在simics上trace vexpress a9x4board 以及 ls1021atwr的uboot启动过程.
1.首先安装simics,uboot,arm交叉编译器,以及opengrok查阅代码
编译 u-boot
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- clean
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- distclean
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- vexpress_defconfig
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- uImage LOADADDR=0x60008000
vexpress-a9x4-linux.simics 脚本文件
add-directory “%script%”
run-command-file “%simics%/targets/versatile-express/vexpress-a9x4-linux.simics”
VExpress.console0.con.close
add-symbol-file /home/wind/work/opengrok/u-boot/u-boot relocation-address = 0x60800000 #重定位地址,代码被copy后PC跳转至此处
b 0x80 #设置断点,因为uboot前面还有一段a.rom从0地址开始执行直到0x80处.
编译 u-boot
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- clean
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- distclean
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- ls1021atwr_nor_defconfig
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- -j
ls1021atwr-linux-common.simics 脚本文件
add-directory “%script%”
run-command-file “%simics%/targets/qoriq-ls1/ls1021atwr-linux-common.simics”
twr_ls1021a.console1.con.close #关闭串口
b 0x80
add-symbol-file /home/wind/workspace/opensrc/u-boot/u-boot relocation-address = 0x67f80000 # readelf -e u-boot =>>Entry point address: 0x67f80000
u-boot 会被重新copy到内存的另一地址处: 打开debug宏可以看到
“Relocating to bff60000, new gd at bef3feb4, sp at bef3fe90”
relocate_code 内存代码地址: 0x67f81508
add-symbol-file /home/wind/workspace/opensrc/u-boot/u-boot relocation-address = 0xbff60000
2.修改.simics并用debug模式启动vexpress-a9x4-linux.simics
3.Ctrl+R运行到指定行(0x80),然后开始跟踪uboot执行代码,可以开启instruction step mode mixed assembly
0x60800000 [u-boot] __image_copy_start(): arch/arm/lib/vectors.S, line 54
0x60800300 [u-boot] reset(): arch/arm/cpu/armv7/start.S, line 36
0x60800350 [u-boot] save_boot_params(): arch/arm/cpu/armv7/start.S, line 100
0x60800304 [u-boot] save_boot_params_ret(): arch/arm/cpu/armv7/start.S, line 42
0x60800354 [u-boot] cpu_init_cp15(): arch/arm/cpu/armv7/start.S, line 116
0x608003a8 [u-boot] cpu_init_crit(): arch/arm/cpu/armv7/start.S, line 248
0x60800590 [u-boot] _main(): arch/arm/lib/crt0.S, line 70
***arch/arm/lib/vectors.S***
----------------------------
#include <config.h>
/*
*************************************************************************
*
* Symbol _start is referenced elsewhere, so make it global
*
*************************************************************************
*/
.globl _start
/*
*************************************************************************
*
* Vectors have their own section so linker script can map them easily
*
*************************************************************************
*/
.section ".vectors", "ax"
/*
*************************************************************************
*
* Exception vectors as described in ARM reference manuals
*
* Uses indirect branch to allow reaching handlers anywhere in memory.
*
*************************************************************************
*/
_start:
#ifdef CONFIG_SYS_DV_NOR_BOOT_CFG
.word CONFIG_SYS_DV_NOR_BOOT_CFG
#endif
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
/*
*************************************************************************
*
* Indirect vectors table
*
* Symbols referenced here must be defined somewhere else
*
*************************************************************************
*/
.globl _undefined_instruction
.globl _software_interrupt
.globl _prefetch_abort
.globl _data_abort
.globl _not_used
.globl _irq
.globl _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
.balignl 16,0xdeadbeef
When the processor is reset then hardware sets the pc to 0x0000 and starts executing by fetching the instruction at 0x0000. When an undefined instruction is executed or tries to be executed the hardware responds by setting the pc to 0x0004 and starts executing the instruction at 0x0004. irq interrupt, the hardware finishes the instruction it is executing starts executing the instruction at address 0x0018. and so on.
00000000 <_start>:
0: ea00000d b 3c <reset>
4: e59ff014 ldr pc, [pc, #20] ; 20 <_undefined_instruction>
8: e59ff014 ldr pc, [pc, #20] ; 24 <_software_interrupt>
c: e59ff014 ldr pc, [pc, #20] ; 28 <_prefetch_abort>
10: e59ff014 ldr pc, [pc, #20] ; 2c <_data_abort>
14: e59ff014 ldr pc, [pc, #20] ; 30 <_not_used> 硬件负责把PC指针指向这些地址当发生这些异常后
18: e59ff014 ldr pc, [pc, #20] ; 34 <_irq>
1c: e59ff014 ldr pc, [pc, #20] ; 38 <_fiq>
/*
b是相对跳转， 这种指令的作用是相对于当前的pc向前或者向后跳转多远。
而ldr pc，xxxx这种是绝对跳转，你写进pc的地址就是你的目的地址。
*/
00000020 <_undefined_instruction>:
20: 00000000 andeq r0, r0, r0
00000024 <_software_interrupt>:
24: 00000000 andeq r0, r0, r0
00000028 <_prefetch_abort>:
28: 00000000 andeq r0, r0, r0
0000002c <_data_abort>:
2c: 00000000 andeq r0, r0, r0
00000030 <_not_used>:
30: 00000000 andeq r0, r0, r0
00000034 <_irq>:
34: 00000000 andeq r0, r0, r0
00000038 <_fiq>:
38: 00000000 andeq r0, r0, r0
Now of course in addition to changing the pc and starting execution from these addresses. The hardware will save the state of the machine, switch processor modes if necessary and then start executing at the new address from the vector table.
