使用ldr命令注意事項(xiàng)
[導(dǎo)讀]今天犯了一個(gè)小錯(cuò)誤，調(diào)試了幾個(gè)小時(shí)；最后重新分析了幾遍反匯編才發(fā)現(xiàn)。萬(wàn)丈高樓平地起、勿以浮沙駐高臺(tái)。錯(cuò)誤代碼：.text
.global?_start
_start:
????b?reset
????
今天犯了一個(gè)小錯(cuò)誤，調(diào)試了幾個(gè)小時(shí)；最后重新分析了幾遍反匯編才發(fā)現(xiàn)。萬(wàn)丈高樓平地起、勿以浮沙駐高臺(tái)。
錯(cuò)誤代碼：
.text
.global?_start
_start:
????b?reset
????ldr?pc,=und_addr
????ldr?pc,=swi_addr
und_addr:
????.word?undef
swi_addr:
????.word?swi_handle
undef:
????ldr?sp,=0x32000000?//處理異常調(diào)用C函數(shù)，設(shè)置und模式下的棧，以前是usr模式
????//保存現(xiàn)場(chǎng)
????stmdb?sp!,{r0-r12,lr}
????bl?print2
????//處理異常
????mrs?r0,cpsr?//und模式下的cpsr寄存器，也可以改為輸出spsr低5位和usr模式的cpsr相同
????ldr?r1,=und_string
????bl?printException
????//恢復(fù)現(xiàn)場(chǎng)
????ldmia?sp!,{r0-r12,pc}^
und_string:
????.string?"undefined?instruction?exception"
.align?4
swi_handle:?
????ldr?sp,=0x33000000?//處理異常調(diào)用C函數(shù)，設(shè)置svc模式下的棧，以前是usr模式
????//保存現(xiàn)場(chǎng)
????stmdb?sp!,{r0-r12,lr}
????//處理異常
????mrs?r0,cpsr?//und模式下的cpsr寄存器，也可以改為輸出spsr低5位和svc模式的cpsr相同
????ldr?r1,=swi_string
????bl?printException
????//恢復(fù)現(xiàn)場(chǎng)
????ldmia?sp!,{r0-r12,pc}^
swi_string:
????.string?"swi?exception"
.align?4????
reset:
//關(guān)看門狗
????ldr?r1,=0x53000000
????ldr?r0,=0
????str?r0,[r1]
//設(shè)置時(shí)鐘
????//①?設(shè)置鎖定時(shí)間LOCKTIME
????ldr?r1,=0x4C000000
????ldr?r0,=0xFFFFFFFF
????str?r0,[r1]
????//②?設(shè)置分頻系數(shù)，使得FCLK:HCLK:PCLK?=?8:4:1
????ldr?r1,=0x4C000014
????ldr?r0,=0x5
????str?r0,[r1]
????//③?根據(jù)2440規(guī)則，由于②中HDIVN!=0?->?CPU總線模式從快速總線模式到異步模式
????mrc?p15,0,r0,c1,c0,0
????orr?r0,r0,#0xc0000000?//R1_nF:OR:R1_iA
????mcr?p15,0,r0,c1,c0,0
????//④?設(shè)置PLL，設(shè)置完P(guān)LL后，PLL開始工作，鎖定時(shí)間內(nèi)CPU停止工作
????/*?設(shè)置FCLK?=?400MHZ，HCLK?=?100MHZ，?PCLK?=?100MHZ?
?????*?Mpll(FCLK)?=?(2*m?*?Fin)?/?(p?*?2^s)
?????*?m?=?M(MDIV)?(the?value?for?divider?M)+?8,
?????*?p?=?P(PDIV)?(the?value?for?divider?P)?+?2,?s?=?SDIV
?????*?MDIV?=?92(0x5c),?PDIV?=?1?,?SDIV?=?1?
