linux kernel的编译过程比较复杂，最后生成的给用户使用的kernel文件也是多种多样。
先来看一段编译ARM kernel时命令行最后的输出，主要可以分为5个步骤：
- 生成vmlinux
- 生成arch/arm/boot/Image
- 生成arch/arm/boot/compressed/piggy.gzip
- 生成arch/arm/boot/compressed/vmlinux
- 生成arch/arm/boot/zImage
LD vmlinux
SYSMAP System.map
OBJCOPY arch/arm/boot/Image
Kernel: arch/arm/boot/Image is ready
Building modules, stage 2.
GZIP arch/arm/boot/compressed/piggy.gzip
MODPOST 1 modules
CC arch/arm/boot/compressed/misc.o
CC drivers/scsi/scsi_wait_scan.mod.o
CC arch/arm/boot/compressed/decompress.o
CC arch/arm/boot/compressed/string.o
LD [M] drivers/scsi/scsi_wait_scan.ko
AS arch/arm/boot/compressed/piggy.gzip.o
LD arch/arm/boot/compressed/vmlinux
OBJCOPY arch/arm/boot/zImage
Kernel: arch/arm/boot/zImage is ready我们来逐步分析每一个步骤：
生成vmlinux
vmlinux生成在kernel的根目录下，大小为28MB。链接文件在arch/arm/kernel/vmlinux.lds。
panzhenjie@panzhenjie-vmware:~/workspace/linux-3.5$ ll
......
-rwxrwxr-x 1 panzhenjie panzhenjie 28289095 9月 1 07:27 vmlinux*这个vmlinux是一个elf格式的文件，通过readelf命令我们可以看到，这个文件的链接地址是0xc0008000。
从第1段.head.text开始到第22段.bss，一共占用了2MB多的空间，而后面20多MB的数据其实都是些不必要的调试信息。
panzhenjie@panzhenjie-vmware:~/workspace/linux-3.5$ readelf -S vmlinux
There are 36 section headers, starting at offset 0x1a09770:
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .head.text PROGBITS c0008000 008000 0001c4 00 AX 0 0 4
[ 2] .text PROGBITS c0008200 008200 1948e4 00 AX 0 0 64
[ 3] .rodata PROGBITS c019d000 19d000 072100 00 A 0 0 64
[ 4] __bug_table PROGBITS c020f100 20f100 0028c8 00 A 0 0 1
[ 5] __ksymtab PROGBITS c02119c8 2119c8 003c28 00 A 0 0 4
[ 6] __ksymtab_gpl PROGBITS c02155f0 2155f0 001988 00 A 0 0 4
[ 7] __ksymtab_strings PROGBITS c0216f78 216f78 00b9b8 00 A 0 0 1
[ 8] __param PROGBITS c0222930 222930 000340 00 A 0 0 4
[ 9] __modver PROGBITS c0222c70 222c70 000390 00 A 0 0 4
[10] .ARM.unwind_idx ARM_EXIDX c0223000 223000 00ea60 00 AL 12 0 4
[11] .ARM.unwind_tab PROGBITS c0231a60 231a60 002964 00 A 0 0 4
[12] .init.text PROGBITS c0235000 235000 0182f8 00 AX 0 0 32
[13] .exit.text PROGBITS c024d2f8 24d2f8 000480 00 AX 0 0 4
[14] .init.arch.info PROGBITS c024d778 24d778 000240 00 A 0 0 4
[15] .init.tagtable PROGBITS c024d9b8 24d9b8 000048 00 A 0 0 4
[16] .init.smpalt PROGBITS c024da00 24da00 0001d8 00 A 0 0 4
[17] .init.pv_table PROGBITS c024dbd8 24dbd8 000488 00 A 0 0 1
[18] .init.data PROGBITS c024e060 24e060 0081ac 00 WA 0 0 8
[19] .data..percpu PROGBITS c0257000 257000 001480 00 WA 0 0 64
[20] .data PROGBITS c025a000 25a000 042760 00 WA 0 0 64
[21] .notes NOTE c029c760 29c760 000024 00 AX 0 0 4
[22] .bss NOBITS c029c7c0 29c784 0319ec 00 WA 0 0 64
[23] .comment PROGBITS 00000000 29c784 00002b 01 MS 0 0 1
[24] .ARM.attributes ARM_ATTRIBUTES 00000000 29c7af 00002d 00 0 0 1
[25] .debug_line PROGBITS 00000000 29c7dc 140a16 00 0 0 1
[26] .debug_info PROGBITS 00000000 3dd1f2 10da832 00 0 0 1
[27] .debug_abbrev PROGBITS 00000000 14b7a24 0b90ba 00 0 0 1
[28] .debug_aranges PROGBITS 00000000 1570ae0 006910 00 0 0 8
[29] .debug_ranges PROGBITS 00000000 15773f0 0a11e0 00 0 0 8
[30] .debug_frame PROGBITS 00000000 16185d0 03ef0c 00 0 0 4
[31] .debug_str PROGBITS 00000000 16574dc 088dbd 01 MS 0 0 1
[32] .debug_loc PROGBITS 00000000 16e0299 32934c 00 0 0 1
[33] .shstrtab STRTAB 00000000 1a095e5 00018a 00 0 0 1
[34] .symtab SYMTAB 00000000 1a09d10 0908d0 10 35 30957 4
[35] .strtab STRTAB 00000000 1a9a5e0 060267 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)生成arch/arm/boot/Image
Image文件被除去了一些不必要的调试信息，是一个已经可以使用了的kernel文件了，只有2.7MB。
panzhenjie@panzhenjie-vmware:~/workspace/linux-3.5$ ll arch/arm/boot/
......
