ARM7ldr指令与ldr伪指令
2015-12-10 10:44 cascle 阅读(588) 评论(0) 编辑 收藏 举报ldr伪指令的第二个操作数之前有个=,意思是第一个操作书 = 第二个操作数,相当明了
核心就在于对于用.word指令在.text段里另外定义一段内存,用ldr r0,[pc + x(可以算出.text段里的内存地址)]这种基于PC的偏移量方式加载内存里的内容到寄存器
看下源代码和反汇编的结果就清楚了
伪指令用于大的常数:
源代码:
1 top: 2 ldr r0,=12345678 3 add r1,r2,r3 4 eor r1,r2,r3 5 eor r1,r2,r3 6 bottom: 7 b top
反汇编:
prebuilts/gcc/linux-x86/arm/arm-eabi-4.8/bin$ ./arm-eabi-objdump -d test.o test.o: file format elf32-littlearm Disassembly of section .text: 00000000 <top>: 0: e59f000c ldr r0, [pc, #12] ; 14 <bottom+0x4> 4: e0821003 add r1, r2, r3 8: e0221003 eor r1, r2, r3 c: e0221003 eor r1, r2, r3 00000010 <bottom>: 10: eafffffa b 0 <top> 14: 00bc614e .word 0x00bc614e
伪指令用于标签:
源代码:
1 top: 2 ldr r0,=bottom 3 add r1,r2,r3 4 eor r1,r2,r3 5 eor r1,r2,r3 6 bottom: 7 b top
反汇编:
prebuilts/gcc/linux-x86/arm/arm-eabi-4.8/bin$ ./arm-eabi-objdump -d test.o test.o: file format elf32-littlearm Disassembly of section .text: 00000000 <top>: 0: e59f000c ldr r0, [pc, #12] ; 14 <bottom+0x4> 4: e0821003 add r1, r2, r3 8: e0221003 eor r1, r2, r3 c: e0221003 eor r1, r2, r3 00000010 <bottom>: 10: eafffffa b 0 <top> 14: 00000010 .word 0x00000010
ldr指令用于常数:
源代码:
1 top: 2 ldr r0,[r0] 3 add r1,r2,r3 4 eor r1,r2,r3 5 eor r1,r2,r3 6 bottom: 7 b top
反汇编:
prebuilts/gcc/linux-x86/arm/arm-eabi-4.8/bin$ ./arm-eabi-objdump -d test.o test.o: file format elf32-littlearm Disassembly of section .text: 00000000 <top>: 0: e5900000 ldr r0, [r0] 4: e0821003 add r1, r2, r3 8: e0221003 eor r1, r2, r3 c: e0221003 eor r1, r2, r3 00000010 <bottom>: 10: eafffffa b 0 <top>
ldr指令用于标签:
源代码:
1 top: 2 ldr r0,bottom 3 add r1,r2,r3 4 eor r1,r2,r3 5 eor r1,r2,r3 6 bottom: 7 b top
反汇编:
prebuilts/gcc/linux-x86/arm/arm-eabi-4.8/bin$ ./arm-eabi-objdump -d test.o test.o: file format elf32-littlearm Disassembly of section .text: 00000000 <top>: 0: e59f0008 ldr r0, [pc, #8] ; 10 <bottom> 4: e0821003 add r1, r2, r3 8: e0221003 eor r1, r2, r3 c: e0221003 eor r1, r2, r3 00000010 <bottom>: 10: eafffffa b 0 <top>