.globl _start//定義_start是全域性標籤，其他檔案也可以使用。.globl是GNU彙編語法。

_start:breset//跳轉到reset標籤出執行，由於reset操作是在MMU工作之前/之後都有可能使用，所以這裡用b來跳轉。

/*下面定義ARM異常向量表對應的跳轉程式碼*/

ldrpc, _undefined_instruction

ldrpc, _software_interrupt

ldrpc, _prefetch_abort

ldrpc, _data_abort

ldrpc, _not_used

ldrpc, _irq

ldrpc, _fiq

_undefined_instruction:.word undefined_instruction

_software_interrupt:.word software_interrupt

_prefetch_abort:.word prefetch_abort

_data_abort:.word data_abort

_not_used:.word not_used

_irq:.word irq

_fiq:.word fiq

/*其中.word的語法是用來指定該變數可以被C語言直接引用*/

.balignl 16,0xdeadbeef /* 這個指令用來填寫一個長字，即內容長度為長字，即

reset:

/*

* set the cpu to SVC32 mode

*/

mrsr0,cpsr/*

bicr0,r0,#0x1f/*位清除指令，即=r0&(~0x1f)*/

orrr0,r0,#0xd3/*邏輯或指令，即=r0|0xd3*/

msrcpsr,r0 /*傳送通用暫存器到CPSR指令*/

/* turn off the watchdog */

#if defined(CONFIG_S3C2400)

# define pWTCON0x15300000

# define INTMSK0x14400008/* Interupt-Controller base addresses */

# define CLKDIVN0x14800014/* clock divisor register */

#elif defined(CONFIG_S3C2410)

# define pWTCON0x53000000

# define INTMOD0X4A000004

# define INTMSK0x4A000008/* Interupt-Controller base addresses */

# define INTSUBMSK0x4A00001C

# define CLKDIVN0x4C000014/* clock divisor register */

#endif

#if defined(CONFIG_S3C2400) || defined(CONFIG_S3C2410)

ldrr0, =pWTCON

movr1, #0x0/*將0x0值傳入r1暫存器*/

str/*r1, [r0] 將r1種的內容寫入到r0中*/

/*

* mask all IRQs by setting all bits in the INTMR - default

*/

movr1, #0xffffffff

ldrr0, =INTMSK

strr1, [r0]

# if defined(CONFIG_S3C2410)

ldrr1, =0x3ff

ldrr0, =INTSUBMSK

strr1, [r0]

# endif

#endif/* CONFIG_S3C2400 || CONFIG_S3C2410 */

#ifndef CONFIG_SKIP_LOWLEVEL_INIT

blcpu_init_crit/*bl是帶跳轉返回指令*/

#endif

接下來看一下cpu_init_crit標籤的實現，如下：

cpu_init_crit:

/*

* flush v4 I/D caches

*/

movr0, #0

mcrp15, 0, r0, c7, c7, 0/* ARM 處理器暫存器到協處理器暫存器的資料傳送指令，清除I、D cache和分支運算快取（BTB）*/

mcrp15, 0, r0, c8, c7, 0/* flush v4 TLB（Translation Lookaside Buffers） */

/*

* disable MMU stuff and caches

*/

mrcp15, 0, r0, c1, c0, 0

bicr0, r0, #[email protected] clear bits 13, 9:8 (--V- --RS)

bicr0, r0, #[email protected] clear bits 7, 2:0 (B--- -CAM)

orrr0, r0, #[email protected] set bit 2 (A) Align

orrr0, r0, #[email protected] set bit 12 (I) I-Cache

mcrp15, 0, r0, c1, c0, 0

/*

* before relocating, we have to setup RAM timing

* because memory timing is board-dependend, you will

* find a lowlevel_init.S in your board directory.

*/

movip, lr

bllowlevel_init /*跳轉到lowlevel_init，處理完返回*/

movlr, ip

movpc, lr

.globl lowlevel_init

lowlevel_init:

/* memory control configuration */

/* make r0 relative the current location so that it */

/* reads SMRDATA out of FLASH rather than memory ! */

ldrr0, =SMRDATA

ldrr1, _TEXT_BASE

subr0, r0, r1

ldrr1, =BWSCON/* Bus Width Status Controller */

addr2, r0, #13*4

0:

ldrr3, [r0], #4

strr3, [r1], #4

cmpr2, r0

bne0b

/* everything is fine now */

movpc, lr

stack_setup:

ldrr0, _TEXT_BASE/* upper 128 KiB: relocated uboot*/

subr0, r0, #CFG_MALLOC_LEN/* malloc area*/

subr0, r0, #CFG_GBL_DATA_SIZE /* bdinfo*/

subsp, r0, #12/* leave 3 words for abort-stack*/

relocate:/* relocate U-Boot to RAM*/

adrr0, _start/* r0 <- current position of code*/

ldrr1, _TEXT_BASE/* test if we run from flash or RAM */

cmpr0, r1/* don't reloc during debug*/

beqclear_bss

ldrr2, _armboot_start

ldrr3, _bss_start

subr2, r3, r2/* r2 <- size of armboot*/

blCopyCode2Ram/* r0: source, r1: dest, r2: size */

clear_bss:

ldrr0, _bss_start/* find start of bss segment*/

ldrr1, _bss_end/* stop here*/

mov r2, #0x00000000/* clear*/

clbss_l:strr2, [r0]/* clear loop...*/

addr0, r0, #4

cmpr0, r1

bleclbss_l

ldrpc, _start_armboot

_start_armboot:.word start_armboot /*start_armboot是C程式碼函式，在lib_arm/board.c中實現*/