一步步写嵌入式操作系统 变参函数传参探究

摘要

为了实现内核的打印函数,需要对变参函数的传参进行分析

文件结构

启动代码start.s

.section .startup
.code 32
.align 0

.global _start
.extern __vector_reset
.extern __vector_undefined
.extern __vector_swi
.extern __vector_prefetch_abort
.extern __vector_data_abort
.extern __vector_reserved
.extern __vector_irq
.extern __vector_fiq

_start:
	ldr pc,_vector_reset
	ldr pc,_vector_undefined
	ldr pc,_vector_swi
	ldr pc,_vector_prefetch_abort
	ldr pc,_vector_data_abort
	ldr pc,_vector_reserved
	ldr pc,_vector_irq
	ldr pc,_vector_fiq

	.align 4

_vector_reset:			.word  __vector_reset
_vector_undefined:		.word  __vector_undefined
_vector_swi:			.word  __vector_swi
_vector_prefetch_abort:	.word  __vector_prefetch_abort
_vector_data_abort:		.word  __vector_data_abort
_vector_reserved:		.word  __vector_reserved
_vector_irq:			.word  __vector_irq
_vector_fiq:			.word  __vector_fiq

异常处理代码abnormal.s

.global __vector_undefined
.global __vector_swi
.global __vector_prefetch_abort
.global __vector_data_abort
.global __vector_reserved
.global __vector_irq
.global __vector_fiq

.text
.code 32

__vector_undefined:
	nop
__vector_swi:
	nop
__vector_prefetch_abort:	
	nop
__vector_data_abort:
	nop
__vector_reserved:
	nop
__vector_irq:
	nop
__vector_fiq:
	nop

初始化代码init.s

.equ DISABLE_IRQ,	0x80
.equ DISABLE_FIQ,	0x40
.equ SYS_MOD,		0x1f
.equ IRQ_MOD,		0x12
.equ FIQ_MOD,		0x11
.equ SVC_MOD,		0x13
.equ ABT_MOD,		0x17
.equ UND_MOD,		0x1b

.equ MEM_SIZE, 		0x800000
.equ TEXT_BASE,		0x30000000
.equ _SVC_STACK,	(TEXT_BASE+MEM_SIZE-4)
.equ _IRQ_STACK,	(_SVC_STACK-0x400)
.equ _FIQ_STACK,	(_IRQ_STACK-0x400)
.equ _ABT_STACK,	(_FIQ_STACK-0x400)
.equ _UND_STACK,	(_ABT_STACK-0x400)
.equ _SYS_STACK,	(_UND_STACK-0x400)

.text
.code 32
.global __vector_reset

.extern plat_boot
.extern __bss_start__
.extern __bss_end__

__vector_reset:
	msr cpsr_c,#(DISABLE_IRQ|DISABLE_FIQ|SVC_MOD)
	ldr sp,=_SVC_STACK
	msr cpsr_c,#(DISABLE_IRQ|DISABLE_FIQ|IRQ_MOD)
	ldr sp,=_IRQ_STACK
	msr cpsr_c,#(DISABLE_IRQ|DISABLE_FIQ|FIQ_MOD)
	ldr sp,=_FIQ_STACK
	msr cpsr_c,#(DISABLE_IRQ|DISABLE_FIQ|ABT_MOD)
	ldr sp,=_ABT_STACK
	msr cpsr_c,#(DISABLE_IRQ|DISABLE_FIQ|UND_MOD)
	ldr sp,=_UND_STACK
	msr cpsr_c,#(DISABLE_IRQ|DISABLE_FIQ|SYS_MOD)
	ldr sp,=_SYS_STACK

_clear_bss:
	ldr r1,_bss_start_
	ldr r3,_bss_end_
	mov r2,#0x0
1:
	cmp r1,r3
	beq _main
	str r2,[r1],#0x4
	b 1b