-rwxrwxr-x 1 panzhenjie panzhenjie 2705284 9月 1 07:27 Image*arch/arm/boot/Makefile
$(obj)/Image: vmlinux FORCE
$(call if_changed,objcopy)
@echo ' Kernel: $@ is ready'scripts/Kbuild.include
# Execute command if command has changed or prerequisite(s) are updated.
#
if_changed = $(if $(strip $(any-prereq) $(arg-check)), \
@set -e; \
$(echo-cmd) $(cmd_$(1)); \
echo 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd)scripts/Makefile.lib
# Objcopy
# ---------------------------------------------------------------------------
quiet_cmd_objcopy = OBJCOPY $@
cmd_objcopy = $(OBJCOPY) $(OBJCOPYFLAGS) $(OBJCOPYFLAGS_$(@F)) $< $@arch/arm/Makefile
OBJCOPYFLAGS :=-O binary -R .comment -S生成arch/arm/boot/compressed/piggy.gzip
为了让kernel更小，下一步就是对kernel进行压缩，压缩出来的kernel变成了1.3MB。
panzhenjie@panzhenjie-vmware:~/workspace/linux-3.5$ ll arch/arm/boot/compressed/
......
-rw-rw-r-- 1 panzhenjie panzhenjie 1311566 9月 1 07:27 piggy.gziparch/arm/boot/compressed/Makefile
suffix_$(CONFIG_KERNEL_GZIP) = gzip
suffix_$(CONFIG_KERNEL_LZO) = lzo
suffix_$(CONFIG_KERNEL_LZMA) = lzma
suffix_$(CONFIG_KERNEL_XZ) = xzkern
$(obj)/piggy.$(suffix_y): $(obj)/../Image FORCE
$(call if_changed,$(suffix_y))scripts/Makefile.lib
# Gzip
# ---------------------------------------------------------------------------
quiet_cmd_gzip = GZIP $@
cmd_gzip = (cat $(filter-out FORCE,$^) | gzip -n -f -9 > $@) || \
(rm -f $@ ; false)生成arch/arm/boot/compressed/vmlinux
piggy.zip只是对kernel进行了压缩，但是还并不能让我们直接使用。想要使用，必须要加上自解压缩代码。
可以看到，生成出来的elf格式的kernel就比刚才压缩过的kernel大一点而已。
panzhenjie@panzhenjie-vmware:~/workspace/linux-3.5$ ll arch/arm/boot/compressed/
......
-rwxrwxr-x 1 panzhenjie panzhenjie 1404652 9月 1 07:27 vmlinux*
arch/arm/boot/compressed/Makefile
$(obj)/vmlinux: $(obj)/vmlinux.lds $(obj)/$(HEAD) $(obj)/piggy.$(suffix_y).o \
$(addprefix $(obj)/, $(OBJS)) $(lib1funcs) $(ashldi3) FORCE
@$(check_for_multiple_zreladdr)
$(call if_changed,ld)
@$(check_for_bad_syms)
$(obj)/piggy.$(suffix_y).o: $(obj)/piggy.$(suffix_y) FORCE生成arch/arm/boot/zImage
最后一步，就是剔除掉新的压缩后的kernel中的调试信息，成为一个真正可以使用的带有自解压功能的kernel。
panzhenjie@panzhenjie-vmware:~/workspace/linux-3.5$ ll arch/arm/boot/
......
-rwxrwxr-x 1 panzhenjie panzhenjie 1329592 9月 1 07:27 zImage*
arch/arm/boot/Makefile
$(obj)/zImage: $(obj)/compressed/vmlinux FORCE
$(call if_changed,objcopy)
@echo ' Kernel: $@ is ready'