?????*?FCLK?=?(2*100*12MHZ)?/?(3*2^1)?=?400MHZ?*/
?????ldr?r1,=0x4C000004
?????ldr?r0,=?((92<<12)?|?(1<<4)?|?(1<<0))
?????str?r0,[r1]
//判斷啟動(dòng)方式
????mov?r1,#0?//?r1?=?0;
????ldr?r0,[r1]?//?r0?=?[0];將0地址的值備份
????str?r1,[r1]?//?將0寫入0地址
????ldr?r2,[r1]?//?再將0地址的值讀出來(lái)給r2
????cmp?r1,r2???//?比較r1和r2，即比較0地址原來(lái)的值和寫入的值
????ldr?sp,=?0x40000000?+?4096?//?先假設(shè)為nor啟動(dòng)
????ldreq?sp,=?4096?//如果r1==r2，假設(shè)不成立為Nand啟動(dòng)
????streq?r0,[r1]???//?還原0地址以前的值
????bl?sdram_init
????//bl?sdram_init2?//用到有初始值的數(shù)組，不是位置無(wú)關(guān)碼????
????bl?copy2sdram???
????bl?clean_bss
????//從復(fù)位后的SVC模式切換到USR模式
????//M[4:0]?SVC:10011?USR:10000?
????mrs?r0,cpsr
????bic?r0,r0,#0xf?//修改低四位，進(jìn)入usr模式
????msr?cpsr,r0
????//設(shè)置usr模式下的棧，sp_usr
????ldr?sp,=0x34000000;
????ldr?pc,=sdram_next
sdram_next:?????
????bl?uart0_init
????mrs?r0,cpsr??//SVC模式下的cpsr寄存器
????bl?print1
????bl?print2
????swi?0x1??/*?執(zhí)行此命令,?觸發(fā)SWI異常,?進(jìn)入0x8執(zhí)行?*/
und_code:
????.word?0xeeadc0de??/*?未定義指令?*/
????bl?main??/*?使用BL命令相對(duì)跳轉(zhuǎn),?程序仍然在NOR/sram執(zhí)行?*/
????//ldr?pc,?=main??/*?絕對(duì)跳轉(zhuǎn),?跳到SDRAM?*/
halt:
????b?halt結(jié)果：
分析：
sdram.elf:?????file?format?elf32-littlearm
Disassembly?of?section?.text:
30000000:
30000000:???ea00001e????b???3000008030000004:???e59ff110????ldr?pc,?[pc,?#272]??;?3000011c?
30000008:???e59ff110????ldr?pc,?[pc,?#272]??;?30000120?
3000000c:
3000000c:???30000014????andcc???r0,?r0,?r4,?lsl?r0
30000010:
30000010:???30000050????andcc???r0,?r0,?r0,?asr?r0
*************************************************
3000011c:???3000000c????andcc???r0,?r0,?ip
30000120:???30000010????andcc???r0,?r0,?r0,?lsl?r0
*************************************************
30000014:
30000014:???e3a0d432????mov?sp,?#838860800??;?0x32000000
30000018:???e92d5fff????stmdb???sp!,?{r0,?r1,?r2,?r3,?r4,?r5,?r6,?r7,?r8,?r9,?sl,?fp,?ip,?lr}
3000001c:???eb000103????bl??3000043030000020:???e10f0000????mrs?r0,?CPSR
30000024:???e59f10f8????ldr?r1,?[pc,?#248]??;?30000124?
30000028:???eb000216????bl??300008883000002c:???e8fd9fff????ldmia???sp!,?{r0,?r1,?r2,?r3,?r4,?r5,?r6,?r7,?r8,?r9,?sl,?fp,?ip,?pc}^
30000030:
30000030:???65646e75????strvsb??r6,?[r4,?#-3701]!
