一.如何写位置无关码：
1.汇编指令中跳转用 b 、bl
2.C语言：不能有全局变量
1.bl指令
pc_new = pc + 偏移
.extern main
.text
.global _start
_start:
Reset:
ldr sp, =4096 @ 设置栈指针，以下都是C函数，调用前需要设好栈
bl disable_watch_dog @ 关闭WATCHDOG，否则CPU会不断重启
//bl 是位置无关码，相当于 PCnew = PC +偏移
// PCnew = (4 + 8) + 0x28 = 0x34
bl clock_init @ 设置MPLL，改变FCLK、HCLK、PCLK
bl memsetup @ 设置存储控制器以使用SDRAM
bl copy_steppingstone_to_sdram @ 复制代码到SDRAM中
ldr pc, =on_sdram @ 跳到SDRAM中继续执行
on_sdram:
ldr sp, =0x34000000 @ 设置栈指针
ldr lr, =halt_loop @ 设置返回地址
ldr pc, =main @ 调用main函数
halt_loop:
b halt_loop
上面代码的反汇编如下
30000000 <_start>:
30000000: e3a0da01 mov sp, #4096 ; 0x1000
30000004:eb00000a bl30000034
30000008: eb00000d bl 30000044
3000000c: eb000026 bl 300000ac
30000010: eb000040 bl 30000118
30000014: e59ff00c ldr pc, [pc, #12] ; 30000028 <.text+0x28>
30000018
30000018: e3a0d30d mov sp, #872415232 ; 0x34000000
3000001c: e59fe008 ldr lr, [pc, #8] ; 3000002c <.text+0x2c>
30000020: e59ff008 ldr pc, [pc, #8] ; 30000030 <.text+0x30>
30000024
30000024: eafffffe b 30000024
30000028: 30000018 andcc r0, r0, r8, lsl r0
3000002c: 30000024 andcc r0, r0, r4, lsr #32
30000030: 30000200 andcc r0, r0, r0, lsl #4
30000034
30000034: e3a02000 mov r2, #0 ; 0x0
30000038: e3a03453 mov r3, #1392508928 ; 0x53000000
3000003c: e5832000 str r2, [r3]
30000040: e1a0f00e mov pc, lr
30000044
30000044: e3a0c313 mov ip, #1275068416 ; 0x4c000000
30000048: e3a03003 mov r3, #3 ; 0x3
3000004c: e52de004 str lr, [sp, #-4]!
30000050: e58c3014 str r3, [ip, #20]
30000054: ee111f10 mrc 15, 0, r1, cr1, cr0, {0}
30000058: e3811103 orr r1, r1, #-1073741824 ; 0xc0000000
3000005c: ee011f10 mcr 15, 0, r1, cr1, cr0, {0}
30000060: e3a0e456 mov lr, #1442840576 ; 0x56000000
30000064: e59e20b0 ldr r2, [lr, #176]
30000068: e3a035c9 mov r3, #843055104 ; 0x32400000
3000006c: e3a015c9 mov r1, #843055104 ; 0x32400000
30000070: e2833801 add r3, r3, #65536 ; 0x10000
30000074: e2811801 add r1, r1, #65536 ; 0x10000
30000078: e3a00917 mov r0, #376832 ; 0x5c000
3000007c: e1520003 cmp r2, r3
30000080: e2800040 add r0, r0, #64 ; 0x40
30000084: e2811002 add r1, r1, #2 ; 0x2
30000088: 0a000005 beq 300000a4
3000008c: e59e30b0 ldr r3, [lr, #176]
30000090: e3a02917 mov r2, #376832 ; 0x5c000
30000094: e2822012 add r2, r2, #18 ; 0x12
30000098: e1530001 cmp r3, r1
3000009c: 158c2004 strne r2, [ip, #4]
300000a0: 149df004 ldrne pc, [sp], #4
300000a4: e58c0004 str r0, [ip, #4]
300000a8: e49df004 ldr pc, [sp], #4
300000ac
300000ac: e52de004 str lr, [sp, #-4]!