_main:
	b plat_boot


_bss_start_:	.word  __bss_start__
_bss_end_:		.word  __bss_end__

.end

mmu功能代码mmu.c

/*mask for page table base addr*/
#define PAGE_TABLE_L1_BASE_ADDR_MASK	(0xffffc000)

#define VIRT_TO_PTE_L1_INDEX(addr)		(((addr)&0xfff00000)>>18)

#define PTE_L1_SECTION_NO_CACHE_AND_WB	(0x0<<2)
#define PTE_L1_SECTION_DOMAIN_DEFAULT	(0x0<<5)
#define PTE_ALL_AP_L1_SECTION_DEFAULT	(0x1<<10)

#define PTE_L1_SECTION_PADDR_BASE_MASK	(0xfff00000)
#define PTE_BITS_L1_SECTION				(0x2)

#define L1_PTR_BASE_ADDR				0x30700000
#define PHYSICAL_MEM_ADDR				0x30000000
#define VIRTUAL_MEM_ADDR				0x30000000
#define MEM_MAP_SIZE					0x800000
#define PHYSICAL_IO_ADDR				0x48000000
#define VIRTUAL_IO_ADDR					0xc8000000
#define IO_MAP_SIZE						0x18000000

void start_mmu(void)
{
	unsigned int ttb = L1_PTR_BASE_ADDR;

	asm (
		"mcr p15,0,%0,c2,c0,0\n"    /* set base address of page table*/
		"mvn r0,#0\n"                  
		"mcr p15,0,r0,c3,c0,0\n"    /* enable all region access*/

		"mov r0,#0x1\n"
		"mcr p15,0,r0,c1,c0,0\n"    /* set back to control register */
		"mov r0,r0\n"
		"mov r0,r0\n"
		"mov r0,r0\n"
		:
		: "r" (ttb)
		:"r0"
	);
}

unsigned int gen_l1_pte(unsigned int paddr)
{
	return (paddr & PTE_L1_SECTION_PADDR_BASE_MASK) | PTE_BITS_L1_SECTION;
}

unsigned int gen_l1_pte_addr(unsigned int baddr, unsigned int vaddr)
{
	return (baddr & PAGE_TABLE_L1_BASE_ADDR_MASK) | VIRT_TO_PTE_L1_INDEX(vaddr);
}

void init_sys_mmu(void)
{
	unsigned int pte;
	unsigned int pte_addr;
	int j;

	for(j = 0; j < MEM_MAP_SIZE>>20; j++) {
		pte = gen_l1_pte(PHYSICAL_MEM_ADDR + (j<<20));
		pte |= PTE_ALL_AP_L1_SECTION_DEFAULT;
		pte |= PTE_L1_SECTION_NO_CACHE_AND_WB;
		pte |= PTE_L1_SECTION_DOMAIN_DEFAULT;
		pte_addr = gen_l1_pte_addr(L1_PTR_BASE_ADDR, VIRTUAL_MEM_ADDR + (j<<20));
		*(volatile unsigned int *)pte_addr = pte;
	}

	for(j = 0; j< IO_MAP_SIZE>>20; j++) {
		pte = gen_l1_pte(PHYSICAL_IO_ADDR+(j<<20));
		pte |= PTE_ALL_AP_L1_SECTION_DEFAULT;
		pte |= PTE_L1_SECTION_NO_CACHE_AND_WB;
		pte |= PTE_L1_SECTION_DOMAIN_DEFAULT;
		pte_addr = gen_l1_pte_addr(L1_PTR_BASE_ADDR, VIRTUAL_IO_ADDR + (j<<20));
		*(volatile unsigned int *)pte_addr = pte;
	}
}

