位置無(wú)關(guān)碼、位置有關(guān)碼、鏈接地址、加載地址

在移植 uboot 時(shí)，接觸到一個(gè)概念叫做位置無(wú)關(guān)碼，那么與它對(duì)應(yīng)的就是位置有關(guān)碼。提到這兩個(gè)概念就還得提一提鏈接地址、加載地址

鏈接地址，鏈接腳本里指定的，理論上程序運(yùn)行時(shí)所處的地址。在編譯時(shí)，編譯器會(huì)根據(jù)鏈接地址來(lái)翻譯位置有關(guān)碼。

加載地址，程序運(yùn)行時(shí)，實(shí)際所處的地址。

位置無(wú)關(guān)碼，位置有關(guān)碼，是相對(duì)于一條指令的正常目的來(lái)說(shuō)的。比如 ldr r0 ,=標(biāo)號(hào)，它的正常目的是取得標(biāo)號(hào)處的地址，對(duì)于這個(gè)目的，它是位置有關(guān)碼，運(yùn)行的地址不對(duì)就獲取不到正確的標(biāo)號(hào)地址，其實(shí)它無(wú)論在哪都是獲取的程序加載地址等于鏈接地址時(shí)，標(biāo)號(hào)的地址，如果你就是想要這個(gè)值，那么用這條指令是非常正確的，就不用理會(huì)什么位置無(wú)關(guān)碼，位置有關(guān)碼的概念了，這一點(diǎn)非常重要。

因此，當(dāng)加載地址不等于鏈接地址時(shí)，并不是不可以用位置無(wú)關(guān)碼，而是要看你用位置無(wú)關(guān)碼是否達(dá)到了你想要的目的。

位置無(wú)關(guān)碼，依賴于程序當(dāng)前運(yùn)行的PC值，進(jìn)行相對(duì)的跳轉(zhuǎn)，導(dǎo)致的結(jié)果就是，無(wú)論代碼在哪，總能達(dá)到指令的正常目的，因此是位置無(wú)關(guān)的。

位置有關(guān)碼，不依賴當(dāng)前PC值，是絕對(duì)跳轉(zhuǎn)，只有程序運(yùn)行在鏈接地址處時(shí)，才能達(dá)到指令的正常目的，因此是位置有關(guān)系的。

下面，我們來(lái)看常用的匯編指令以及C語(yǔ)言中哪些操作是位置有關(guān)碼，哪些是位置無(wú)關(guān)碼。

SECTIONS { . = 0x33f80000; .text : { *(.text) } . = ALIGN(4); .rodata : {*(.rodata*)} . = ALIGN(4); .data : { *(.data) } . = ALIGN(4); __bss_start = .; .bss : { *(.bss) *(COMMON) } __bss_end = .; }

SECTIONS {??

????. = 0x33f80000;??

????.text : { *(.text) }??

??????

????. = ALIGN(4);??

????.rodata : {*(.rodata*)}??

??????

????. = ALIGN(4);??

????.data : { *(.data) }??

??????

????. = ALIGN(4);??

????__bss_start = .;??

????.bss : { *(.bss)??*(COMMON) }??

????__bss_end = .;??

}

.text .global _start _start: bl close_watch_dog /* 相對(duì)跳轉(zhuǎn)，位置無(wú)關(guān) */ bl _start adr r0, close_watch_dog /* 獲取標(biāo)號(hào)地址，位置無(wú)關(guān) */ ldr r0, SMRDATA /* 獲取標(biāo)號(hào)處的值，位置無(wú)關(guān) */ ldr r0, =0x12345678 ldr r0, =SMRDATA /* 獲取標(biāo)號(hào)地址，位置有關(guān) */ ldr r0, =main /* 獲取函數(shù)名的地址，位置有關(guān) */ ldr r0 ,=__bss_start /* 獲取鏈接腳本里標(biāo)號(hào)的地址，位置有關(guān) */ close_watch_dog: mov r1, #0 str r1, [r0] mov pc, lr SMRDATA: .word 0x22111120

.text??

.global _start??

_start:??

????bl close_watch_dog??????/* 相對(duì)跳轉(zhuǎn)，位置無(wú)關(guān) */??

????bl _start??

????adr r0, close_watch_dog /* 獲取標(biāo)號(hào)地址，位置無(wú)關(guān) */??

??????

????ldr r0, SMRDATA???????? /* 獲取標(biāo)號(hào)處的值，位置無(wú)關(guān) */??

