bootloader理论上被分成了stage1(boot code)和stage2(main code)两部分，而boot code部分代码比较难看懂，都是汇编，我也是弄了好些天才搞懂的，真是不易，拿出来大家分享下吧！
我用的是三星的一款开发板，s3c2440，基于arm9的处理器，由于字数限制，一下我只列出了比较难懂的部分。
;=========================================
; NAME: 2440INIT.S
; DESC: C start up codes
; Configure memory, ISR ,stacks
; Initialize C-variables
; HISTORY:
; 2002.02.25:kwtark: ver 0.0
; 2002.03.20:purnnamu: Add some functions for testing STOP,Sleep mode
; 2003.03.14:DonGo: Modified for 2440.
;=========================================
GET option.inc
GET memcfg.inc ;nand flash config
GET 2440addr.inc ;定义内部寄存器地址对应的变量
BIT_SELFREFRESH EQU (1<<22)
;Pre-defined constants
;对arm处理器模式寄存器对应值的常数定义，arm处理器中有一个CPSR程序状态寄存器，它的后五位决定目前处理器模式
USERMODE EQU 0x10
FIQMODE EQU 0x11
IRQMODE EQU 0x12
SVCMODE EQU 0x13
ABORTMODE EQU 0x17
UNDEFMODE EQU 0x1b
MODEMASK EQU 0x1f
NOINT EQU 0xc0
;The location of stacks
;定义各模式下SDRAM中堆栈起始地址，均是物理地址值
UserStack EQU (_STACK_BASEADDRESS-0x3800) ;0x33ff4800 ~
SVCStack EQU (_STACK_BASEADDRESS-0x2800) ;0x33ff5800 ~
UndefStack EQU (_STACK_BASEADDRESS-0x2400) ;0x33ff5c00 ~
AbortStack EQU (_STACK_BASEADDRESS-0x2000) ;0x33ff6000 ~
IRQStack EQU (_STACK_BASEADDRESS-0x1000) ;0x33ff7000 ~
FIQStack EQU (_STACK_BASEADDRESS-0x0) ;0x33ff8000 ~
;Check if tasm.exe(armasm -16 ...@ADS 1.0) is used.
;这一段是为了统一处理器工作状态和软件编译方式（16位编译环境使用tasm.exe编译）
;arm有两种工作状态1.32位，该状态执行字对准的arm指令；2.16位，该状态执行半字对准的Thumb指令
;不同的工作状态，编译器编译方式也不同
GBLL THUMBCODE
[ {CONFIG} = 16 ;if config
THUMBCODE SETL {TRUE}
CODE32 ;转入32位编译模式
| ;else
THUMBCODE SETL {FALSE} ;设置为FALSE
]
MACRO
MOV_PC_LR
[ THUMBCODE
bx lr
|
mov pc,lr
]
MEND
MACRO
MOVEQ_PC_LR
[ THUMBCODE
bxeq lr
|
moveq pc,lr
]
MEND
;下面这个宏用于把中断服务程序的首地址装载到pc中，加载程序，属于向量中断模式
MACRO ;声明一个宏
$HandlerLabel HANDLER $HandleLabel
$HandlerLabel ;中断服务入口地址
sub sp,sp,#4 ;decrement sp(to store jump address)
;将要使用的r0寄存器入栈
stmfd sp!,{r0} ;PUSH the work register to stack(lr does't push because it return to original address)
;间接寻址方式获得中断函数入口地址
ldr r0,=$HandleLabel;load the address of HandleXXX to r0
ldr r0,[r0] ;load the contents(service routine start address) of HandleXXX
;将对应的中断向量首地址入栈
str r0,[sp,#4] ;store the contents(ISR) of HandleXXX to stack
;将中断函数首地址出栈，放入程序指针中，系统将跳转到对应中断处理函数
ldmfd sp!,{r0,pc} ;POP the work register and pc(jump to ISR)
MEND ;宏结束
;对于arm由RO,RW,ZI三个段组成 其中RO为代码段 RW是已经初始化的全局变量 ZI是未初始化的全局变量
;bootloader要将RW段复制到RAM中并将ZI段清零
;编译器使用下列段来记录各段的起始和结束地址
IMPORT |Image$$RO$$Base| ; Base of ROM code
IMPORT |Image$$RO$$Limit| ; End of ROM code (=start of ROM data)
IMPORT |Image$$RW$$Base| ; Base of RAM to initialise
IMPORT |Image$$ZI$$Base| ; Base and limit of area
IMPORT |Image$$ZI$$Limit| ; to zero initialise
IMPORT MMU_SetAsyncBusMode
IMPORT MMU_SetFastBusMode ;hzh
IMPORT Main ; The main entry of mon program引入外部函数
;定义arm汇编程序段，段名Init,程序段为只读代码段
AREA Init,CODE,READONLY
;程序入口
;异常中断向量表（每个表项占4个字节）
;一旦系统运行时有中断发生 处理器已经把控制权交给了操作系统 处理器还是会跳转到从0x0开始执行中断向量表中某个中断表项
;板子上电和复位后 程序开始从位于0x0处开始执行 即从这里开始执行跳转到标号为ResetHandler处执行
ENTRY
EXPORT __ENTRY
__ENTRY
ResetEntry
;1)The code, which converts to Big-endian, should be in little endian code.
