移植嵌入式Linux到ARM处理器:BootLoader(8)

本节简介:

BootLoader指系统启动后，在操作系统内核运行之前运行的一段小程序。通过BootLoader，我们可以初始化硬件设备、建立内存空间的映射图，从而将系统的软硬件环境带到一个合适的状态，以便为最终调用操作系统内核准备好正确的环境。通常，BootLoader 是严重地依赖于硬件而实现的，特别是在嵌入式世界。因此，在嵌入式世界里建立一个通用的 BootLoader 几乎是不可能的。尽管如此，我们仍然可以对BootLoader归纳出一些通用的概念来，以指导用户特定的BootLoader设计与实现。
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3.2Bootloader拷贝

　　配置为从NAND FLASH启动，需要将NAND FLASH中的vivi代码copy到RAM中：

#ifdef CONFIG_S3C2410_NAND_BOOT
bl copy_myself

@ jump to ram
ldr r1, =on_the_ram
add pc, r1, #0
nop
nop
1: b 1b @ infinite loop

#ifdef CONFIG_S3C2410_NAND_BOOT
@
@ copy_myself: copy vivi to ram
@
copy_myself:
mov r10, lr

@ reset NAND
mov r1, #NAND_CTL_BASE
ldr r2, =0xf830 @ initial value
str r2, [r1, #oNFCONF]
ldr r2, [r1, #oNFCONF]
bic r2, r2, #0x800 @ enable chip
str r2, [r1, #oNFCONF]
mov r2, #0xff @ RESET command
strb r2, [r1, #oNFCMD]
mov r3, #0 @ wait
1: add r3, r3, #0x1
cmp r3, #0xa
blt 1b
2: ldr r2, [r1, #oNFSTAT] @ wait ready
tst r2, #0x1
beq 2b
ldr r2, [r1, #oNFCONF]
orr r2, r2, #0x800 @ disable chip
str r2, [r1, #oNFCONF]

@ get read to call C functions (for nand_read())
ldr sp, DW_STACK_START @ setup stack pointer
mov fp, #0 @ no previous frame, so fp=0

@ copy vivi to RAM
ldr r0, =VIVI_RAM_BASE
mov r1, #0x0
mov r2, #0x20000
bl nand_read_ll

tst r0, #0x0
beq ok_nand_read
#ifdef CONFIG_DEBUG_LL
bad_nand_read:
ldr r0, STR_FAIL
ldr r1, SerBase
bl PrintWord
1: b 1b @ infinite loop
#endif

ok_nand_read:
#ifdef CONFIG_DEBUG_LL
ldr r0, STR_OK
ldr r1, SerBase
bl PrintWord
#endif

@ verify
mov r0, #0
ldr r1, =0x33f00000
mov r2, #0x400 @ 4 bytes * 1024 = 4K-bytes
go_next:
ldr r3, [r0], #4
ldr r4, [r1], #4
teq r3, r4
bne notmatch
subs r2, r2, #4
beq done_nand_read
bne go_next
notmatch:
#ifdef CONFIG_DEBUG_LL
sub r0, r0, #4
ldr r1, SerBase
bl PrintHexWord
ldr r0, STR_FAIL
ldr r1, SerBase
bl PrintWord
#endif
1: b 1b
done_nand_read:

#ifdef CONFIG_DEBUG_LL
ldr r0, STR_OK
ldr r1, SerBase
bl PrintWord
#endif

mov pc, r10

@ clear memory
@ r0: start address
@ r1: length
mem_clear:
mov r2, #0
mov r3, r2
mov r4, r2
mov r5, r2
mov r6, r2
mov r7, r2
mov r8, r2
mov r9, r2

clear_loop:
stmia r0!, {r2-r9}
subs r1, r1, #(8 * 4)
bne clear_loop

mov pc, lr

#endif @ CONFIG_S3C2410_NAND_BOOT

　　3.3进入C代码

　　首先要设置堆栈指针sp，堆栈指针的设置是为了执行C语言代码作好准备。设置好堆栈后，调用C语言的main函数：

@ get read to call C functions
ldr sp, DW_STACK_START @ setup stack pointer
mov fp, #0 @ no previous frame, so fp=0
mov a2, #0 @ set argv to NULL

bl main @ call main

mov pc, #FLASH_BASE @ otherwise, reboot

作者:宋宝华   更新日期:2006-11-21