????ldr r0, =0x12345678??

????ldr r0, =SMRDATA????????/* 獲取標(biāo)號(hào)地址，位置有關(guān) */??

????ldr r0, =main?????????? /* 獲取函數(shù)名的地址，位置有關(guān) */??

????ldr r0 ,=__bss_start????/* 獲取鏈接腳本里標(biāo)號(hào)的地址，位置有關(guān) */??

??????

close_watch_dog:??

????mov r1, #0??

????str r1, [r0]??

????mov pc, lr??

??????

SMRDATA:??

????.word??0x22111120

int a; void abc(){ a = 2; } int main(){ int b; a =1 ; b =1 ; abc(); return 0; }

int a;??

void abc(){??

????a = 2;??

}??

int main(){??

????int b;??

????a =1 ;??

????b =1 ;??

????abc();??

????return 0;??

}

如果加載地址為 0 ，那么代碼將按照下面的順序排放

00000000 <_start>: 00000000: eb000006 bl 33f80020 00000004: ebfffffd bl 33f80000 <_start> 00000008: e28f0010 add r0, pc, #16 0000000c: e59f0018 ldr r0, [pc, #24] ; 00000010: e59f0018 ldr r0, [pc, #24] ; 00000014: e59f0018 ldr r0, [pc, #24] ; 00000018: e59f0018 ldr r0, [pc, #24] ; 0000001c: e59f0018 ldr r0, [pc, #24] ; 00000020 : 00000020: e3a01000 mov r1, #0 00000024: e5801000 str r1, [r0] 00000028: e1a0f00e mov pc, lr 0000002c : 0000002c: 22111120 andscs r1, r1, #8 00000030: 12345678 eorsne r5, r4, #125829120 ; 0x7800000 00000034: 33f8002c mvnscc r0, #44 ; 0x2c 00000038: 33f80064 mvnscc r0, #100 ; 0x64 0000003c: 33f800a0 mvnscc r0, #160 ; 0xa0 00000040 : 00000040: e52db004 push {fp} ; (str fp, [sp, #-4]!) 00000044: e28db000 add fp, sp, #0 00000048: e59f3010 ldr r3, [pc, #16] ; 33f80060 0000004c: e3a02002 mov r2, #2 00000050: e5832000 str r2, [r3] 00000054: e28bd000 add sp, fp, #0 00000058: e8bd0800 pop {fp} 0000005c: e12fff1e bx lr 00000060: 33f800a0 mvnscc r0, #160 ; 0xa0 00000064

: 00000064: e92d4800 push {fp, lr} 00000068: e28db004 add fp, sp, #4 0000006c: e24dd008 sub sp, sp, #8 00000070: e59f3024 ldr r3, [pc, #36] ; 33f8009c 00000074: e3a02001 mov r2, #1 00000078: e5832000 str r2, [r3] 0000007c: e3a03001 mov r3, #1 00000080: e50b3008 str r3, [fp, #-8] 00000084: ebffffed bl 33f80040 00000088: e3a03000 mov r3, #0 0000008c: e1a00003 mov r0, r3 00000090: e24bd004 sub sp, fp, #4 00000094: e8bd4800 pop {fp, lr} 00000098: e12fff1e bx lr 0000009c: 33f800a0 mvnscc r0, #160 ; 0xa0

00000000 <_start>:??

00000000:?? eb000006????bl??33f80020 <close_watch_dog>??

00000004:?? ebfffffd????bl??33f80000 <_start>??

00000008:?? e28f0010????add r0, pc, #16??

0000000c:?? e59f0018????ldr r0, [pc, #24]?? ;??

00000010:?? e59f0018????ldr r0, [pc, #24]?? ;??

00000014:?? e59f0018????ldr r0, [pc, #24]?? ;??

00000018:?? e59f0018????ldr r0, [pc, #24]?? ;??

0000001c:?? e59f0018????ldr r0, [pc, #24]?? ;??

00000020 <close_watch_dog>:??

00000020:?? e3a01000????mov r1, #0??

00000024:?? e5801000????str r1, [r0]??

00000028:?? e1a0f00e????mov pc, lr??

0000002c <SMRDATA>:??

0000002c:?? 22111120????andscs??r1, r1, #8??

00000030:?? 12345678????eorsne??r5, r4, #125829120??; 0x7800000??

00000034:?? 33f8002c????mvnscc??r0, #44 ; 0x2c??

