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
--------------------------
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.
*/