300000b0: e3a01422 mov r1, #570425344 ; 0x22000000
300000b4: e3a0c723 mov ip, #9175040 ; 0x8c0000
300000b8: e2811a11 add r1, r1, #69632 ; 0x11000
300000bc: e3a03312 mov r3, #1207959552 ; 0x48000000
300000c0: e3a02c07 mov r2, #1792 ; 0x700
300000c4: e28cce4f add ip, ip, #1264 ; 0x4f0
300000c8: e3a00906 mov r0, #98304 ; 0x18000
300000cc: e2811e11 add r1, r1, #272 ; 0x110
300000d0: e2800005 add r0, r0, #5 ; 0x5
300000d4: e3a0e030 mov lr, #48 ; 0x30
300000d8: e5831000 str r1, [r3]
300000dc: e28cc004 add ip, ip, #4 ; 0x4
300000e0: e5832004 str r2, [r3, #4]
300000e4: e5832008 str r2, [r3, #8]
300000e8: e583200c str r2, [r3, #12]
300000ec: e5832010 str r2, [r3, #16]
300000f0: e5832014 str r2, [r3, #20]
300000f4: e5832018 str r2, [r3, #24]
300000f8: e3a020b1 mov r2, #177 ; 0xb1
300000fc: e583001c str r0, [r3, #28]
30000100: e5830020 str r0, [r3, #32]
30000104: e583c024 str ip, [r3, #36]
30000108: e5832028 str r2, [r3, #40]
3000010c: e583e02c str lr, [r3, #44]
30000110: e583e030 str lr, [r3, #48]
30000114: e49df004 ldr pc, [sp], #4
30000118
30000118: e3a00eff mov r0, #4080 ; 0xff0
3000011c: e280000f add r0, r0, #15 ; 0xf
30000120: e3a02000 mov r2, #0 ; 0x0
30000124: e3a01203 mov r1, #805306368 ; 0x30000000
30000128: e4923004 ldr r3, [r2], #4
3000012c: e1520000 cmp r2, r0
30000130: e4813004 str r3, [r1], #4
30000134: 9afffffb bls 30000128
30000138: e1a0f00e mov pc, lr
3000013c
3000013c: e3a01456 mov r1, #1442840576 ; 0x56000000
30000140: e5913070 ldr r3, [r1, #112]
30000144: e38330a0 orr r3, r3, #160 ; 0xa0
30000148: e5813070 str r3, [r1, #112]
3000014c: e3a0c205 mov ip, #1342177280 ; 0x50000000
30000150: e3a0200c mov r2, #12 ; 0xc
30000154: e3a03003 mov r3, #3 ; 0x3
30000158: e3a00000 mov r0, #0 ; 0x0
3000015c: e5812078 str r2, [r1, #120]
30000160: e242224b sub r2, r2, #-1342177276 ; 0xb0000004
30000164: e3a01005 mov r1, #5 ; 0x5
30000168: e58c3000 str r3, [ip]
3000016c: e2833215 add r3, r3, #1342177281 ; 0x50000001
30000170: e4831008 str r1, [r3], #8
30000174: e5820000 str r0, [r2]
30000178: e3a0201a mov r2, #26 ; 0x1a
3000017c: e5830000 str r0, [r3]
30000180: e58c2028 str r2, [ip, #40]
30000184: e1a0f00e mov pc, lr
30000188
30000188: e20000ff and r0, r0, #255 ; 0xff
3000018c: e3a02205 mov r2, #1342177280 ; 0x50000000
30000190: e5923010 ldr r3, [r2, #16]
30000194: e3130004 tst r3, #4 ; 0x4
30000198: 0afffffc beq 30000190
3000019c: e5c20020 strb r0, [r2, #32]
300001a0: e1a0f00e mov pc, lr
300001a4
300001a4: e3a02205 mov r2, #1342177280 ; 0x50000000
300001a8: e5923010 ldr r3, [r2, #16]
300001ac: e3130001 tst r3, #1 ; 0x1
300001b0: 0afffffc beq 300001a8
300001b4: e5d20024 ldrb r0, [r2, #36]
300001b8: e1a0f00e mov pc, lr
300001bc
300001bc: e2400030 sub r0, r0, #48 ; 0x30
300001c0: e20000ff and r0, r0, #255 ; 0xff
300001c4: e3500009 cmp r0, #9 ; 0x9
300001c8: 83a00000 movhi r0, #0 ; 0x0
300001cc: 93a00001 movls r0, #1 ; 0x1
300001d0: e1a0f00e mov pc, lr
300001d4
300001d4: e20000ff and r0, r0, #255 ; 0xff
300001d8: e2403061 sub r3, r0, #97 ; 0x61
300001dc: e3530019 cmp r3, #25 ; 0x19
300001e0: e2400041 sub r0, r0, #65 ; 0x41
300001e4: e3a03001 mov r3, #1 ; 0x1
300001e8: 9a000002 bls 300001f8
300001ec: e3500019 cmp r0, #25 ; 0x19
300001f0: 83a03000 movhi r3, #0 ; 0x0
300001f4: 93a03001 movls r3, #1 ; 0x1
300001f8: e1a00003 mov r0, r3
300001fc: e1a0f00e mov pc, lr
30000200
30000200: e92d4010 stmdb sp!, {r4, lr}
30000204: ebffffcc bl 3000013c
30000208: ea000002 b 30000218
3000020c: e2840001 add r0, r4, #1 ; 0x1
30000210: e20000ff and r0, r0, #255 ; 0xff
30000214: ebffffdb bl 30000188
30000218: ebffffe1 bl 300001a4
3000021c: e20040ff and r4, r0, #255 ; 0xff
30000220: e1a00004 mov r0, r4
30000224: ebffffe4 bl 300001bc
30000228: e3500000 cmp r0, #0 ; 0x0
3000022c: e1a00004 mov r0, r4
30000230: 1afffff5 bne 3000020c
30000234: ebffffe6 bl 300001d4
30000238: e3500000 cmp r0, #0 ; 0x0
3000023c: 0afffff5 beq 30000218
30000240: eafffff1 b 3000020c
Disassembly of section .comment:
00000000 <.comment>:
0: 43434700 cmpmi r3, #0 ; 0x0
4: 4728203a undefined
8: 2029554e eorcs r5, r9, lr, asr #10
c: 2e342e33 mrccs 14, 1, r2, cr4, cr3, {1}
10: 47000035 smladxmi r0, r5, r0, r0
14: 203a4343 eorcss r4, sl, r3, asr #6
18: 554e4728 strplb r4, [lr, #-1832]
1c: 2e332029 cdpcs 0, 3, cr2, cr3, cr9, {1}
20: 00352e34 eoreqs r2, r5, r4, lsr lr
24: 43434700 cmpmi r3, #0 ; 0x0
28: 4728203a undefined
2c: 2029554e eorcs r5, r9, lr, asr #10
30: 2e342e33 mrccs 14, 1, r2, cr4, cr3, {1}
34: Address 0x34 is out of bounds.
eb00000a
1110 101 1 a
偏移地址 = a 左移2位 = 0xa * 4 = 0x28
disable_watch_dog = 30000000 + 8 +偏移地址 = 30000034