Linux混入了mmu內存管理之後,ARM的中斷是怎麼樣的呢？和我們在裸板上的中斷有沒有區別？讓我們從源代碼入手,做一個粗略的分析:
init/main.c->start_kernel()->trap_init()
//-----------------------------------------------
1.trap_init()
//gliethttp函數位於arch/arm/kernel/traps.c
void __init trap_init(void)
{
extern void __trap_init(unsigned long);
unsigned long base = vectors_base(); //返回中斷base基址0xffff0000
__trap_init(base); //以base為vector基址,初始化中斷向量表
if (base != 0)
printk(KERN_DEBUG "Relocating machine vectors to 0x%08lx\n",
base);
#ifdef CONFIG_CPU_32
modify_domain(DOMAIN_USER, DOMAIN_CLIENT);
#endif
}
//--------------------------------------
2.vectors_base()
//gliethttp include/arch/asm-arm/proc-armv/system.h
extern unsigned long cr_alignment;
#if __LINUX_ARM_ARCH__ >= 4 //at91rm9200是ARMV4結構
#define vectors_base() ((cr_alignment & CR_V) ？ 0xffff0000 : 0)
#else
#define vectors_base() (0)
#endif
可以看到ARMv4以下的版本,該地址固定為0;ARMv4及以上版本，ARM中斷向量表的地址由CP15協處理器c1寄存器中V位(bit[13])控制,V和中斷向量表的對應關系如下:
V=0 ～ 0x00000000~0x0000001C
V=1 ～ 0xffff0000~0xffff001C
//------------------------------------------
2.1 cr_alignment
//gliethttp arch/arm/kernel/entry-armv.S
ENTRY(stext)
mov r12, r0
mov r0, #F_BIT | I_BIT | MODE_SVC @ make sure svc mode
msr cpsr_c, r0 @ and all irqs disabled
//__lookup_processor_type 查詢處理器類型,[gliethttp 以後補上<淺析head-armv.S>]返回值
//2007-07-04
//r9 = processor ID //讀取cp15的c0寄存器
//r10 = pointer to processor structure //下面會add pc, r10, #12,跳轉到__arm920_setup
//gliethttp 在vmlinux-armv.lds.in中
//__proc_info_begin = .;
// *(.proc.info)
// __proc_info_end = .;
//見2.2
bl __lookup_processor_type
teq r10, #0 @ invalid processor？
moveq r0, #'p' @ yes, error 'p'
beq __error
bl __lookup_architecture_type
teq r7, #0 @ invalid architecture？
moveq r0, #'a' @ yes, error 'a'
beq __error
//__create_page_tables 創建arm啟動臨時使用的前4M頁表
bl __create_page_tables
adr lr, __ret @ return address
add pc, r10, #12 @ initialise processor
.type __switch_data, %object
__switch_data: .long __mmap_switched
.long SYMBOL_NAME(__bss_start)
.long SYMBOL_NAME(_end)
.long SYMBOL_NAME(processor_id)
.long SYMBOL_NAME(__machine_arch_type)
.long SYMBOL_NAME(cr_alignment)
.long SYMBOL_NAME(init_task_union)+8192
/*
* Enable the MMU. This completely changes the structure of the visible
* memory space. You will not be able to trace execution through this.
* If you have an enquiry about this, *please* check the linux-arm-kernel
* mailing list archives BEFORE sending another post to the list.
*/
.type __ret, %function
__ret: ldr lr, __switch_data
mcr p15, 0, r0, c1, c0
//將__arm920_setup中設置的r0值,置入cp15協處理器c1寄存器中
mrc p15, 0, r0, c1, c0, 0 @ read it back.
mov r0, r0
//填充armv4中的三級流水線:mov r0,
r0 對應一個nop,所以對應2個nop和一個mov pc,lr剛好三個"無用"操作
mov r0, r0
mov pc, lr
//跳轉到__mmap_switched函數 gliethtttp
/*
* The following fragment of code is executed with the MMU on, and uses
* absolute addresses; this is not position independent.
*
* r0 = processor control register
* r1 = machine ID
* r9 = processor ID
*/
.align 5
__mmap_switched:
adr r3, __switch_data + 4
ldmia r3, {r4, r5, r6, r7, r8, sp}@ r2 = compat
//2007-07-04 gliethttp
//r4 ～ __bss_start
//r5 ～ _end
//r6 ～ processor_id
//r7 ～ __machine_arch_type
//r8 ～ cr_alignment
//sp ～ (init_task_union)+8192
//以下幾步操作對processor_id,__machine_arch_type,cr_alignment賦值gliethttp
mov fp, #0 @ Clear BSS (and zero fp)
1: cmp r4, r5 //bss區清0
strcc fp, [r4],#4
bcc 1b
str r9, [r6] @ Save processor ID
str r1, [r7] @ Save machine type
#ifdef CONFIG_ALIGNMENT_TRAP
orr r0, r0, #2 @ ...........A.
#endif
bic r2, r0, #2 @ Clear 'A' bit
//r2存放 禁用TRAP隊列故障 後的r0值
//r8->cr_alignment,cr_no_alignment
//所以stmia r8, {r0, r2}後,cr_alignment = r0,cr_no_alignment = r2
stmia r8, {r0, r2} @ Save control register values
b SYMBOL_NAME(start_kernel) //進入內核C程序
//--------------------------------------
2.2 __arm920_proc_info
//gliethttp arch/arm/mm/proc-arm920.S
.section ".proc.info", #alloc, #execinstr
.type __arm920_proc_info,#object
__arm920_proc_info:
//該地址存儲到r10中
.long 0x41009200
.long 0xff00fff0
.long 0x00000c1e
sp; .LCsirq: .word __temp_irq
如果你現在有這樣一種疑惑？程序為什麼編譯地址是0xc0008000,將其直接拷貝到0xffff0000和0xffff2000為什麼還能順利執行,請參考我的另一篇blog《arm相對跳轉到底是怎麼回事》,主要原因是b指令是相對跳轉指令,adr也是基於pc的前後偏移指令,當然對於ldr pc, __real_stubs_start + (.LCvswi - __stubs_start)是絕對地址賦值,所以最後pc會跳轉到0xc000xxxx空間執行代碼,其他的跳轉如:b __real_stubs_start + (vector_IRQ - __stubs_start)都會到0xffff2xxxx相應的vector_IRQ處執行向量中斷處理函數,還有一個要分析的問題是:.equ __real_stubs_start,
.LCvectors + 0x200,語句b __real_stubs_start
+ (vector_undefinstr - __stubs_start)就是跳轉到LCvectors+0x200空間執行。舉一個例子:
org 0x8000
reset b InitRest
...
InitRest:
...
那麼reset標號的地址為0x8000,他的意思是在0x8000處向前跳轉到InitRest, 我們也可以這樣來構造跳轉:
org 0x8000
reset b (0x8000+(.InitRest - .reset))
...
InitRest:
...
以上的構造語句同樣實現了相對0x8000地址的跳轉
