1：上一节解释了什么是位置无关码、位置有关码。当程序的加载地址与链接地址不一致的时候，如果使用位置有关码则需要重定位。
2：重定位的操作实质就是把链接脚本中的想要重定位的代码段如：.text、 .data段的内容复制到链接地址处。
3：具体分析一下位置有关码，位置无关码：位置无关码和位置有关码实质的区别是操作使用的是绝对地址，还是PC + offset 的相对地址，主要集中在取址、和跳转两个操作上：
b、BL、都是用的相对地址，所以是位置无关码， ldr pc =main 因为main标志在编译的时候会受到链接脚本的链接位置的影响，所以main是链接后的绝对地址，所以是位置有关码
ldr r0, 0xe0002700 这个操作的作用是取0xe0002700内存地址中的值赋值给r0，因为这个内存地址不会受到链接脚本中链接位置的影响，所以是位置无关码
#define WTCON 0xE2700000
#define PS_HOLD_CONTROL 0xE010E81C
#define SVC_STACK 0xD0037D80
ldr r0, =WTCON
ldr r0, =PS_HOLD_CONTROL
同样这几句代码不会受到链接脚本中链接位置的影响，所以也位置无关码
ldr r0, =bss_start
这句代码的作用是把bss_start的地址放入r0中，因为会受到链接脚本中链接位置的影响，所以是位置有关码
看一下反汇编之后的汇编代码
led.elf: file format elf32-littlearm
Disassembly of section .text:
d0020010 <_start>:
d0020010: e59f00dc ldr r0, [pc, #220] ; d00200f4 <delay_loop+0x10> //pc + 220 = 18 + DC = F4， d00200F4中的值是 0xe2700000 位置无关码
d0020014: e3a01000 mov r1, #0
d0020018: e5801000 str r1, [r0] //位置无关
d002001c: e59f00d4 ldr r0, [pc, #212] ; d00200f8 <delay_loop+0x14> //pc + 212 = 24 + D4 = F8, d00200F8中的值是0xe010e81c 位置无关
d0020020: e5901000 ldr r1, [r0]F
d0020024: e59f20d0 ldr r2, [pc, #208] ; d00200fc <delay_loop+0x18>
d0020028: e1811002 orr r1, r1, r2
d002002c: e5801000 str r1, [r0]
d0020030: e59fd0c8 ldr sp, [pc, #200] ; d0020100 <delay_loop+0x1c>
d0020034 <bss_clean>:
d0020034: e59f00c8 ldr r0, [pc, #200] ; d0020104 <delay_loop+0x20> //这是原汇编代码是：ldr r0, =bss_start，可以看到 bss_start的值为d002013f明显位 // 置有关
d0020038: e59f10c8 ldr r1, [pc, #200] ; d0020108 <delay_loop+0x24>
d002003c: e3a02000 mov r2, #0
d0020040: e1500001 cmp r0, r1
d0020044: 0a000002 beq d0020054 <led_blink>
d0020048 <bss_clean_loop>:
d0020048: e4802004 str r2, [r0], #4
d002004c: e1500001 cmp r0, r1
d0020050: 1afffffc bne d0020048 <bss_clean_loop>
d0020054 <led_blink>:
d0020054: e59f00b0 ldr r0, [pc, #176] ; d002010c <delay_loop+0x28>
d0020058: e59f10b0 ldr r1, [pc, #176] ; d0020110 <delay_loop+0x2c>
d002005c: e5801000 str r1, [r0]
d0020060: e59f00ac ldr r0, [pc, #172] ; d0020114 <delay_loop+0x30>
d0020064: e5901000 ldr r1, [r0]
d0020068: e3c110f0 bic r1, r1, #240 ; 0xf0
d002006c: e3811010 orr r1, r1, #16
d0020070: e5801000 str r1, [r0]
d0020074 <led_flash>:
d0020074: e59f009c ldr r0, [pc, #156] ; d0020118 <delay_loop+0x34>
d0020078: e5901000 ldr r1, [r0]
d002007c: e3811002 orr r1, r1, #2
d0020080: e5801000 str r1, [r0]
d0020084: e59f0090 ldr r0, [pc, #144] ; d002011c <delay_loop+0x38>
d0020088: e3a01030 mov r1, #48 ; 0x30
d002008c: e5801000 str r1, [r0]
d0020090: eb000011 bl d00200dc <delay>
d0020094: e59f0080 ldr r0, [pc, #128] ; d002011c <delay_loop+0x38>
d0020098: e3a01028 mov r1, #40 ; 0x28
d002009c: e5801000 str r1, [r0]
d00200a0: eb00000d bl d00200dc <delay>
d00200a4: e59f0070 ldr r0, [pc, #112] ; d002011c <delay_loop+0x38>
d00200a8: e3a01018 mov r1, #24
d00200ac: e5801000 str r1, [r0]
d00200b0: eb000009 bl d00200dc <delay>
d00200b4: e59f0060 ldr r0, [pc, #96] ; d002011c <delay_loop+0x38>
d00200b8: e3a01038 mov r1, #56 ; 0x38
d00200bc: e5801000 str r1, [r0]
