ARM的嵌入式Linux移植体验之BootLoader

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