;2)The following little endian code will be compiled in Big-Endian mode.
; The code byte order should be changed as the memory bus width.
;3)The pseudo instruction,DCD can't be used here because the linker generates error.
;总线宽度判断
;条件编译,在编译成机器码前就设定好
ASSERT :DEF:ENDIAN_CHANGE ;判断ENDIAN_CHANGE是否已定义
[ ENDIAN_CHANGE
ASSERT :DEF:ENTRY_BUS_WIDTH
[ ENTRY_BUS_WIDTH=32
b ChangeBigEndian ;DCD 0xea000007
]
[ ENTRY_BUS_WIDTH=16
andeq r14,r7,r0,lsl #20 ;DCD 0x0007ea00也是b ChangeBigEndian指令，只是由于总线不一样而取机器码顺序不同罢了
]
[ ENTRY_BUS_WIDTH=8
streq r0,[r0,-r10,ror #1] ;DCD 0x070000ea
]
|
b ResetHandler
]
;跳转指令，异常中断处理向量
b HandlerUndef ;handler for Undefined mode
b HandlerSWI ;handler for SWI interrupt
b HandlerPabort ;handler for PAbort
b HandlerDabort ;handler for DAbort
b . ;reserved
b HandlerIRQ ;handler for IRQ interrupt
b HandlerFIQ ;handler for FIQ interrupt
;@0x20
b EnterPWDN ; Must be @0x20.
;大小端判断
ChangeBigEndian
;@0x24
[ ENTRY_BUS_WIDTH=32
DCD 0xee110f10 ;0xee110f10 => mrc p15,0,r0,c1,c0,0
DCD 0xe3800080 ;0xe3800080 => orr r0,r0,#0x80; //Big-endian
DCD 0xee010f10 ;0xee010f10 => mcr p15,0,r0,c1,c0,0
]
[ ENTRY_BUS_WIDTH=16
DCD 0x0f10ee11
DCD 0x0080e380
DCD 0x0f10ee01
]
[ ENTRY_BUS_WIDTH=8
DCD 0x100f11ee
DCD 0x800080e3
DCD 0x100f01ee
]
DCD 0xffffffff ;swinv 0xffffff is similar with NOP and run well in both endian mode.
DCD 0xffffffff
DCD 0xffffffff
DCD 0xffffffff
DCD 0xffffffff
b ResetHandler
;下面是具体的中断处理函数跳转的宏
;通过上面的$HandlerLabel宏定义展开后跳转到对应的中断处理函数
HandlerFIQ HANDLER HandleFIQ
HandlerIRQ HANDLER HandleIRQ
HandlerUndef HANDLER HandleUndef
HandlerSWI HANDLER HandleSWI
HandlerDabort HANDLER HandleDabort
HandlerPabort HANDLER HandlePabort
;下面这段程序是用来处理非向量中断
;具体判断I_ISPR中个位是否置1，置1表示等待
;采用INTOFFSET寄存器判定IRQ中断源
IsrIRQ
sub sp,sp,#4 ;reserved for PC
stmfd sp!,{r8-r9}
ldr r9,=INTOFFSET
ldr r9,[r9] ;载入I_ISR
ldr r8,=HandleEINT0
add r8,r8,r9,lsl #2 ;r8=r8+(r9<<2)
ldr r8,[r8]
str r8,[sp,#8]
ldmfd sp!,{r8-r9,pc}
LTORG
;=======
; ENTRY
;=======
;板子上电和复位后 程序开始从位于0x0执行 b ResetHandler程序处跳转到这里执行
ResetHandler
;1.禁止看门狗 屏蔽所有中断
ldr r0,=WTCON ;watch dog disable
ldr r1,=0x0
str r1,[r0]
ldr r0,=INTMSK
ldr r1,=0xffffffff ;all interrupt disable
str r1,[r0]
ldr r0,=INTSUBMSK
ldr r1,=0x7fff ;all sub interrupt disable
str r1,[r0]
;系统启动指示灯显示
[ {FALSE}
; rGPFDAT = (rGPFDAT & ~(0xf<<4)) | ((~data & 0xf)<<4);
; Led_Display
ldr r0,=GPFCON
ldr r1,=0x5500
str r1,[r0]
ldr r0,=GPFDAT
ldr r1,=0x10
str r1,[r0]
]
;2.根据工作频率设置PLL
;mpll=(2*m*Fin)/(p*2~s) m=M_MDIV+8 p=M_PDIV+2 s="M"_SDIV
;m=92+8 p=1+2 s=1 mpll=400M
;CLKDIV_VAL = 1:4:8 hclk=100M pclk=50M
;To reduce PLL lock time, adjust the LOCKTIME register.
