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原文出自:http://www.linuxforum.net
作者:opera
=============================
概述
----
1) Linux的初始内核映象以gzip压缩文件的格式存放在zImage或bzImage之中, 内核的自举
代码将它解压到1M内存开始处. 在内核初始化时, 如果加载了压缩的initrd映象, 内核会将
它解压到内存盘中, 这两处解压过程都使用了lib/inflate.c文件.
2) inflate.c是从gzip源程序中分离出来的, 包含了一些对全局数据的直接引用, 在使用时
需要直接嵌入到代码中. gzip压缩文件时总是在前32K字节的范围内寻找重复的字符串进行
编码, 在解压时需要一个至少为32K字节的解压缓冲区, 它定义为window[WSIZE].
inflate.c使用get_byte()读取输入文件, 它被定义成宏来提高效率. 输入缓冲区指针必须
定义为inptr, inflate.c中对之有减量操作. inflate.c调用flush_window()来输出window
缓冲区中的解压出的字节串, 每次输出长度用outcnt变量表示. 在flush_window()中, 还必
须对输出字节串计算CRC并且刷新crc变量. 在调用gunzip()开始解压之前, 调用makecrc()
初始化CRC计算表. 最后gunzip()返回0表示解压成功.
3) zImage或bzImage由16位引导代码和32位内核自解压映象两个部分组成. 对于zImage, 内
核自解压映象被加载到物理地址0x1000, 内核被解压到1M的部位. 对于bzImage, 内核自解
压映象被加载到1M开始的地方, 内核被解压为两个片段, 一个起始于物理地址0x2000-0x90000,
另一个起始于高端解压映象之后, 离1M开始处不小于低端片段最大长度的区域. 解压完成后,
这两个片段被合并到1M的起始位置.
解压根内存盘映象文件的代码
--------------------------
- ; drivers/block/rd.c
- #ifdef BUILD_CRAMDISK
- /*
- * gzip declarations
- */
- #define OF(args) args ; 用于函数原型声明的宏
- #ifndef memzero
- #define memzero(s, n) memset ((s), 0, (n))
- #endif
- typedef unsigned char uch; 定义inflate.c所使用的3种数据类型
- typedef unsigned short ush;
- typedef unsigned long ulg;
- #define INBUFSIZ 4096 用户输入缓冲区尺寸
- #define WSIZE 0x8000 /* window size--must be a power of two, and */
- /* at least 32K for zip's deflate method */
- static uch *inbuf; 用户输入缓冲区,与inflate.c无关
- static uch *window; 解压窗口
- static unsigned insize; /* valid bytes in inbuf */
- static unsigned inptr; /* index of next byte to be processed in inbuf */
- static unsigned outcnt; /* bytes in output buffer */
- static int exit_code;
- static long bytes_out; 总解压输出长度,与inflate.c无关
- static struct file *crd_infp, *crd_outfp;
- #define get_byte() (inptr < insize ? inbuf[inptr++] : fill_inbuf()) 读取输入缓冲区中一个字节
-
- /* Diagnostic functions (stubbed out) */ 一些调试宏
- #define Assert(cond,msg)
- #define Trace(x)
- #define Tracev(x)
- #define Tracevv(x)
- #define Tracec(c,x)
- #define Tracecv(c,x)
- #define STATIC static
- static int fill_inbuf(void);
- static void flush_window(void);
- static void *malloc(int size);
- static void free(void *where);
- static void error(char *m);
- static void gzip_mark(void **);
- static void gzip_release(void **);
- #include "../../lib/inflate.c"
- static void __init *malloc(int size)
- {
- return kmalloc(size, GFP_KERNEL);
- }
- static void __init free(void *where)
- {
- kfree(where);
- }
- static void __init gzip_mark(void **ptr)
- {
- ; 读取用户一个标记
- }
- static void __init gzip_release(void **ptr)
- {
- ; 归还用户标记
- }
- /* ===========================================================================
- * Fill the input buffer. This is called only when the buffer is empty
- * and at least one byte is really needed.
- */
- static int __init fill_inbuf(void) 填充输入缓冲区
- {
- if (exit_code) return -1;
-
- insize = crd_infp->;f_op->;read(crd_infp, inbuf, INBUFSIZ,
- &crd_infp->;f_pos);
- if (insize == 0) return -1;
- inptr = 1;
- return inbuf[0];
- }
- /* ===========================================================================
- * Write the output window window[0..outcnt-1] and update crc and bytes_out.