打印功能代码print.c

void print_char(char ch)
{
	*(volatile unsigned int *)(0xd0000020) = ch;
}

void print_char_n_up(char *pChar,int n)
{
	int i = 0;
	for (i = 0; i < n; i ++)
	{
		if (*(pChar + i) != 'x')
		{
			print_char('-');
		}
		else
		{
			print_char(*(pChar + i));
		}
	}
}


void print_char_n_down(char *pChar,int n)
{
	int i = 0;
	for (i = 0; i < n; i ++)
	{
		if (*(pChar - i) != 'x')
		{
			print_char('-');
		}
		else
		{
			print_char(*(pChar - i));
		}
	}
}

void test_char(char argv1,...)
{
	print_char_n_up((char *)&argv1,16);
	print_char('\n');
	print_char_n_down((char *)&argv1,16);
}

void test_short(short argv1,...)
{
	print_char_n_up((char *)&argv1,16);
}

void test_int(int argv1,...)
{
	print_char_n_up((char *)&argv1,16);
}

void test_long(long argv1,...)
{
	print_char_n_up((char *)&argv1,16);
}

void test_pointer(char *argv1,...)
{
	print_char_n_up((char *)&argv1,16);
}

void test_printk(void)
{
	test_char('a','x');
	print_char('\n');
	test_short('a','x');
	print_char('\n');
	test_int('a','x');
	print_char('\n');
	test_long('a','x');
	print_char('\n');
	test_pointer("a",'x');
}

// 现象:
// ----x-----------
// ----------------
// ----x-----------
// ----x-----------
// ----x-----------
// ----x-----------

开机代码boot.c

typedef void (*init_func)(void);

#define UFCON0	((volatile unsigned int *)(0x50000020))

void helloworld(void)
{
	const char *p = "4.2_helloworld\n";
	while(*p) {
		*UFCON0 = *p++;
	};
}

static init_func init[] = {
	helloworld,
	0,
};

void test_mmu(void)
{
	const char *p = "4.2_test_mmu\n";
	while (*p) {
		*(volatile unsigned int *)0xd0000020 = *p++;
	};
}

extern void test_printk(void);

void plat_boot(void)
{
	int i;
	for(i = 0; init[i]; i++) {
		init[i]();
	}
	init_sys_mmu();
	start_mmu();
	test_mmu();
	test_printk();

	while(1);
}

编译脚本makefile

CC = arm-none-eabi-gcc
LD = arm-none-eabi-ld
OBJCOPY = arm-none-eabi-objcopy

CFLAGS = -O0 -g
ASFLAGS = -O0 -g
LDFLAGS = -T leeos.lds -Ttext 30000000 

OBJS = init.o start.o boot.o abnormal.o mmu.o print.o

.c.o:
	$(CC) $(CFLAGS) -c $<
.s.o:
	$(CC) $(ASFLAGS) -c $<

leeos.elf:$(OBJS)
	$(CC) -static -nostartfiles -nostdlib $(LDFLAGS) $? -o $@ -lgcc
	$(OBJCOPY) -O binary $@ leeos.bin

clean:
	rm -f *.o leeos.elf leeos.bin

链接脚本leeos.lds

OUTPUT_ARCH(arm)
ENTRY(_start)

SECTIONS
{
	. = 0x00000000;
	.text :
	{
		*(.startup)
		*(.text)
	}
	. = ALIGN(32);
	.data :
	{
		*(.data)
	}
	. = ALIGN(32);
	__bss_start__ = .;
	.bss :
	{
		*(.bss)
	}
	__bss_end__ = .;
}

仿真器配置脚本skyeye.conf

cpu:  arm920t
mach: s3c2410x
  
#physical memory
mem_bank: map=M, type=RW, addr=0x30000000, size=0x00800000, file=./leeos.bin,boot=yes
mem_bank: map=I, type=RW, addr=0x48000000, size=0x20000000

现象

结论

针对print功能的现象:

----x-----------
----------------
----x-----------
----x-----------
----x-----------

可知

1.当前环境下变参函数压栈顺序为从右到左(系统为满减栈)

2.当前环境下变参函数的每个参数占据固定4个字节地址空间,也就是占4个字节

有了这些结论,可以写出类似于printf的内核打印函数了