ldr r0,=LOCKTIME
ldr r1,=0xffffff
str r1,[r0]
[ PLL_ON_START
; Added for confirm clock divide. for 2440.
; Setting value Fclk:Hclk:Pclk
ldr r0,=CLKDIVN
ldr r1,=CLKDIV_VAL ; 0=1:1:1, 1=1:1:2, 2=1:2:2, 3=1:2:4, 4=1:4:4, 5=1:4:8, 6=1:3:3, 7=1:3:6.
str r1,[r0]
; MMU_SetAsyncBusMode and MMU_SetFastBusMode over 4K, so dont' call here
; call it after copy, hzh
; [ CLKDIV_VAL>1 ; means Fclk:Hclk is not 1:1.
; bl MMU_SetAsyncBusMode
; |
; bl MMU_SetFastBusMode ; default value.
; ]
;program has not been copied, so use these directly, hzh
[ CLKDIV_VAL>1 ; means Fclk:Hclk is not 1:1.
mrc p15,0,r0,c1,c0,0
orr r0,r0,#0xc0000000;R1_nF:OR:R1_iA
mcr p15,0,r0,c1,c0,0
|
mrc p15,0,r0,c1,c0,0
bic r0,r0,#0xc0000000;R1_iA:OR:R1_nF
mcr p15,0,r0,c1,c0,0
]
;Configure UPLL
ldr r0,=UPLLCON
ldr r1,=((U_MDIV<<12)+(U_PDIV<<4)+U_SDIV)
str r1,[r0]
nop ; Caution: After UPLL setting, at least 7-clocks delay must be inserted for setting hardware be completed.
nop
nop
nop
nop
nop
nop
;Configure MPLL
ldr r0,=MPLLCON
ldr r1,=((M_MDIV<<12)+(M_PDIV<<4)+M_SDIV) ;Fin=16.9344MHz
str r1,[r0]
]
;Check if the boot is caused by the wake-up from SLEEP mode.
ldr r1,=GSTATUS2
ldr r0,[r1]
tst r0,#0x2
;In case of the wake-up from SLEEP mode, go to SLEEP_WAKEUP handler.
bne WAKEUP_SLEEP
EXPORT StartPointAfterSleepWakeUp
StartPointAfterSleepWakeUp
;3.设置存储相关寄存器的程序
;SDRAM,FLASH存储器连接和工作时序的程序，片选定义的程序
;SMRDATA map在下面的程序中定义，另外参考memcfg.s程序
;Set memory control registers
;ldr r0,=SMRDATA
adrl r0, SMRDATA ;be careful!, hzh
ldr r1,=BWSCON ;BWSCON Address
add r2, r0, #52 ;End address of SMRDATA
0
ldr r3, [r0], #4
str r3, [r1], #4
cmp r2, r0
bne %B0
;===delay, hzh
mov r0, #&1000
1
subs r0, r0, #1
bne %B1
;===
;5.设置缺省中断处理函数
; Setup IRQ handler
ldr r0,=HandleIRQ ;This routine is needed
ldr r1,=IsrIRQ ;if there isn't 'subs pc,lr,#4' at 0x18, 0x1c
str r1,[r0]
; ;Copy and paste RW data/zero initialized data
; ldr r0, =|Image$$RO$$Limit| ; Get pointer to ROM data
; ldr r1, =|Image$$RW$$Base| ; and RAM copy
; ldr r3, =|Image$$ZI$$Base|
;
;6.将数据段拷贝到RAM中 将零初始化数据段清零 跳入C语言的main函数执行
;到这步结束bootloader 的stage1引导
; ;Zero init base => top of initialised data
; cmp r0, r1 ; Check that they are different
; beq %F2
;1
; cmp r1, r3 ; Copy init data
; ldrcc r2, [r0], #4 ;--> LDRCC r2, [r0] + ADD r0, r0, #4
; strcc r2, [r1], #4 ;--> STRCC r2, [r1] + ADD r1, r1, #4
; bcc %B1
;2
; ldr r1, =|Image$$ZI$$Limit| ; Top of zero init segment
; mov r2, #0
;3
; cmp r3, r1 ; Zero init
; strcc r2, [r3], #4
; bcc %B3
[ :LNOT:THUMBCODE
bl Main ;Don't use main() because ......