d00200c0: e59f0050 ldr r0, [pc, #80] ; d0020118 <delay_loop+0x34>
d00200c4: e5901000 ldr r1, [r0]
d00200c8: e3c11002 bic r1, r1, #2
d00200cc: e5801000 str r1, [r0]
d00200d0: eb000001 bl d00200dc <delay>
d00200d4: eaffffe6 b d0020074 <led_flash>
d00200d8: eafffffe b d00200d8 <led_flash+0x64>
d00200dc <delay>:
d00200dc: e59f003c ldr r0, [pc, #60] ; d0020120 <delay_loop+0x3c>
d00200e0: e3a01000 mov r1, #0
d00200e4 <delay_loop>:
d00200e4: 12400001 subne r0, r0, #1
d00200e8: e1500001 cmp r0, r1
d00200ec: 1afffffc bne d00200e4 <delay_loop>
d00200f0: e1a0f00e mov pc, lr
d00200f4: e2700000 rsbs r0, r0, #0
d00200f8: e010e81c ands lr, r0, ip, lsl r8
d00200fc: 00000301 andeq r0, r0, r1, lsl #6
d0020100: d0037d80 andle r7, r3, r0, lsl #27
d0020104: d002013f andle r0, r2, pc, lsr r1
d0020108: d002013f andle r0, r2, pc, lsr r1
d002010c: e0200240 eor r0, r0, r0, asr #4
d0020110: 11111111 tstne r1, r1, lsl r1
d0020114: e02000a0 eor r0, r0, r0, lsr #1
d0020118: e02000a4 eor r0, r0, r4, lsr #1
d002011c: e0200244 eor r0, r0, r4, asr #4
d0020120: 00989680 addseq r9, r8, r0, lsl #13
d0020124: 00001a41 andeq r1, r0, r1, asr #20
d0020128: 61656100 cmnvs r5, r0, lsl #2
d002012c: 01006962 tsteq r0, r2, ror #18
d0020130: 00000010 andeq r0, r0, r0, lsl r0
d0020134: 45543505 ldrbmi r3, [r4, #-1285] ; 0x505
d0020138: 08040600 stmdaeq r4, {r9, sl}
d002013c: Address 0xd002013c is out of bounds.
如：链接脚本为以下链接脚本，而我们程序执行的起始位置为0xd0020010,，我们只需要把从_start 到data_end的代码复制到链接链接的地址0xd0024000处即可；接着道0xd0024000中执行程序即可，注意重定位之前的汇编代码
一定要使用位置无关码。
SECTIONS
{
. = 0xd0024000;
.text : {
start.o
*(.text)
}
.data : {
*(.data)
}
data_end = .;
bss_start = .;
.bss : {
*(.bss)
}
bss_end = .;
}
3：代码实战：
注：加载地址0xd0020010，连接地址：0xd0026000
/*
* s5pv210裸机实验
*
* 重定位
*
*/
#define WTCON 0xE2700000
#define PS_HOLD_CONTROL 0xE010E81C
#define SVC_STACK 0xD0037D80
//LED相关寄存器
#define _GPJ0CON 0xE0200240
#define _GPJ0DAT 0xE0200244
#define _GPD0CON 0xE02000A0
#define _GPD0DAT 0xE02000A4
#define _GPJ0CON_EN 0x11111111
#define LED_1_ON ((0<<3) | (1<<4) | (1<<5))
#define LED_2_ON ((1<<3) | (0<<4) | (1<<5))
#define LED_3_ON ((1<<3) | (1<<4) | (0<<5))
#define LED_OFF ((1<<3) | (1<<4) | (1<<5))
#define DELAY_TIME 10000000
.global _start
_start:
//第一步关看门狗
ldr r0, =WTCON
ldr r1, =0x0
str r1, [r0]
//第二步电源开关置锁，PS_HOLD_CONTROL寄存器中的0、8、9三位置1
ldr r0, =PS_HOLD_CONTROL
ldr r1, [r0]
ldr r2, =0x301
orr r1, r1, r2
str r1, [r0]
//设置svc模式下的栈
ldr sp, =SVC_STACK
//重定位
adr r0, _start
ldr r1, =_start
ldr r3, =bss_end
cmp r0, r1
beq bss_clean
relocate_loop:
ldr r2, [r0], #4
str r2, [r1], #4
cmp r1, r3
bne relocate_loop
ldreq pc, =bss_clean
//清bss 因为清bss中的代码为位置有关码，所以要先重定位，
bss_clean:
ldr r0, =bss_start //把bss的起始地址读到r0中
ldr r1, =bss_end //把bss的结束地址读到r1中
mov r2, #0x0 //把r2寄存器写入
cmp r0, r1 //对比r0 和r1 如果相等则跳转到led_blink
ldr pc, =led_blink
bss_clean_loop:
str r2, [r0], #4 //不等则把r0地址写入0，r0 + 4
cmp r0, r1 //判断r0 r1是否相等 不相等跳转到bss_clean_loop循环
bne bss_clean_loop
//bl led_blink
long_jump:
ldr pc, =led_blink
//设置死循环放置跑飞
b .