- * (Used for the decompressed data only.)
- */
- static void __init flush_window(void) 输出window缓冲区中outcnt个字节串
- {
- ulg c = crc; /* temporary variable */
- unsigned n;
- uch *in, ch;
-
- crd_outfp->;f_op->;write(crd_outfp, window, outcnt, &crd_outfp->;f_pos);
- in = window;
- for (n = 0; n < outcnt; n++) {
- ch = *in++;
- c = crc_32_tab[((int)c ^ ch) & 0xff] ^ (c >;>; 8); 计算输出串的CRC
- }
- crc = c;
- bytes_out += (ulg)outcnt; 刷新总字节数
- outcnt = 0;
- }
- static void __init error(char *x) 解压出错调用的函数
- {
- printk(KERN_ERR "%s", x);
- exit_code = 1;
- }
- static int __init
- crd_load(struct file * fp, struct file *outfp)
- {
- int result;
- insize = 0; /* valid bytes in inbuf */
- inptr = 0; /* index of next byte to be processed in inbuf */
- outcnt = 0; /* bytes in output buffer */
- exit_code = 0;
- bytes_out = 0;
- crc = (ulg)0xffffffffL; /* shift register contents */
- crd_infp = fp;
- crd_outfp = outfp;
- inbuf = kmalloc(INBUFSIZ, GFP_KERNEL);
- if (inbuf == 0) {
- printk(KERN_ERR "RAMDISK: Couldn't allocate gzip buffer\n");
- return -1;
- }
- window = kmalloc(WSIZE, GFP_KERNEL);
- if (window == 0) {
- printk(KERN_ERR "RAMDISK: Couldn't allocate gzip window\n");
- kfree(inbuf);
- return -1;
- }
- makecrc();
- result = gunzip();
- kfree(inbuf);
- kfree(window);
- return result;
- }
- #endif /* BUILD_CRAMDISK */
- 32位内核自解压代码
- ------------------
- ; arch/i386/boot/compressed/head.S
- .text
- #include
- #include
- .globl startup_32 对于zImage该入口地址为0x1000; 对于bzImage为0x101000
-
- startup_32:
- cld
- cli
- movl $(__KERNEL_DS),%eax
- movl %eax,%ds
- movl %eax,%es
- movl %eax,%fs
- movl %eax,%gs
- lss SYMBOL_NAME(stack_start),%esp # 自解压代码的堆栈为misc.c中定义的16K字节的数组
- xorl %eax,%eax
- 1: incl %eax # check that A20 really IS enabled
- movl %eax,0x000000 # loop forever if it isn't
- cmpl %eax,0x100000
- je 1b
- /*
- * Initialize eflags. Some BIOS's leave bits like NT set. This would
- * confuse the debugger if this code is traced.
- * XXX - best to initialize before switching to protected mode.
- */
- pushl $0
- popfl
- /*
- * Clear BSS 清除解压程序的BSS段
- */
- xorl %eax,%eax
- movl $ SYMBOL_NAME(_edata),%edi
- movl $ SYMBOL_NAME(_end),%ecx
- subl %edi,%ecx
- cld
- rep
- stosb
- /*
- * Do the decompression, and jump to the new kernel..
- */
- subl $16,%esp # place for structure on the stack
- movl %esp,%eax
- pushl %esi # real mode pointer as second arg
- pushl %eax # address of structure as first arg
- call SYMBOL_NAME(decompress_kernel)
- orl %eax,%eax # 如果返回非零,则表示为内核解压为低端和高端的两个片断
- jnz 3f
- popl %esi # discard address
- popl %esi # real mode pointer
- xorl %ebx,%ebx
- ljmp $(__KERNEL_CS), $0x100000 # 运行start_kernel
- /*
- * We come here, if we were loaded high.
- * We need to move the move-in-place routine down to 0x1000
- * and then start it with the buffer addresses in registers,
- * which we got from the stack.
