六． 引导zImage
据我了解，u-boot本身是不支持直接引导zImage的，但是天嵌提供的u-boot就可以直接引导zImage，我就对源码研究了会，结合bootm命令的实现，发现原来引导zImage是如此的简单。
为了实现直接引导zImage，我添加了一个u-boot的命令boot_zImage，命令添加的方法到处都是，可以到这里看一看：http://qiang.ws/index.php?p=537。我主要说下这个命令的实现原理。
因为天嵌把nand flash分了三个区，内核映像就烧在第二个分区，第二个分区的起始地址为0x200000，所以从u-boot需要从nand flash的0x200000处读取内核文件，拷贝到SDRAM的0x30008000处，然后在地址gd->bd->bi_boot_params处设置传递给内核的参数，最后跳转到0x30008000执行，下面我贴出代码进行详细的说明：
/*
* 使用 tag list方式设置传递给内核的参数
* pram_base: base address of linux paramter
*/
static void setup_linux_param(long param_base)
{
/* start of tags */
struct tag *params = (struct tag *)param_base;

params->hdr.tag = ATAG_CORE;
params->hdr.size = tag_size (tag_core);

params->u.core.flags = 0;
params->u.core.pagesize = 0;
params->u.core.rootdev = 0;

params = tag_next (params);

/* !!! importart set SDRAM */
params->hdr.tag = ATAG_MEM;
params->hdr.size = tag_size (tag_mem32);

params->u.mem.start = PHYS_SDRAM_1;
params->u.mem.size = PHYS_SDRAM_1_SIZE;

params = tag_next (params);

/* set bootargs */
char *commandline = getenv (“bootargs”);
if (!commandline)
goto end;
params->hdr.tag = ATAG_CMDLINE;
params->hdr.size = (sizeof (struct tag_header) + strlen (commandline) + 1 + 4) >> 2;

strcpy (params->u.cmdline.cmdline, commandline);

params = tag_next (params);

end:
/* end of tags */
params->hdr.tag = ATAG_NONE;
params->hdr.size = 0;

}

/*
* 将内核映像从nand flash拷贝到SDRAM中
* dst: destination address
* src: source
* size: size to copy
* mt: type of storage device
*/
static inline int copy_kernel_img(ulong dst, const char *src, size_t size)
{
int ret = 0;
if (NF_ReadID() == 0x76) {
ret = nand_read_ll((unsigned char *)dst,
(unsigned long)src, (int)size);
} else {
ret = nand_read_ll_lp((unsigned char *)dst,
(unsigned long)src, (int)size);
}
return ret;
}

int do_boot_zImage (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
{
int ret;
ulong from=0x200000; //这个是内核映像在nand flash中的其实地址
ulong to=0x30008000; //内核在SDRAM中的起始地址
size_t size=0x300000; //拷贝内核映像的大小

/* copy kernel image */
printf(“Copy linux kernel from 0x%08lx to 0x%08lx, size = 0x%08lx … “,
from, to, size);
ret = copy_kernel_img(to, (char *)from, size);
if (ret) {
printf(“failed\n”);
return -1;
} else {
printf(“Copy Kernel to SDRAM done,”);
}

//这里进行魔数的判断，我觉得做不做无所谓，只是检查一下到底是不是zImage格式的映像而已
#define LINUX_ZIMAGE_MAGIC 0x016f2818

if (*(ulong *)(to + 9*4) != LINUX_ZIMAGE_MAGIC) {
printf(“Warning: this binary is not compressed linux kernel image\n”);
printf(“zImage magic = 0x%08lx\n”, *(ulong *)(to + 9*4));
} else {
printf(“zImage magic = 0x%08lx\n”, *(ulong *)(to + 9*4));
;
}

printf(“NOW, Booting Linux……\n”);

/* set atag */
setup_linux_param(gd->bd->bi_boot_params);

/* run kernel */
void(*kernel)(int zero, int arch, uint params);
kernel = (void(*)(int, int, uint))(to);
//跳转到0x30008000，这个传递了三个参数，分别是0，机器码和传递给内核的参数的地址，为什么是这三个参数呢？详情自己看内核的源码arch/arm/boot/compressed/head.S
kernel(0, gd->bd->bi_arch_number,gd->bd->bi_boot_params);

return 0;
}

七． 总结
没有做移植之前移植认为bootloader很神秘，做完一遍后发现原来bootloader也很简单，并没有之前想象的那么难，希望本文档对你有所帮助，同时也希望大家给我指出不足和错误之处。