led.elf: file format elf32-littlearm Disassembly of section .text: d0026000 <_start>: d0026000: e59f006c ldr r0, [pc, #108] ; d0026074 <long_jump+0x8> d0026004: e3a01000 mov r1, #0 d0026008: e5801000 str r1, [r0] d002600c: e59f0064 ldr r0, [pc, #100] ; d0026078 <long_jump+0xc> d0026010: e5901000 ldr r1, [r0] d0026014: e59f2060 ldr r2, [pc, #96] ; d002607c <long_jump+0x10> d0026018: e1811002 orr r1, r1, r2 d002601c: e5801000 str r1, [r0] d0026020: e59fd058 ldr sp, [pc, #88] ; d0026080 <long_jump+0x14> d0026024: e24f002c sub r0, pc, #44 ; 0x2c d0026028: e59f1054 ldr r1, [pc, #84] ; d0026084 <long_jump+0x18> d002602c: e59f3054 ldr r3, [pc, #84] ; d0026088 <long_jump+0x1c> d0026030: e1500001 cmp r0, r1 d0026034: 0a000004 beq d002604c <bss_clean> d0026038 <relocate_loop>: d0026038: e4902004 ldr r2, [r0], #4 d002603c: e4812004 str r2, [r1], #4 d0026040: e1510003 cmp r1, r3 d0026044: 1afffffb bne d0026038 <relocate_loop> d0026048: 059ff03c ldreq pc, [pc, #60] ; d002608c <long_jump+0x20> d002604c <bss_clean>: d002604c: e59f003c ldr r0, [pc, #60] ; d0026090 <long_jump+0x24> d0026050: e59f1030 ldr r1, [pc, #48] ; d0026088 <long_jump+0x1c> d0026054: e3a02000 mov r2, #0 d0026058: e1500001 cmp r0, r1 d002605c: e59ff030 ldr pc, [pc, #48] ; d0026094 <long_jump+0x28> d0026060 <bss_clean_loop>: d0026060: e4802004 str r2, [r0], #4 d0026064: e1500001 cmp r0, r1 d0026068: 1afffffc bne d0026060 <bss_clean_loop> d002606c <long_jump>: d002606c: e59ff020 ldr pc, [pc, #32] ; d0026094 <long_jump+0x28> d0026070: eafffffe b d0026070 <long_jump+0x4> d0026074: e2700000 rsbs r0, r0, #0 d0026078: e010e81c ands lr, r0, ip, lsl r8 d002607c: 00000301 andeq r0, r0, r1, lsl #6 d0026080: d0037d80 andle r7, r3, r0, lsl #27 d0026084: d0026000 andle r6, r2, r0 d0026088: d0026168 andle r6, r2, r8, ror #2 d002608c: d002604c andle r6, r2, ip, asr #32 d0026090: d0026168 andle r6, r2, r8, ror #2 d0026094: d00260b4 strhle r6, [r2], -r4 d0026098: 00001a41 andeq r1, r0, r1, asr #20 d002609c: 61656100 cmnvs r5, r0, lsl #2 d00260a0: 01006962 tsteq r0, r2, ror #18 d00260a4: 00000010 andeq r0, r0, r0, lsl r0 d00260a8: 45543505 ldrbmi r3, [r4, #-1285] ; 0x505 d00260ac: 08040600 stmdaeq r4, {r9, sl} d00260b0: 00010901 andeq r0, r1, r1, lsl #18 d00260b4 <led_blink>: d00260b4: e59f3068 ldr r3, [pc, #104] ; d0026124 <led_blink+0x70> d00260b8: e92d41f0 push {r4, r5, r6, r7, r8, lr} d00260bc: e59350a0 ldr r5, [r3, #160] ; 0xa0 d00260c0: e59f1060 ldr r1, [pc, #96] ; d0026128 <led_blink+0x74> d00260c4: e3c500f0 bic r0, r5, #240 ; 0xf0 d00260c8: e3802010 orr r2, r0, #16 d00260cc: e1a04003 mov r4, r3 d00260d0: e3a08030 mov r8, #48 ; 0x30 d00260d4: e3a07028 mov r7, #40 ; 0x28 d00260d8: e3a06018 mov r6, #24 d00260dc: e3a05038 mov r5, #56 ; 0x38 d00260e0: e5831240 str r1, [r3, #576] ; 0x240 d00260e4: e58320a0 str