- */
- 3:
- movl $move_routine_start,%esi
- movl $0x1000,%edi
- movl $move_routine_end,%ecx
- subl %esi,%ecx
- addl $3,%ecx
- shrl $2,%ecx # 按字取整
- cld
- rep
- movsl # 将内核片断合并代码复制到0x1000区域, 内核的片段起始为0x2000
- popl %esi # discard the address
- popl %ebx # real mode pointer
- popl %esi # low_buffer_start 内核低端片段的起始地址
- popl %ecx # lcount 内核低端片段的字节数量
- popl %edx # high_buffer_start 内核高端片段的起始地址
- popl %eax # hcount 内核高端片段的字节数量
- movl $0x100000,%edi 内核合并的起始地址
- cli # make sure we don't get interrupted
- ljmp $(__KERNEL_CS), $0x1000 # and jump to the move routine
- /*
- * Routine (template) for moving the decompressed kernel in place,
- * if we were high loaded. This _must_ PIC-code !
- */
- move_routine_start:
- movl %ecx,%ebp
- shrl $2,%ecx
- rep
- movsl # 按字拷贝第1个片段
- movl %ebp,%ecx
- andl $3,%ecx
- rep
- movsb # 传送不完全字
- movl %edx,%esi
- movl %eax,%ecx # NOTE: rep movsb won't move if %ecx == 0
- addl $3,%ecx
- shrl $2,%ecx # 按字对齐
- rep
- movsl # 按字拷贝第2个片段
- movl %ebx,%esi # Restore setup pointer
- xorl %ebx,%ebx
- ljmp $(__KERNEL_CS), $0x100000 # 运行start_kernel
- move_routine_end:
- ; arch/i386/boot/compressed/misc.c
- /*
- * gzip declarations
- */
- #define OF(args) args
- #define STATIC static
- #undef memset
- #undef memcpy
- #define memzero(s, n) memset ((s), 0, (n))
- typedef unsigned char uch;
- typedef unsigned short ush;
- typedef unsigned long ulg;
- #define WSIZE 0x8000 /* Window size must be at least 32k, */
- /* and a power of two */
- static uch *inbuf; /* input buffer */
- static uch window[WSIZE]; /* Sliding window buffer */
- static unsigned insize = 0; /* valid bytes in inbuf */
- static unsigned inptr = 0; /* index of next byte to be processed in inbuf */
- static unsigned outcnt = 0; /* bytes in output buffer */
- /* gzip flag byte */
- #define ASCII_FLAG 0x01 /* bit 0 set: file probably ASCII text */
- #define CONTINUATION 0x02 /* bit 1 set: continuation of multi-part gzip file */
- #define EXTRA_FIELD 0x04 /* bit 2 set: extra field present */
- #define ORIG_NAME 0x08 /* bit 3 set: original file name present */
- #define COMMENT 0x10 /* bit 4 set: file comment present */
- #define ENCRYPTED 0x20 /* bit 5 set: file is encrypted */
- #define RESERVED 0xC0 /* bit 6,7: reserved */
- #define get_byte() (inptr < insize ? inbuf[inptr++] : fill_inbuf())
-
- /* Diagnostic functions */
- #ifdef DEBUG
- # define Assert(cond,msg) {if(!(cond)) error(msg);}
- # define Trace(x) fprintf x
- # define Tracev(x) {if (verbose) fprintf x ;}
- # define Tracevv(x) {if (verbose>;1) fprintf x ;}
- # define Tracec(c,x) {if (verbose && (c)) fprintf x ;}
- # define Tracecv(c,x) {if (verbose>;1 && (c)) fprintf x ;}
- #else
- # define Assert(cond,msg)
- # define Trace(x)
- # define Tracev(x)
- # define Tracevv(x)
- # define Tracec(c,x)
- # define Tracecv(c,x)
- #endif
- static int fill_inbuf(void);
- static void flush_window(void);
- static void error(char *m);
- static void gzip_mark(void **);
- static void gzip_release(void **);
-
- /*
- * This is set up by the setup-routine at boot-time
- */
- static unsigned char *real_mode; /* Pointer to real-mode data */
- #define EXT_MEM_K (*(unsigned short *)(real_mode + 0x2))
- #ifndef STANDARD_MEMORY_BIOS_CALL
- #define ALT_MEM_K (*(unsigned long *)(real_mode + 0x1e0))