;ldr pc, =Main ;hzh
b .
]
[ THUMBCODE ;for start-up code for Thumb mode
orr lr,pc,#1
bx lr
CODE16
bl Main ;Don't use main() because ......
b .
CODE32
]
;===========================================================
LTORG ;声明一个数据缓冲池的开始
;GCS0->SST39VF1601
;GCS1->16c550
;GCS2->IDE
;GCS3->CS8900
;GCS4->DM9000
;GCS5->CF Card
;GCS6->SDRAM
;GCS7->unused
;这是上面提到的对存储器初始化的数据map
SMRDATA DATA
; Memory configuration should be optimized for best performance
; The following parameter is not optimized.
; Memory access cycle parameter strategy
; 1) The memory settings is safe parameters even at HCLK="75Mhz".
; 2) SDRAM refresh period is for HCLK<=75Mhz.
DCD (0+(B1_BWSCON<<4)+(B2_BWSCON<<8)+(B3_BWSCON<<12)+(B4_BWSCON<<16)+(B5_BWSCON<<20)+(B6_BWSCON<<24)+(B7_BWSCON<<28))
DCD ((B0_Tacs<<13)+(B0_Tcos<<11)+(B0_Tacc<<8)+(B0_Tcoh<<6)+(B0_Tah<<4)+(B0_Tacp<<2)+(B0_PMC)) ;GCS0
DCD ((B1_Tacs<<13)+(B1_Tcos<<11)+(B1_Tacc<<8)+(B1_Tcoh<<6)+(B1_Tah<<4)+(B1_Tacp<<2)+(B1_PMC)) ;GCS1
DCD ((B2_Tacs<<13)+(B2_Tcos<<11)+(B2_Tacc<<8)+(B2_Tcoh<<6)+(B2_Tah<<4)+(B2_Tacp<<2)+(B2_PMC)) ;GCS2
DCD ((B3_Tacs<<13)+(B3_Tcos<<11)+(B3_Tacc<<8)+(B3_Tcoh<<6)+(B3_Tah<<4)+(B3_Tacp<<2)+(B3_PMC)) ;GCS3
DCD ((B4_Tacs<<13)+(B4_Tcos<<11)+(B4_Tacc<<8)+(B4_Tcoh<<6)+(B4_Tah<<4)+(B4_Tacp<<2)+(B4_PMC)) ;GCS4
DCD ((B5_Tacs<<13)+(B5_Tcos<<11)+(B5_Tacc<<8)+(B5_Tcoh<<6)+(B5_Tah<<4)+(B5_Tacp<<2)+(B5_PMC)) ;GCS5
DCD ((B6_MT<<15)+(B6_Trcd<<2)+(B6_SCAN)) ;GCS6
DCD ((B7_MT<<15)+(B7_Trcd<<2)+(B7_SCAN)) ;GCS7
DCD ((REFEN<<23)+(TREFMD<<22)+(Trp<<20)+(Tsrc<<18)+(Tchr<<16)+REFCNT)
DCD 0x32 ;SCLK power saving mode, BANKSIZE 128M/128M
;DCD 0x02 ;SCLK power saving disable, BANKSIZE 128M/128M
DCD 0x30 ;MRSR6 CL="3clk"
DCD 0x30 ;MRSR7 CL="3clk"
BaseOfROM DCD |Image$$RO$$Base|
TopOfROM DCD |Image$$RO$$Limit|
BaseOfBSS DCD |Image$$RW$$Base|
BaseOfZero DCD |Image$$ZI$$Base|
EndOfBSS DCD |Image$$ZI$$Limit|
ALIGN ;通过添加补丁字节使当前位置满足一定的对齐方式
;Function for entering power down mode
; 1. SDRAM should be in self-refresh mode.
; 2. All interrupt should be maksked for SDRAM/DRAM self-refresh.
; 3. LCD controller should be disabled for SDRAM/DRAM self-refresh.
; 4. The I-cache may have to be turned on.
; 5. The location of the following code may have not to be changed.
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