r2, [r3, #160] ; 0xa0 d00260e8: e59430a4 ldr r3, [r4, #164] ; 0xa4 d00260ec: e5848244 str r8, [r4, #580] ; 0x244 d00260f0: e383e002 orr lr, r3, #2 d00260f4: e584e0a4 str lr, [r4, #164] ; 0xa4 d00260f8: eb00000b bl d002612c <delay> d00260fc: e5847244 str r7, [r4, #580] ; 0x244 d0026100: eb000009 bl d002612c <delay> d0026104: e5846244 str r6, [r4, #580] ; 0x244 d0026108: eb000007 bl d002612c <delay> d002610c: e59420a4 ldr r2, [r4, #164] ; 0xa4 d0026110: e5845244 str r5, [r4, #580] ; 0x244 d0026114: e3c2c002 bic ip, r2, #2 d0026118: e584c0a4 str ip, [r4, #164] ; 0xa4 d002611c: eb000002 bl d002612c <delay> d0026120: eafffff0 b d00260e8 <led_blink+0x34> d0026124: e0200000 eor r0, r0, r0 d0026128: 11111111 tstne r1, r1, lsl r1 d002612c <delay>: d002612c: e24dd008 sub sp, sp, #8 d0026130: e59f002c ldr r0, [pc, #44] ; d0026164 <delay+0x38> d0026134: e58d0004 str r0, [sp, #4] d0026138: e59d3004 ldr r3, [sp, #4] d002613c: e3530000 cmp r3, #0 d0026140: 0a000005 beq d002615c <delay+0x30> d0026144: e59dc004 ldr ip, [sp, #4] d0026148: e24c2001 sub r2, ip, #1 d002614c: e58d2004 str r2, [sp, #4] d0026150: e59d1004 ldr r1, [sp, #4] d0026154: e3510000 cmp r1, #0 d0026158: 1afffff9 bne d0026144 <delay+0x18> d002615c: e28dd008 add sp, sp, #8 d0026160: e12fff1e bx lr d0026164: 000dbba0 andeq fp, sp, r0, lsr #23 Disassembly of section .comment: 00000000 <.comment>: 0: 43434700 movtmi r4, #14080 ; 0x3700 4: 5328203a teqpl r8, #58 ; 0x3a 8: 6372756f cmnvs r2, #465567744 ; 0x1bc00000 c: 20797265 rsbscs r7, r9, r5, ror #4 10: 202b2b47 eorcs r2, fp, r7, asr #22 14: 6574694c ldrbvs r6, [r4, #-2380]! ; 0x94c 18: 30303220 eorscc r3, r0, r0, lsr #4 1c: 2d337139 ldfcss f7, [r3, #-228]! ; 0xffffff1c 20: 20293736 eorcs r3, r9, r6, lsr r7 24: 2e342e34 mrccs 14, 1, r2, cr4, cr4, {1} 28: 47000031 smladxmi r0, r1, r0, r0 2c: 203a4343 eorscs r4, sl, r3, asr #6 30: 756f5328 strbvc r5, [pc, #-808]! ; fffffd10 <bss_end+0x2ffd9ba8> 34: 72656372 rsbvc r6, r5, #-939524095 ; 0xc8000001 38: 2b472079 blcs 11c8224 <_start-0xcee5dddc> 3c: 694c202b stmdbvs ip, {r0, r1, r3, r5, sp}^ 40: 32206574 eorcc r6, r0, #486539264 ; 0x1d000000 44: 71393030 teqvc r9, r0, lsr r0 48: 37362d33 undefined instruction 0x37362d33 4c: 2e342029 cdpcs 0, 3, cr2, cr4, cr9, {1} 50: 00312e34 eorseq r2, r1, r4, lsr lr Disassembly of section .ARM.attributes: 00000000 <.ARM.attributes>: 0: 00002641 andeq r2, r0, r1, asr #12 4: 61656100 cmnvs r5, r0, lsl #2 8: 01006962 tsteq r0, r2, ror #18 c: 0000001c andeq r0, r0, ip, lsl r0 10: 45543505 ldrbmi r3, [r4, #-1285] ; 0x505 14: 08040600 stmdaeq r4, {r9, sl} 18: 12010901 andne r0, r1, #16384 ; 0x4000 1c: 15011404 strne r1, [r1, #-1028] ; 0x404 20: 18031701 stmdane r3, {r0, r8, r9, sl, ip} 24: Address 0x00000024 is out of bounds.
采用长跳转的话，直接跳转到d00260b4中去执行led_blink，如果不重定位则不会亮，重定位则亮