- #endif
- #define SCREEN_INFO (*(struct screen_info *)(real_mode+0))
- extern char input_data[];
- extern int input_len;
- static long bytes_out = 0;
- static uch *output_data;
- static unsigned long output_ptr = 0;
-
- static void *malloc(int size);
- static void free(void *where);
- static void error(char *m);
- static void gzip_mark(void **);
- static void gzip_release(void **);
-
- static void puts(const char *);
-
- extern int end;
- static long free_mem_ptr = (long)&end;
- static long free_mem_end_ptr;
- #define INPLACE_MOVE_ROUTINE 0x1000 内核片段合并代码的运行地址
- #define LOW_BUFFER_START 0x2000 内核低端解压片段的起始地址
- #define LOW_BUFFER_MAX 0x90000 内核低端解压片段的终止地址
- #define HEAP_SIZE 0x3000 为解压低码保留的堆的尺寸,堆起始于BSS的结束
- static unsigned int low_buffer_end, low_buffer_size;
- static int high_loaded =0;
- static uch *high_buffer_start /* = (uch *)(((ulg)&end) + HEAP_SIZE)*/;
- static char *vidmem = (char *)0xb8000;
- static int vidport;
- static int lines, cols;
- #include "../../../../lib/inflate.c"
- static void *malloc(int size)
- {
- void *p;
- if (size <0) error("Malloc error\n");
- if (free_mem_ptr <= 0) error("Memory error\n");
- free_mem_ptr = (free_mem_ptr + 3) & ~3; /* Align */
- p = (void *)free_mem_ptr;
- free_mem_ptr += size;
- if (free_mem_ptr >;= free_mem_end_ptr)
- error("\nOut of memory\n");
- return p;
- }
- static void free(void *where)
- { /* Don't care */
- }
- static void gzip_mark(void **ptr)
- {
- *ptr = (void *) free_mem_ptr;
- }
- static void gzip_release(void **ptr)
- {
- free_mem_ptr = (long) *ptr;
- }
-
- static void scroll(void)
- {
- int i;
- memcpy ( vidmem, vidmem + cols * 2, ( lines - 1 ) * cols * 2 );
- for ( i = ( lines - 1 ) * cols * 2; i < lines * cols * 2; i += 2 )
- vidmem[ i ] = ' ';
- }
- static void puts(const char *s)
- {
- int x,y,pos;
- char c;
- x = SCREEN_INFO.orig_x;
- y = SCREEN_INFO.orig_y;
- while ( ( c = *s++ ) != '\0' ) {
- if ( c == '\n' ) {
- x = 0;
- if ( ++y >;= lines ) {
- scroll();
- y--;
- }
- } else {
- vidmem [ ( x + cols * y ) * 2 ] = c;
- if ( ++x >;= cols ) {
- x = 0;
- if ( ++y >;= lines ) {
- scroll();
- y--;
- }
- }
- }
- }
- SCREEN_INFO.orig_x = x;
- SCREEN_INFO.orig_y = y;
- pos = (x + cols * y) * 2; /* Update cursor position */
- outb_p(14, vidport);
- outb_p(0xff & (pos >;>; 9), vidport+1);
- outb_p(15, vidport);
- outb_p(0xff & (pos >;>; 1), vidport+1);
- }
- void* memset(void* s, int c, size_t n)
- {
- int i;
- char *ss = (char*)s;
- for (i=0;i return s;
- }
- void* memcpy(void* __dest, __const void* __src,
- size_t __n)
- {
- int i;
- char *d = (char *)__dest, *s = (char *)__src;
- for (i=0;i<__n;i++) d[ i ] = s[ i ];
- return __dest;
- }
- /* ===========================================================================
- * Fill the input buffer. This is called only when the buffer is empty
- * and at least one byte is really needed.
- */
- static int fill_inbuf(void)
- {
- if (insize != 0) {
- error("ran out of input data\n");
- }
- inbuf = input_data;
- insize = input_len;
- inptr = 1;
- return inbuf[0];
- }
- /* ===========================================================================
- * Write the output window window[0..outcnt-1] and update crc and bytes_out.
- * (Used for the decompressed data only.)
