1.       内核启动地址

ZTEXTADDR

解压代码运行的开始地址。没有物理地址和虚拟地址之分，因为此时MMU处于关闭状态。这个地址不一定时RAM的地址，可以是支持读写寻址的flash等存储中介。

Start address of decompressor. here's no point in talking about virtual or physical addresses here, since the MMU will be off at the time when you call the decompressor code. You normally call   the kernel at this address to start it booting. This doesn't have to be located in RAM, it can be in flash or other read-only or      read-write addressable medium.

在arch/arm/boot/compressed/Makefile中说的很明确

#                            
# We now have a PIC decompressor implementation.  Decompressors running
# from RAM should not define ZTEXTADDR.  Decompressors running directly
# from ROM or Flash must define ZTEXTADDR (preferably via the config)   
# FIXME: Previous assignment to ztextaddr-y is lost here. See SHARK
ifeq ($(CONFIG_ZBOOT_ROM),y) 
ZTEXTADDR   := $(CONFIG_ZBOOT_ROM_TEXT)
ZBSSADDR    := $(CONFIG_ZBOOT_ROM_BSS)
else                         
ZTEXTADDR   := 0             
ZBSSADDR    := ALIGN(8)      
endif                        

      

ZRELADDR

       内核启动在RAM中的地址。压缩的内核映像被解压到这个地址，然后执行。

This is the address where the decompressed kernel will be written, and eventually executed. The following constraint must be valid:

              __virt_to_phys(TEXTADDR) == ZRELADDR

       The initial part of the kernel is carefully coded to be position independent.

一般定义在项目目录下，比如：

arch/arm/mach-at91/Makefile.boot:   zreladdr-y	+= 0x70008000
arch/arm/mach-at91/Makefile.boot:   zreladdr-y	+= 0x20008000
arch/arm/mach-cns3xxx/Makefile.boot:   zreladdr-y	+= 0x00008000
arch/arm/mach-davinci/Makefile.boot:   zreladdr-y	+= 0xc0008000
arch/arm/mach-davinci/Makefile.boot:   zreladdr-y	+= 0x80008000
arch/arm/mach-dove/Makefile.boot:   zreladdr-y	+= 0x00008000
arch/arm/mach-ebsa110/Makefile.boot:   zreladdr-y	+= 0x00008000
arch/arm/mach-exynos/Makefile.boot:   zreladdr-y	+= 0x40008000
arch/arm/mach-footbridge/Makefile.boot:   zreladdr-y	+= 0x00008000
arch/arm/mach-gemini/Makefile.boot:   zreladdr-y	+= 0x00008000
arch/arm/mach-gemini/Makefile.boot:   zreladdr-y	+= 0x10008000
arch/arm/mach-integrator/Makefile.boot:   zreladdr-y	+= 0x00008000
arch/arm/mach-iop13xx/Makefile.boot:   zreladdr-y   += 0x00008000

  
  

   
   

  
  

在arch/arm/boot/Makefile中被赋值：

ZRELADDR    := $(zreladdr-y)
PARAMS_PHYS := $(params_phys-y)
INITRD_PHYS := $(initrd_phys-y)
...  ...

ifneq ($(LOADADDR),)   
  UIMAGE_LOADADDR=$(LOADADDR)
else                   
  ifeq ($(CONFIG_ZBOOT_ROM),y)
    UIMAGE_LOADADDR=$(CONFIG_ZBOOT_ROM_TEXT)
  else                 
    UIMAGE_LOADADDR=$(ZRELADDR)              
  endif                
endif                        

  
  

   
   

  
  
  
  

   
   check_for_multiple_loadaddr = \
   
   
if [ $(words $(UIMAGE_LOADADDR)) -ne 1 ]; then \
   
   
    echo 'multiple (or no) load addresses: $(UIMAGE_LOADADDR)'; \         
   
   
    echo 'This is incompatible with uImages'; \
   
   
   
   
    echo 'Specify LOADADDR on the commandline to build an uImage'; \
   
   
    false; \
   
   
fi  
  
  

从最后一句红色说明可以看出，如果没有在代码中指定zreladdr-y，也可以在编译的命令行中指定LOADADDR来确定内核加载的实际物理地址。

TEXTADDR

       内核启动的虚拟地址，与ZRELADDR相对应。一般内核启动的虚拟地址为RAM的第一个bank地址加上0x8000。

       TEXTADDR = PAGE_OFFSET + TEXTOFFST

       Virtual start address of kernel, normally PAGE_OFFSET + 0x8000.This is where the kernel image ends up. With the latest kernels, it must be located at 32768 bytes into a 128MB region. Previous kernels placed a restriction of 256MB here.

TEXTOFFSET

       内核偏移地址。在arch/arm/makefile中设定。

PHYS_OFFSET

       RAM第一个bank的物理起始地址。

Physical start address of the first bank of RAM.

PAGE_OFFSET

RAM第一个bank的虚拟起始地址。

       Virtual start address of the first bank of RAM. During the kernel

       boot phase, virtual address PAGE_OFFSET will be mapped to physical

       address PHYS_OFFSET, along with any other mappings you supply.

       This should be the same value as TASK_SIZE.