........發(fā)生異常的時(shí)候，異常向量表中pc沒有跳轉(zhuǎn)到相應(yīng)的執(zhí)行函數(shù)地址，因?yàn)槲也捎昧碎g接的賦值。兩次異常都是跳轉(zhuǎn)到und_addr或者swi_addr開始往后面執(zhí)行，所以兩次都執(zhí)行了undef處理函數(shù)。
更正測(cè)試：
①直接賦值到對(duì)應(yīng)執(zhí)行函數(shù)處
.text .global?_start _start: ????b?reset ????ldr?pc,=undef ????ldr?pc,=swi_handle
反匯編：
sdram.elf:?????file?format?elf32-littlearm
Disassembly?of?section?.text:
***************************************
3000011c:???30000014????andcc???r0,?r0,?r4,?lsl?r0
30000120:???30000050????andcc???r0,?r0,?r0,?asr?r0
***************************************
30000000:
30000000:???ea00001e????b???3000008030000004:???e59ff110????ldr?pc,?[pc,?#272]??;?3000011c?
30000008:???e59ff110????ldr?pc,?[pc,?#272]??;?30000120?
3000000c:
3000000c:???30000014????andcc???r0,?r0,?r4,?lsl?r0
30000010:
30000010:???30000050????andcc???r0,?r0,?r0,?asr?r0
30000014:
30000014:???e3a0d432????mov?sp,?#838860800??;?0x32000000
30000018:???e92d5fff????stmdb???sp!,?{r0,?r1,?r2,?r3,?r4,?r5,?r6,?r7,?r8,?r9,?sl,?fp,?ip,?lr}
3000001c:???eb000103????bl??3000043030000020:???e10f0000????mrs?r0,?CPSR
30000024:???e59f10f8????ldr?r1,?[pc,?#248]??;?30000124?
30000028:???eb000216????bl??300008883000002c:???e8fd9fff????ldmia???sp!,?{r0,?r1,?r2,?r3,?r4,?r5,?r6,?r7,?r8,?r9,?sl,?fp,?ip,?pc}^
30000030:
...
30000050:
30000050:???e3a0d433????mov?sp,?#855638016??;?0x33000000
...結(jié)果：
② 間接賦值
.text .global?_start _start: ????b?reset ????ldr?pc,und_addr ????ldr?pc,swi_addr
反匯編：
sdram.elf:?????file?format?elf32-littlearm
Disassembly?of?section?.text:
30000000:
30000000:???ea00001e????b???3000008030000004:???e59ff110????ldr?pc,?[pc,?#272]??;?3000011c?
30000008:???e59ff110????ldr?pc,?[pc,?#272]??;?30000120?
*********************
3000011c:???30000014????andcc???r0,?r0,?r4,?lsl?r0
30000120:???30000050????andcc???r0,?r0,?r0,?asr?r0
*********************
3000000c:
3000000c:???30000014????andcc???r0,?r0,?r4,?lsl?r0
30000010:
30000010:???30000050????andcc???r0,?r0,?r0,?asr?r0
30000014:
30000014:???e3a0d432????mov?sp,?#838860800??;?0x32000000
30000018:???e92d5fff????stmdb???sp!,?{r0,?r1,?r2,?r3,?r4,?r5,?r6,?r7,?r8,?r9,?sl,?fp,?ip,?lr}
3000001c:???eb000103????bl??3000043030000020:???e10f0000????mrs?r0,?CPSR
30000024:???e59f10f8????ldr?r1,?[pc,?#248]??;?30000124?
30000028:???eb000216????bl??300008883000002c:???e8fd9fff????ldmia???sp!,?{r0,?r1,?r2,?r3,?r4,?r5,?r6,?r7,?r8,?r9,?sl,?fp,?ip,?pc}^
30000030:
...
30000050:
30000050:???e3a0d433????mov?sp,?#855638016??;?0x33000000
...結(jié)果：
總結(jié)：
注意ldr pc,=czg與ldr pc,czg 用法：
① ?ldr pc,=czg =》 pc = czg (就是czg標(biāo)號(hào)的地址)
② ? ldr pc,czg =》 pc = *(czg) (czg標(biāo)號(hào)地址中放的值)
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