- */
- static void flush_window_low(void)
- {
- ulg c = crc; /* temporary variable */
- unsigned n;
- uch *in, *out, ch;
-
- in = window;
- out = &output_data[output_ptr];
- for (n = 0; n < outcnt; n++) {
- ch = *out++ = *in++;
- c = crc_32_tab[((int)c ^ ch) & 0xff] ^ (c >;>; 8);
- }
- crc = c;
- bytes_out += (ulg)outcnt;
- output_ptr += (ulg)outcnt;
- outcnt = 0;
- }
- static void flush_window_high(void)
- {
- ulg c = crc; /* temporary variable */
- unsigned n;
- uch *in, ch;
- in = window;
- for (n = 0; n < outcnt; n++) {
- ch = *output_data++ = *in++;
- if ((ulg)output_data == low_buffer_end) output_data=high_buffer_start;
- c = crc_32_tab[((int)c ^ ch) & 0xff] ^ (c >;>; 8);
- }
- crc = c;
- bytes_out += (ulg)outcnt;
- outcnt = 0;
- }
- static void flush_window(void)
- {
- if (high_loaded) flush_window_high();
- else flush_window_low();
- }
- static void error(char *x)
- {
- puts("\n\n");
- puts(x);
- puts("\n\n -- System halted");
- while(1); /* Halt */
- }
- #define STACK_SIZE (4096)
- long user_stack [STACK_SIZE];
- struct {
- long * a;
- short b;
- } stack_start = { & user_stack [STACK_SIZE] , __KERNEL_DS };
- void setup_normal_output_buffer(void) 对于zImage, 直接解压到1M
- {
- #ifdef STANDARD_MEMORY_BIOS_CALL
- if (EXT_MEM_K < 1024) error("Less than 2MB of memory.\n");
- #else
- if ((ALT_MEM_K >; EXT_MEM_K ? ALT_MEM_K : EXT_MEM_K) < 1024) error("Less than 2MB of memory.\n");
- #endif
- output_data = (char *)0x100000; /* Points to 1M */
- free_mem_end_ptr = (long)real_mode;
- }
- struct moveparams {
- uch *low_buffer_start; int lcount;
- uch *high_buffer_start; int hcount;
- };
- void setup_output_buffer_if_we_run_high(struct moveparams *mv)
- {
- high_buffer_start = (uch *)(((ulg)&end) + HEAP_SIZE); 内核高端片段的最小起始地址
- #ifdef STANDARD_MEMORY_BIOS_CALL
- if (EXT_MEM_K < (3*1024)) error("Less than 4MB of memory.\n");
- #else
- if ((ALT_MEM_K >; EXT_MEM_K ? ALT_MEM_K : EXT_MEM_K) < (3*1024)) error("Less than 4MB of memory.\n");
- #endif
- mv->;low_buffer_start = output_data = (char *)LOW_BUFFER_START;
- low_buffer_end = ((unsigned int)real_mode >; LOW_BUFFER_MAX
- ? LOW_BUFFER_MAX : (unsigned int)real_mode) & ~0xfff;
- low_buffer_size = low_buffer_end - LOW_BUFFER_START;
- high_loaded = 1;
- free_mem_end_ptr = (long)high_buffer_start;
- if ( (0x100000 + low_buffer_size) >; ((ulg)high_buffer_start)) {
- ; 如果高端片段的最小起始地址小于它实际应加载的地址,则将它置为实际地址,
- ; 这样高端片段就无需再次移动了,否则它要向前移动
- high_buffer_start = (uch *)(0x100000 + low_buffer_size);
- mv->;hcount = 0; /* say: we need not to move high_buffer */
- }
- else mv->;hcount = -1; 待定
- mv->;high_buffer_start = high_buffer_start;
- }
- void close_output_buffer_if_we_run_high(struct moveparams *mv)
- {
- if (bytes_out >; low_buffer_size) {
- mv->;lcount = low_buffer_size;
- if (mv->;hcount)
- mv->;hcount = bytes_out - low_buffer_size; 求出高端片段的字节数
- } else { 如果解压后内核只有低端的一个片段
- mv->;lcount = bytes_out;
- mv->;hcount = 0;
- }
- }
- int decompress_kernel(struct moveparams *mv, void *rmode)
- {
- real_mode = rmode;
- if (SCREEN_INFO.orig_video_mode == 7) {
- vidmem = (char *) 0xb0000;
- vidport = 0x3b4;
- } else {
- vidmem = (char *) 0xb8000;
- vidport = 0x3d4;
- }
- lines = SCREEN_INFO.orig_video_lines;
- cols = SCREEN_INFO.orig_video_cols;
- if (free_mem_ptr < 0x100000) setup_normal_output_buffer();
- else setup_output_buffer_if_we_run_high(mv);
- makecrc();
- puts("Uncompressing Linux... ");
- gunzip();
- puts("Ok, booting the kernel.\n");
- if (high_loaded) close_output_buffer_if_we_run_high(mv);
- return high_loaded;
- }
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