这个值由make menuconfig进行配置

1.2.   内核启动地址确定
内核启动引导地址由bootp.lds决定。 Bootp.lds : arch/arm/bootp

OUTPUT_ARCH(arm)

ENTRY(_start)

SECTIONS

{

. = 0;

.text : {

   _stext = .;

   *(.start)

   *(.text)

   initrd_size = initrd_end - initrd_start;

   _etext = .;

}

}

由上 .= 0可以确定解压代码运行的开始地址在0x0的位置。ZTEXTADDR的值决定了这个值得选取。

Makefile : arch/arm/boot/compressed

如果设定内核从ROM中启动的话，可以在make menuconfig 的配置界面中设置解压代码的起始地址，否则解压代码的起始地址为0x0。实际上，默认从ROM启动时，解压代码的起始地址也是0x0。

ifeq ($(CONFIG_ZBOOT_ROM),y)

ZTEXTADDR := $(CONFIG_ZBOOT_ROM_TEXT)

ZBSSADDR    := $(CONFIG_ZBOOT_ROM_BSS)

else

ZTEXTADDR :=0                                                      

ZBSSADDR := ALIGN(4)

endif

SEDFLAGS    = s/TEXT_START/$(ZTEXTADDR)/;s/BSS_START/$(ZBSSADDR)/                                                                                                                                             

……

$(obj)/vmlinux.lds: $(obj)/vmlinux.lds.in arch/arm/mach-s3c2410/Makefile .config

       @sed "$(SEDFLAGS)" < $< > $@

@sed "$(SEDFLAGS)" < $< > $@ 规则将TEXT_START设定为ZTEXTADDR。TEXT_START在arch/arm/boot/compressed/vmlinux.lds.in 中被用来设定解压代码的起始地址。

OUTPUT_ARCH(arm)

ENTRY(_start)

SECTIONS

{

. = TEXT_START;  

_text = .;

.text : {

    _start = .;

    *(.start)

    *(.text)

*(.text.*)

……

}

}

内核的编译依靠vmlinux.lds，vmlinux.lds由vmlinux.lds.s 生成。从下面代码可以看出内核启动的虚拟地址被设置为PAGE_OFFSET + TEXT_OFFSET，而内核启动的物理地址ZRELADDR在arch/arm/boot/Makefile中设定。

OUTPUT_ARCH(arm)

ENTRY(stext)

SECTIONS

{

#ifdef CONFIG_XIP_KERNEL

       . = XIP_VIRT_ADDR(CONFIG_XIP_PHYS_ADDR);

#else

       . = PAGE_OFFSET + TEXT_OFFSET;   

#endif

       .init : {                  /* Init code and data             */

              _stext = .;

                     _sinittext = .;

                     *(.init.text)

                     _einittext = .;

……

       }

}

# arch/arm/boot/Makefile

# Note: the following conditions must always be true:

#   ZRELADDR == virt_to_phys(PAGE_OFFSET + TEXT_OFFSET)

#   PARAMS_PHYS must be within 4MB of ZRELADDR

#   INITRD_PHYS must be in RAM

ZRELADDR    := $(zreladdr-y)                                      

#---> zrealaddr-y is specified with 0x30008000 in arch/arm/boot/makefile.boot

PARAMS_PHYS := $(params_phys-y)

INITRD_PHYS := $(initrd_phys-y)

export ZRELADDR INITRD_PHYS PARAMS_PHYS

通过下面的命令编译内核映像，由参数-a, -e设置其入口地址为ZRELADDR，此值在上面ZRELADDR    := $(zreladdr-y)指定。

quiet_cmd_uimage= UIMAGE $@                                                                     

      cmd_uimage = $(CONFIG_SHELL) $(MKIMAGE) -A arm -O linux -T kernel \

                 -C none -a $(ZRELADDR) -e $(ZRELADDR) \

                 -n 'Linux-$(KERNELRELEASE)' -d $< $@

1.3.   小结
从上面分析可知道，linux内核被bootloader拷贝到RAM后，解压代码从ZTEXTADDR开始运行（这段代码是与位置无关的PIC( Position independent code)）。内核被解压缩到ZREALADDR处，也就是内核启动的物理地址处。相应地，内核启动的虚拟地址被设定为TEXTADDR，满足如下条件：

TEXTADDR = PAGE_OFFSET + TEXT_OFFSET

内核启动的物理地址和虚拟地址满足入下条件：

ZRELADDR == virt_to_phys(PAGE_OFFSET + TEXT_OFFSET)= virt_to_phys(TEXTADDR)

假定开发板为smdk2410，则有：

内核启动的虚拟地址

TEXTADDR     = 0xC0008000

内核启动的物理地址  

ZRELADDR     = 0x30008000

如果直接从flash中启动还需要设置ZTEXTADDR地址。

2.       内核启动过程分析
内核启动过程经过大体可以分为两个阶段：内核映像的自引导；linux内核子模块的初始化。

start

Decompress_kernel()

Call_kernel

Stext:

Prepare_namespace

Do_basic_setup

init

Rest_init

Setup_arch ……

Start_kernel

_enable_mmu

Execve(“/sbin/init”))