial bad block就是在三星出厂时就是坏块的。为什么出厂会有坏块？这个很正常，因为nand就是会有坏块，比LCD有坏点的几率大的多。但是可以放心的是三星在出厂前都对nand进行了测试，erase了所有的block，所以内容会都是0xFF，同时标记了bad block，这些即是initial bad block。每一种容量的nand允许的最多坏块数都是有规定的，低于某个值是不会出厂的，同时低于98%的可用块，基本就会认为这块nand很不稳定了。

　　initial bad block的标记是保证坏块的前2个page里的spare area里的第一个字节的内容不会是0xFF。即如果前2个page其中一个的2048地址（从0开始计数，以后都是这种计数方式）上的数据（就是spare area的第一个字节）是0xFF，那么这个块不是initial bad block。值得注意的一点是bad block的标记会被erase掉成0xFF，所以有可能会误被认为是good block。所以在对每一个block earse之前一定要判断是不是bad block，否则将bad block的标志erase了，以后使用的时候会误被当作good block，从而有可能带来数据损失。

　　对于runtime的bad block，就是使用中出现的bad block，可以从erase或program后对成功与否的判断来决定这个block是否变成bad block，如果是的话则标记。一般标记的法则和initial bad block一样，在前两个page的spare area的第一个byte上写非0xFF值，一般写0。

　　对于bad block的处理办法就是驱动层的问题了，做的好的话会很复杂，这个以后再介绍诸如三星的pocketstoreII这样的专门的nand driver来提供高可靠性和高perfermance。最简单的就是遇到坏块向后跳，但也是效率最差的，因为访问第n块时，你要知道0–(n-1)块一共有多少个bad block才能决定操作的偏移量，而扫描n-1块对于n比较大的情况无疑会很慢。

cnmaizi said on: 2011 年 05 月 24 日 11:09

（编辑：李大同）

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