DIY自己的arm11（6410）的bootloader

现我们仿照着uboot对ARM11所做的初始化操作，编写裸机bootloader。

1.首先编写makefile代码

all: start.o
arm-linux-ld -Tgboot.lds -o gboot.elf $^
arm-linux-objcopy -O binary gboot.elf gboot.bin

%.o : %.S
arm-linux-gcc -g -c $^

%.o : %.c
arm-linux-gcc -g -c $^

.PHONY: clean
clean:
rm *.o *.elf *.bin

2.然后编写链接器脚本diyboot.lds

OUTPUT_ARCH(arm)//arm内核
ENTRY(_start)//代码入口start
SECTIONS {
. = 0x50008000;

. = ALIGN(4);
.text :  //代码段
{
start.o (.text) //start.s是初始化代码
*(.text)
}

. = ALIGN(4); //数据段，4字节对齐
.data :
{
*(.data)
}

. = ALIGN(4);
bss_start = .;  //当前代码所在地址
.bss :
{
*(.bss)
}
bss_end = .; //当前代码结束地址
}

3. 开始编写我们自己的start.S

a. 添加向量表

查看三星官方的ARM Architecture Reference Manual.pdf文件，可以看到ARM11共有七种模式

设计代码如下：

.text
.global _start
_start:
b   reset
ldr pc, _undefined_instruction
ldr pc, _software_interrupt
ldr pc, _prefetch_abort
ldr pc, _data_abort
ldr pc, _not_used
ldr pc, _irq
ldr pc, _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

undefined_instruction:
nop

software_interrupt:
nop

prefetch_abort:
nop

data_abort:
nop

not_used:
nop

irq:
nop

fiq:
nop

reset:
nop

b. 设置SVC模式

如手册上表所示

要把CSRP的后五位M[4：0]设置为10011

另外要屏蔽中断和快速中断，他们的屏蔽室I为和F位

要把I位和F位设置为1 因此CSRP的后8为设置为11010011即为0xD3

因此修改代码如下：

.text
.global _start
_start:
b   reset
ldr pc, _undefined_instruction
ldr pc, _software_interrupt
ldr pc, _prefetch_abort
ldr pc, _data_abort
ldr pc, _not_used
ldr pc, _irq
ldr pc, _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

undefined_instruction:
nop

software_interrupt:
nop

prefetch_abort:
nop

data_abort:
nop

not_used:
nop

irq:
nop

fiq:
nop

reset:
bl set_svc

set_svc:
mrs r0, cpsr  //把CPSR导入到r0寄存器
bic r0, r0, #0x1f  //把CPSR的后五位清零
orr r0, r0, #0xd3//把CPSR的后八位设置为0XD3
msr cpsr, r0//把r0值导入到CPSR
mov pc, lr//退出CPSR寄存器

c.关闭看门狗

在嵌入式领域，有些系统需要长期运行在无人看守的环境。在运行过程中，难免不出现系统死机的情况，这时就需要系统自身带有一种自动重启的功能。watchdog一般是一个硬件模块，其作用就是在系统死机时，帮助系统实现自动重启。Watchdog在硬件上实现了计时功能，启动计时后，用户（软件）必须在计时结束前重新开始计时，俗称“喂狗”，如果到超时的时候还没有重新开始计时，那么它就认为系统是死机了，就自动重启系统。

关闭看门狗和看门狗中断就是设置WTCON的0位和2位为0，修改代码如下：

.text
.global _start
_start:
b   reset
ldr pc, _undefined_instruction
ldr pc, _software_interrupt
ldr pc, _prefetch_abort
ldr pc, _data_abort
ldr pc, _not_used
ldr pc, _irq
ldr pc, _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

undefined_instruction:
nop

software_interrupt:
nop

prefetch_abort:
nop

data_abort:
nop

not_used:
nop

irq:
nop

fiq:
nop

reset:
bl set_svc
bl disable_watchdog

set_svc:
mrs r0, cpsr
bic r0, r0, #0x1f
orr r0, r0, #0xd3
msr cpsr, r0
mov pc, lr

#define pWTCON 0x7e004000   //定义WTCON的地址
disable_watchdog:
ldr r0, =pWTCON  //把地址装入r0
mov r1, #0x0     //r1清零
str r1, [r0]      //把r1的0装入ro装入的WTCON所在的地址
mov pc, lr       //返回

d.关闭中断

中断使能寄存器如图所示：

很简单只要把VIC0INTENABLE和VIC1INTENABLE全部设置为0即可，修改代码如下：

.text
.global _start
_start:
b reset
ldr pc, _undifined_instruction
ldr pc, _software_interrupt
ldr pc, _prefetch_abort
ldr pc, _data_abort
ldr pc, _not_used
ldr pc, _irq
ldr pc, _fiq

_undifined_instruction: .word undifined_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 reset

undifined_instruction:
nop

software_interrupt:
nop

prefetch_abort:
nop

data_abort:
nop

not_used:
nop

irq:
nop

fiq:
nop

reset:
bl set_svc
bl disable_watchdog
bl disable_interrupt

set_svc:
mrs r0, cpsr
bic r0, r0,#0x1f
orr r0, r0,#0xd3
msr cpsr, r0
mov pc, lr

#define pWTCON 0x7e004000
disable_watchdog:
ldr r0, =pWTCON
mov r1, #0x0
str r1, [r0]
mov pc, lr

disable_interrupt:      //把VIC0INTENABLE和VIC1INTENABLE全部设置为0
mvn r1,#0x0
ldr r0,=0x71200014
str r1,[r0]

ldr r0,=0x71300014
str r1,[r0]
mov pc, lr

e.关闭MMU

MMU作用就是把虚拟地址映射到物理地址上去。

初始化完成前必须先关掉。

关闭mmu按照CP15的寄存器说明进行：

设计代码如下：

.text
.global _start
_start:
b reset
ldr pc, _undifined_instruction
ldr pc, _software_interrupt
ldr pc, _prefetch_abort
ldr pc, _data_abort
ldr pc, _not_used
ldr pc, _irq
ldr pc, _fiq

_undifined_instruction: .word undifined_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 reset

undifined_instruction:
nop

software_interrupt:
nop

prefetch_abort:
nop

data_abort:
nop

not_used:
nop

irq:
nop

fiq:
nop

reset:
bl set_svc
bl disable_watchdog
bl disable_interrupt
bl disable_mmu

set_svc:
mrs r0, cpsr
bic r0, r0,#0x1f
orr r0, r0,#0xd3
msr cpsr, r0
mov pc, lr

#define pWTCON 0x7e004000
disable_watchdog:
ldr r0, =pWTCON
mov r1, #0x0
str r1, [r0]
mov pc, lr

disable_interrupt:
mvn r1,#0x0
ldr r0,=0x71200014
str r1,[r0]

ldr r0,=0x71300014
str r1,[r0]
mov pc, lr

disable_mmu:
mcr p15,0,r0,c7,c7,0
mrc p15,0,r0,c1,c0,0
bic r0, r0, #0x00000007
mcr p15,0,r0,c1,c0,0
mov pc, lr