移植嵌入式Linux到ARM处理器:BootLoader(7)

3.BootLoader第一阶段

　　3.1硬件设备初始化

　　基本的硬件初始化工作包括：

　　·屏蔽所有的中断；

　　·设置CPU的速度和时钟频率；

　　·RAM初始化；

　　·初始化LED

　　ARM的中断向量表设置在0地址开始的8个字空间中，如下表：

　　每当其中的某个异常发生后即将PC值置到相应的中断向量处，每个中断向量处放置一个跳转指令到相应的中断服务程序去进行处理，中断向量表的程序如下：

@ 0x00: Reset
b Reset
@ 0x04: Undefined instruction exception
UndefEntryPoint:
b HandleUndef
@ 0x08: Software interrupt exception
SWIEntryPoint:
b HandleSWI
@ 0x0c: Prefetch Abort (Instruction Fetch Memory Abort)
PrefetchAbortEnteryPoint:
b HandlePrefetchAbort
@ 0x10: Data Access Memory Abort
DataAbortEntryPoint:
b HandleDataAbort
@ 0x14: Not used
NotUsedEntryPoint:
b HandleNotUsed
@ 0x18: IRQ(Interrupt Request) exception
IRQEntryPoint:
b HandleIRQ
@ 0x1c: FIQ(Fast Interrupt Request) exception
FIQEntryPoint:
b HandleFIQ

　　复位时关闭看门狗定时器、屏蔽所有中断：

Reset:
@ disable watch dog timer
mov r1, #0x53000000
mov r2, #0x0
str r2, [r1]
@ disable all interrupts
mov r1, #INT_CTL_BASE
mov r2, #0xffffffff
str r2, [r1, #oINTMSK]
ldr r2, =0x7ff
str r2, [r1, #oINTSUBMSK]

　　设置系统时钟：

@init clk
@ 1:2:4
mov r1, #CLK_CTL_BASE
mov r2, #0x3
str r2, [r1, #oCLKDIVN]
mrc p15, 0, r1, c1, c0, 0 @ read ctrl register
orr r1, r1, #0xc0000000 @ Asynchronous
mcr p15, 0, r1, c1, c0, 0 @ write ctrl register
@ now, CPU clock is 200 Mhz
mov r1, #CLK_CTL_BASE
ldr r2, mpll_200mhz
str r2, [r1, #oMPLLCON]

　　点亮所有的用户LED：

@ All LED on
mov r1, #GPIO_CTL_BASE
add r1, r1, #oGPIO_F
ldr r2,=0x55aa
str r2, [r1, #oGPIO_CON]
mov r2, #0xff
str r2, [r1, #oGPIO_UP]
mov r2, #0x00
str r2, [r1, #oGPIO_DAT]

　　设置（初始化）内存映射：

ENTRY(memsetup)
@ initialise the static memory

@ set memory control registers
mov r1, #MEM_CTL_BASE
adrl r2, mem_cfg_val
add r3, r1, #52
1: ldr r4, [r2], #4
str r4, [r1], #4
cmp r1, r3
bne 1b

mov pc, lr

　　设置（初始化）UART：

@ set GPIO for UART
mov r1, #GPIO_CTL_BASE
add r1, r1, #oGPIO_H
ldr r2, gpio_con_uart
str r2, [r1, #oGPIO_CON]
ldr r2, gpio_up_uart
str r2, [r1, #oGPIO_UP]
bl InitUART

@ Initialize UART
@
@ r0 = number of UART port
InitUART:
ldr r1, SerBase
mov r2, #0x0
str r2, [r1, #oUFCON]
str r2, [r1, #oUMCON]
mov r2, #0x3
str r2, [r1, #oULCON]
ldr r2, =0x245
str r2, [r1, #oUCON]
#define UART_BRD ((50000000 / (UART_BAUD_RATE * 16)) - 1)
mov r2, #UART_BRD
str r2, [r1, #oUBRDIV]
mov r3, #100
mov r2, #0x0
1: sub r3, r3, #0x1
tst r2, r3
bne 1b

#if 0
mov r2, #'U'
str r2, [r1, #oUTXHL]

1: ldr r3, [r1, #oUTRSTAT]
and r3, r3, #UTRSTAT_TX_EMPTY
tst r3, #UTRSTAT_TX_EMPTY
bne 1b

mov r2, #'0'
str r2, [r1, #oUTXHL]

1: ldr r3, [r1, #oUTRSTAT]
and r3, r3, #UTRSTAT_TX_EMPTY
tst r3, #UTRSTAT_TX_EMPTY
bne 1b
#endif

mov pc, lr

　　此外，vivi还提供了几个汇编情况下通过串口打印字符的函数PrintChar、PrintWord和PrintHexWord：

@ PrintChar : prints the character in R0
@ r0 contains the character
@ r1 contains base of serial port
@ writes ro with XXX, modifies r0,r1,r2
@ TODO : write ro with XXX reg to error handling
PrintChar:
TXBusy:
ldr r2, [r1, #oUTRSTAT]
and r2, r2, #UTRSTAT_TX_EMPTY
tst r2, #UTRSTAT_TX_EMPTY
beq TXBusy
str r0, [r1, #oUTXHL]
mov pc, lr

@ PrintWord : prints the 4 characters in R0
@ r0 contains the binary word
@ r1 contains the base of the serial port
@ writes ro with XXX, modifies r0,r1,r2
@ TODO : write ro with XXX reg to error handling
PrintWord:
mov r3, r0
mov r4, lr
bl PrintChar

mov r0, r3, LSR #8 /* shift word right 8 bits */
bl PrintChar

mov r0, r3, LSR #16 /* shift word right 16 bits */
bl PrintChar

mov r0, r3, LSR #24 /* shift word right 24 bits */
bl PrintChar

mov r0, #'\r'
bl PrintChar

mov r0, #'\n'
bl PrintChar

mov pc, r4

@ PrintHexWord : prints the 4 bytes in R0 as 8 hex ascii characters
@ followed by a newline
@ r0 contains the binary word
@ r1 contains the base of the serial port
@ writes ro with XXX, modifies r0,r1,r2
@ TODO : write ro with XXX reg to error handling
PrintHexWord:
mov r4, lr
mov r3, r0
mov r0, r3, LSR #28
bl PrintHexNibble
mov r0, r3, LSR #24
bl PrintHexNibble
mov r0, r3, LSR #20
bl PrintHexNibble
mov r0, r3, LSR #16
bl PrintHexNibble
mov r0, r3, LSR #12
bl PrintHexNibble
mov r0, r3, LSR #8
bl PrintHexNibble
mov r0, r3, LSR #4
bl PrintHexNibble
mov r0, r3
bl PrintHexNibble

mov r0, #'\r'
bl PrintChar

mov r0, #'\n'
bl PrintChar

mov pc, r4

作者:宋宝华   更新日期:2006-11-21