start.S 清bss段

 start.S

.global _start
.global _end
.align 2
.text

_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 und
_software_interrupt:    .word svc
_prefetch_abort:    .word pre_abort
_data_abort:        .word data_abort
_not_used:        .word 0x12345678
_irq:            .word irq
_fiq:            .word fiq

reset:    
    mov ip,sp
    push {fp,ip,lr,pc}
    sub fp,ip,#4

    bl clr_bss

    bl entry

    sub sp,sp,#200
    sub sp,sp,#200
    sub sp,sp,#200
    sub sp,sp,#200
    sub sp,sp,#200

    sub sp,fp,#12
    ldm sp,{fp,sp,pc}
 
und:
    ldr sp, =0x56000000
    stmfd sp!, {r0-r12, lr}
    mov lr, pc
    ldr pc, do_und_
    
    ldmfd sp!, {r0-r12, pc}^
    do_und_:
    .word do_und
svc:
    stmfd sp!, {r0-r12, lr} @异常发生时arm会总动保存返回地址到lr = pc +4
    sub lr, #4  @取出c程序中svc异常源的指令地址
    mov r0, lr  @使用r0进行参数传递
    mov lr, pc
    ldr pc, do_svc_
    
    ldmfd sp!, {r0-r12, pc}

    do_svc_:
    .word do_svc
pre_abort:
data_abort:
irq:
    ldr sp, =0x58000000
    sub lr , #4 @irq是打断正在执行的程序，因此返回必须到断点处
    stmfd sp!, {r0-r12, lr}
    ldr r0, =0x110002e0
    ldr r1, =0x1111
    str r1, [r0]
    ldr r0, =0x110002e4
    ldr r1, =0x0
    str r1, [r0]
    mov lr, pc
    ldr pc, do_irq_
    
    ldmfd sp!, {r0-r12, pc}^
    do_irq_:
    .word do_irq
fiq:

.globl _bss_start
_bss_start:
        .word __bss_start

.globl _bss_end
_bss_end:
        .word __bss_end


clr_bss:
    
    mov ip, sp
    push {fp,ip,lr,pc}
    sub fp, ip, #4

    ldr r0,_bss_start
    ldr r1,_bss_end

    mov r2,#0

    forclrbss:
        cmp r0,r1
        bge endclrbss
        strb r2,[r0],#1    
    b forclrbss

    endclrbss:

    sub sp, fp, #12
    ldm sp, {fp, sp, pc}


_end:

.end

 链接脚本 ld.lds

OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm","elf32-littlearm")
OUTPUT_ARCH(arm)
ENTRY(_start)
SECTIONS
{
/*这样可以备注吗？*/
    . = 0x40008000;
    
    .text : {
        start.o(.text);
        *(.text);
    }

    . = ALIGN(4);

    .data : {
        *(.data);
    }

    . = ALIGN(4);
    
    __bss_start = .;
    .bss : {
        *(.bss);
    }
    __bss_end = .;
}

2016-01-18