嵌入式Linux開發(fā)環(huán)境的搭建之：U-Boot移植

2．U-Boot重要代碼

（1）cpu/arm920t/start.S

這是U-Boot的起始位置。在這個文件中設(shè)置了處理器的狀態(tài)、初始化中斷向量和內(nèi)存時序等，從Flash中跳轉(zhuǎn)到定位好的內(nèi)存位置執(zhí)行。

.globl_start（起始位置：中斷向量設(shè)置）

_start:breset

ldrpc,_undefined_instruction

ldrpc,_software_interrupt

ldrpc,_prefetch_abort

ldrpc,_data_abort

ldrpc,_not_used

ldrpc,_irq

ldrpc,_fiq

_undefined_instruction:.wordundefined_instruction

_software_interrupt:.wordsoftware_interrupt

_prefetch_abort:.wordprefetch_abort

_data_abort:.worddata_abort

_not_used:.wordnot_used

_irq:.wordirq

_fiq:.wordfiq

_TEXT_BASE:（代碼段起始位置）

.wordTEXT_BASE

.globl_armboot_start

_armboot_start:

.word_start

/*

*Thesearedefinedintheboard-specificlinkerscript.

*/

.globl_bss_start（BSS段起始位置）

_bss_start:

.word__bss_start

.globl_bss_end

_bss_end:

.word_end

reset:（執(zhí)行入口）

/*

*setthecputoSVC32mode;使處理器進(jìn)入特權(quán)模式

*/

mrsr0,cpsr

bicr0,r0,#0x1f

orrr0,r0,#0xd3

msrcpsr,r0

relocate:（代碼的重置）/*relocateU-BoottoRAM*/

adrr0,_start/*r0-currentpositionofcode*/

ldrr1,_TEXT_BASE/*testifwerunfromflashorRAM*/

cmpr0,r1/*dontrelocduringdebug*/

beqstack_setup

ldrr2,_armboot_start

ldrr3,_bss_start

subr2,r3,r2/*r2-sizeofarmboot*/

addr2,r0,r2/*r2-sourceendaddress*/

copy_loop:（拷貝過程）

ldmiar0!,{r3-r10}/*copyfromsourceaddress[r0]*/

stmiar1!,{r3-r10}/*copytotargetaddress[r1]*/

cmpr0,r2/*untilsourceendaddreee[r2]*/

blecopy_loop

/*Setupthestack；設(shè)置堆棧*/

stack_setup:

ldrr0,_TEXT_BASE/*upper128KiB:relocateduboot*/

subr0,r0,#CFG_MALLOC_LEN/*mallocarea*/

subr0,r0,#CFG_GBL_DATA_SIZE/*bdinfo*/

clear_bss:（清空BSS段）

ldrr0,_bss_start/*findstartofbsssegment*/

ldrr1,_bss_end/*stophere*/

movr2,#0x00000000/*clear*/

clbss_l:strr2,[r0]/*clearloop...*/

addr0,r0,#4

cmpr0,r1

bneclbss_l

ldrpc,_start_armboot

_start_armboot:.wordstart_armboot

（2）interrupts.c

這個文件是處理中斷的，如打開和關(guān)閉中斷等。

#ifdefCONFIG_USE_IRQ

/*enableIRQinterrupts；中斷使能函數(shù)*/

voidenable_interrupts(void)

{

unsignedlongtemp;

__asm____volatile__(mrs%0,cpsrn

bic%0,%0,#0x80n

msrcpsr_c,%0

:=r(temp)

:

:memory);

}

/*

*disableIRQ/FIQinterrupts；中斷屏蔽函數(shù)

*returnstrueifinterruptshadbeenenabledbeforewedisabledthem

*/

intdisable_interrupts(void)

{

unsignedlongold,temp;

__asm____volatile__(mrs%0,cpsrn

orr%1,%0,#0xc0n

msrcpsr_c,%1

:=r(old),=r(temp)

:

:memory);

return(old0x80)==0;

}

#endif

voidshow_regs(structpt_regs*regs)

{

unsignedlongflags;

constchar*processor_modes[]={

USER_26,FIQ_26,IRQ_26,SVC_26,

UK4_26,UK5_26,UK6_26,UK7_26,

UK8_26,UK9_26,UK10_26,UK11_26,

UK12_26,UK13_26,UK14_26,UK15_26,

USER_32,FIQ_32,IRQ_32,SVC_32,

UK4_32,UK5_32,UK6_32,ABT_32,

UK8_32,UK9_32,UK10_32,UND_32,

UK12_32,UK13_32,UK14_32,SYS_32,

};

…

}

/*在U-Boot啟動模式下，在原則上要禁止中斷處理，所以如果發(fā)生中斷，當(dāng)作出錯處理*/

voiddo_fiq(structpt_regs*pt_regs)

{

printf(fastinterruptrequestn);

show_regs(pt_regs);

bad_mode();

}

voiddo_irq(structpt_regs*pt_regs)

{

printf(interruptrequestn);

show_regs(pt_regs);

bad_mode();

}

（3）cpu.c

這個文件是對處理器進(jìn)行操作，如下所示：

intcpu_init(void)

{

/*

*setupupstacksifnecessary；設(shè)置需要的堆棧

*/

#ifdefCONFIG_USE_IRQ

DECLARE_GLOBAL_DATA_PTR;

IRQ_STACK_START=_armboot_start-CFG_MALLOC_LEN-CFG_GBL_DATA_SIZE-4;

FIQ_STACK_START=IRQ_STACK_START-CONFIG_STACKSIZE_IRQ;

#endif

return0;

}

intcleanup_before_linux(void)/*準(zhǔn)備加載linux*/

{

/*

*thisfunctioniscalledjustbeforewecalllinux

*itpreparestheprocessorforlinux

*

*weturnoffcachesetc...

*/

unsignedlongi;

disable_interrupts();

/*turnoffI/D-cache：關(guān)閉cache*/

asm(mrcp15,0,%0,c1,c0,0:=r(i));

i=~(C1_DC|C1_IC);

asm(mcrp15,0,%0,c1,c0,0::r(i));

/*flushI/D-cache*/

i=0;

asm(mcrp15,0,%0,c7,c7,0::r(i));

return(0);

}

OUTPUT_ARCH(arm)

ENTRY(_start)

SECTIONS

{

.=0x00000000;

.=ALIGN(4);

.text:

{

cpu/arm920t/start.o(.text)

*(.text)

}

.=ALIGN(4);

.rodata:{*(.rodata)}

.=ALIGN(4);

.data:{*(.data)}

.=ALIGN(4);

.got:{*(.got)}

__u_boot_cmd_start=.;

.u_boot_cmd:{*(.u_boot_cmd)}

__u_boot_cmd_end=.;

.=ALIGN(4);

__bss_start=.;

.bss:{*(.bss)}

_end=.;

}