00000038:?? 33f80064????mvnscc??r0, #100????; 0x64??

0000003c:?? 33f800a0????mvnscc??r0, #160????; 0xa0??

00000040 <abc>:??

00000040:?? e52db004????push????{fp}????????; (str fp, [sp, #-4]!)??

00000044:?? e28db000????add fp, sp, #0??

00000048:?? e59f3010????ldr r3, [pc, #16]?? ; 33f80060 ??

0000004c:?? e3a02002????mov r2, #2??

00000050:?? e5832000????str r2, [r3]??

00000054:?? e28bd000????add sp, fp, #0??

00000058:?? e8bd0800????pop {fp}??

0000005c:?? e12fff1e????bx??lr??

00000060:?? 33f800a0????mvnscc??r0, #160????; 0xa0??

00000064 <main>:??

00000064:?? e92d4800????push????{fp, lr}??

00000068:?? e28db004????add fp, sp, #4??

0000006c:?? e24dd008????sub sp, sp, #8??

00000070:?? e59f3024????ldr r3, [pc, #36]?? ; 33f8009c ??

00000074:?? e3a02001????mov r2, #1??

00000078:?? e5832000????str r2, [r3]??

0000007c:?? e3a03001????mov r3, #1??

00000080:?? e50b3008????str r3, [fp, #-8]??

00000084:?? ebffffed????bl??33f80040 <abc>??

00000088:?? e3a03000????mov r3, #0??

0000008c:?? e1a00003????mov r0, r3??

00000090:?? e24bd004????sub sp, fp, #4??

00000094:?? e8bd4800????pop {fp, lr}??

00000098:?? e12fff1e????bx??lr??

0000009c:?? 33f800a0????mvnscc??r0, #160????; 0xa0

如果加載地址為0x33f80000 則按照下邊的順序排放

33f80000 <_start>: 33f80000: eb000006 bl 33f80020 33f80004: ebfffffd bl 33f80000 <_start> 33f80008: e28f0010 add r0, pc, #16 33f8000c: e59f0018 ldr r0, [pc, #24] ; 33f8002c 33f80010: e59f0018 ldr r0, [pc, #24] ; 33f80030 33f80014: e59f0018 ldr r0, [pc, #24] ; 33f80034 33f80018: e59f0018 ldr r0, [pc, #24] ; 33f80038 33f8001c: e59f0018 ldr r0, [pc, #24] ; 33f8003c 33f80020 : 33f80020: e3a01000 mov r1, #0 33f80024: e5801000 str r1, [r0] 33f80028: e1a0f00e mov pc, lr 33f8002c : 33f8002c: 22111120 andscs r1, r1, #8 33f80030: 12345678 eorsne r5, r4, #125829120 ; 0x7800000 33f80034: 33f8002c mvnscc r0, #44 ; 0x2c 33f80038: 33f80064 mvnscc r0, #100 ; 0x64 33f8003c: 33f800a0 mvnscc r0, #160 ; 0xa0 33f80040 : 33f80040: e52db004 push {fp} ; (str fp, [sp, #-4]!) 33f80044: e28db000 add fp, sp, #0 33f80048: e59f3010 ldr r3, [pc, #16] ; 33f80060 33f8004c: e3a02002 mov r2, #2 33f80050: e5832000 str r2, [r3] 33f80054: e28bd000 add sp, fp, #0 33f80058: e8bd0800 pop {fp} 33f8005c: e12fff1e bx lr 33f80060: 33f800a0 mvnscc r0, #160 ; 0xa0 33f80064

: 33f80064: e92d4800 push {fp, lr} 33f80068: e28db004 add fp, sp, #4 33f8006c: e24dd008 sub sp, sp, #8 33f80070: e59f3024 ldr r3, [pc, #36] ; 33f8009c 33f80074: e3a02001 mov r2, #1 33f80078: e5832000 str r2, [r3] 33f8007c: e3a03001 mov r3, #1 33f80080: e50b3008 str r3, [fp, #-8] 33f80084: ebffffed bl 33f80040 33f80088: e3a03000 mov r3, #0 33f8008c: e1a00003 mov r0, r3 33f80090: e24bd004 sub sp, fp, #4 33f80094: e8bd4800 pop {fp, lr} 33f80098: e12fff1e bx lr 33f8009c: 33f800a0 mvnscc r0, #160 ; 0xa0 Disassembly of section .bss: 33f800a0 : 33f800a0: 00000000 andeq r0, r0, r0