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  程序主要作用是通过串口下载VxWorks到ARM Flash ROM,主要过程是初始化ARM硬件,初始化串口UART1,

从串口接收VxWorks image到DRAM,初始化Flash ROM.最后将DRAM中的VxWorks写入Flash ROM. ARM 串口

与PC机串口连接,由PC机下载(download)VxWorks 到主板Flash ROM.

         该程序对学习和理解ARM编程很有帮助.为了便于理解,这里我全部采用实际地址,不用宏定义.

         程序语言为 ARM 汇编,具体过程参考ARM编程,具体寄存器接口定义参见ARM硬件手册

VramBaseAddress EQU 0xC0000000                ;DRAM起始地址,在ARM硬件地址映射定义,见ARM编程
Length1    EQU        0x100                  

FlashBase         EQU         0x70000000      ;Flash的基地址,在Boot模式下,为0x70000000
MMUCP             CP         15                ;协处理器15

    AREA |C$$code|, CODE, READONLY

    ENTRY

    EXPORT main

main
        MOV         r14, #0x70                ;设置MMU无效, 32位模式, Little endian
        MCR         MMUCP, 0, r14, c1, c0, 0

        MRS         r14, CPSR
        BIC         r14, r14, #0x1f             ;Mask
        ORR         r14, r14, #0xc0 + 0x13   ;关闭所有 IRQ FIQ, 用SVC32 模式
        MSR         CPSR, r14

        LDR         r11, =0x80000000         ;内部寄存器基地址 0x80000000
        LDR         r12, =0x80001000

        MOV         r0, #0

        STR         r0, [r11, #0x280]        ;disable 所有中断
        STR         r0, [r12, #0x280]

        LDR         r0, =0x840100            ;在系统控制寄存器SYSCON1中
        STR         r0, [r11, #0x100]        ;使能外部时钟 EXCKEN 和串口 UART1

　
        LDR         r0, =0x06                ;在系统控制寄存器SYSCON2中
        STR         r0, [r12, #0x100]        ;使能16-bit DRAM, 键盘KBD6

        LDR         r0, =0x03010100          ;在存储控制寄存器MEMCFG1中
        STR         r0, [r11, #0x180]        ;设置外设数据线宽度CS0:16位,CS1,2:32位,CS3:8位

;特别注意:

如设置线宽为16位,由于CPU是32位,CPU会把32位数据分两次操作写入,每次写16位,这样对32位的外设来

说是一种浪费
例如:       LDR      r0, =0x10000000
            LDR      r1, =0x12345678
            STR      r1,[r0]
实际情况是 第一次写入地址0x10000000的数据是 0x1234
          第二次写入地址0x10000002的数据是 0x5678

示波器上观察的情况是一个片选信号/CS低电平范围内,有两个写入信号/WE低电平.

如设置线宽为32位,CPU会把32位数据一次写入,如果外设是16位的话,如ISA网卡,就会造成高16位丢失.
例如:       LDR      r0, =0x10000000
            LDR      r1, =0x12345678
            STR      r1,[r0]
实际情况是 一次写入地址0x10000000的数据是 0x12345678
          

        LDR         r0, = 0xff               ;DRAM刷新率refresh rate,设置要适当,否则数据丢

失
        STR         r0, [r11, #0x200]

        LDR         r13, =0xc0020000         ;设置堆栈 Stack, r13 为ARM定义的堆栈指针
;Init OK
        LDR         r0, =receiving_msg
        BL         printmsg

        BL         dram_test                 ;跳转指令,执行测试DRAM
 LDR         r0, =startloader_cmdmsg
        BL         printmsg

        BL         Beep

        LDR         r0, =0x60005             ;设置波特率 38400, 8位
        LDR         r11, =0x80000000
        STR         r0, [r11, #0x4c0]        ;初始化串口 UART
;Receive data and save it to buffer        ;开始接收数据
        LDR         r12, =SaveAddr             ;First 8 bytes are start address and length
;Receive address
        LDR         r5, =0x4                 ;Read first 4 bytes
        MOV         r0, #0
rx_loop1
        LDR         r1, [r11, #0x140]        ;检查系统状态寄存器System Status Register 1
        TST         r1, #0x00400000           ;UART1 Rx 是否为空
        BNE         rx_loop1                  ;等待数据 Wait data
        LDR         r1, [r11, #0x480]         ;读数据 Read data
        AND         r1, r1, #0xff
        ORR         r0, r1, r0, ROR #8
        SUBS    r5, r5, #1
        BNE         rx_loop1
        MOV         r0, r0, ROR #8

        STR         r0, [r12]
        ADD         r12, r12, #4

;Receive count
        LDR         r12, =Count
        LDR         r5, =0x4                 ;Read second 4 bytes
        MOV         r0, #0
rx_loop2
        LDR         r1, [r11, #0x140]        ;检查系统状态寄存器System Status Register 1
        TST         r1, #0x00400000           ;UART1 Rx 是否为空
        BNE         rx_loop2                  ;等待数据 Wait data
        LDR         r1, [r11, #0x480]        ;读数据 Read data
        AND         r1, r1, #0xff
        ORR         r0, r1, r0, ROR #8
        SUBS    r5, r5, #1
        BNE         rx_loop2
        MOV         r0, r0, ROR #8

        STR         r0, [r12]
        MOV         r6, r0                     ;Save the count to r6

;Receive data
        MOV         r5, r0
        MOV         r9, #0
        LDR         r10, =BufferBase
        MOV         r12, #0
rx_loop3
        LDR         r1, [r11, #0x140]         ;System Status Register 1
        TST         r1, #0x00400000           ;UART1 Rx Empty
        BNE         rx_loop3                  ;Wait data
        LDR         r1, [r11, #0x480]         ;Read data
        AND         r1, r1, #0xff
        STRB    r1, [r10, r12]
        ADD         r12, r12, #1

nextbyte                                     ;读下一个字节
        SUBS    r5, r5, #1
        BNE         rx_loop3
;Receive data OK                            ;数据接收完毕

        MOV         r1, #0x31
        BL         send_char
        MOV         r1, #0x0d
        BL         send_char
        MOV         r1, #0x0a
        BL         send_char

;Save it to Flash ROM                         把数据写入Flash ROM