Our job as programmers is to build the binary such that the instructions we want to be run for each of these instructions is at the right address. The hardware provides one word, one instruction for each location. Now if you never expect to ever have any of these exceptions, you dont have to have a branch at address zero for example you can just have your program start, there is nothing magic about the memory at these addresses. If you expect to have these exceptions, then you have two choices for instructions that are one word and can jump out of the way of the exception that follows. One is a branch the other is a load pc. There are pros and cons to each.
When the hardware takes an exception, the program counter (PC) is automatically set to the address of the relevant exception vector and the processor begins executing instructions from that address. When the processor comes out of reset, the PC is automatically set to base+0. An undefined instruction sets the PC to base+4, etc. The base address of the vector table (base) is either 0x00000000, 0xFFFF0000, or VBAR depending on the processor and configuration. Note that this provides limited flexibility in where the vector table gets placed and you'll need to consult the ARM documentation in conjunction with the reference manual for the device that you are using to get the right value to be used.
The layout of the table (4 bytes per exception) makes it necessary to immediately branch from the vector to the actual exception handler. The reasons for the LDR PC, label approach are twofold - because a PC-relative branch is limited to (24 << 2) bits (+/-32MB) using B would constrain the layout of the code in memory somewhat; by loading an absolute address the handler can be located anywhere in memory. Secondly it makes it very simple to change exception handlers at runtime, by simply writing a different address to that location, rather than having to assemble and hotpatch a branch instruction.
There's little value to having a remappable reset vector in this way, however, which is why you tend to see that one implemented as a simple branch to skip over the rest of the vectors to the real entry point code.
***arch/arm/cpu/armv7/start.S***
--------------------------------
/*************************************************************************
*
* Startup Code (reset vector)
*
* do important init only if we don't start from memory!
* setup Memory and board specific bits prior to relocation.
* relocate armboot to ram
* setup stack
*
*************************************************************************/
.globl reset
.globl save_boot_params_ret
reset:
/* Allow the board to save important registers */
b save_boot_params
save_boot_params_ret:
/*
* disable interrupts (FIQ and IRQ), also set the cpu to SVC32 mode,
* except if in HYP mode already
*/
mrs r0, cpsr
and r1, r0, #0x1f @ mask mode bits
teq r1, #0x1a @ test for HYP mode
bicne r0, r0, #0x1f @ clear all mode bits
orrne r0, r0, #0x13 @ set SVC mode
orr r0, r0, #0xc0 @ disable FIQ and IRQ
msr cpsr,r0
/*
* Setup vector:
* (OMAP4 spl TEXT_BASE is not 32 byte aligned.
* Continue to use ROM code vector only in OMAP4 spl)
*/
#if !(defined(CONFIG_OMAP44XX) && defined(CONFIG_SPL_BUILD))
/* Set V=0 in CP15 SCTLR register - for VBAR to point to vector */
mrc p15, 0, r0, c1, c0, 0 @ Read CP15 SCTLR Register
bic r0, #CR_V @ V = 0
mcr p15, 0, r0, c1, c0, 0 @ Write CP15 SCTLR Register
/* Set vector address in CP15 VBAR register */
ldr r0, =_start
mcr p15, 0, r0, c12, c0, 0 @Set VBAR
#endif
/* the mask ROM code should have PLL and others stable */
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
bl cpu_init_cp15
bl cpu_init_crit
#endif
bl _main
***_main(): arch/arm/lib/crt0.S***
----------------------------
/*
* crt0 - C-runtime startup Code for ARM U-Boot
*
* Copyright (c) 2012 Albert ARIBAUD <albert.u.boot@aribaud.net>
*
* SPDX-License-Identifier: GPL-2.0+
*/
#include <config.h>
#include <asm-offsets.h>
#include <linux/linkage.h>
/*
* This file handles the target-independent stages of the U-Boot
* start-up where a C runtime environment is needed. Its entry point
* is _main and is branched into from the target's start.S file.
*
* _main execution sequence is:
*
* 1. Set up initial environment for calling board_init_f().
* This environment only provides a stack and a place to store
* the GD ('global data') structure, both located in some readily
* available RAM (SRAM, locked cache...). In this context, VARIABLE
* global data, initialized or not (BSS), are UNAVAILABLE; only
* CONSTANT initialized data are available.
*
* 2. Call board_init_f(). This function prepares the hardware for
* execution from system RAM (DRAM, DDR...) As system RAM may not
* be available yet, , board_init_f() must use the current GD to
* store any data which must be passed on to later stages. These
* data include the relocation destination, the future stack, and
* the future GD location.
*
* (the following applies only to non-SPL builds)
*
* 3. Set up intermediate environment where the stack and GD are the
* ones allocated by board_init_f() in system RAM, but BSS and
* initialized non-const data are still not available.
*
* 4. Call relocate_code(). This function relocates U-Boot from its
* current location into the relocation destination computed by
* board_init_f().
*
* 5. Set up final environment for calling board_init_r(). This
* environment has BSS (initialized to 0), initialized non-const
* data (initialized to their intended value), and stack in system
* RAM. GD has retained values set by board_init_f(). Some CPUs
* have some work left to do at this point regarding memory, so
* call c_runtime_cpu_setup.
*
* 6. Branch to board_init_r().
*/
/*
* entry point of crt0 sequence
*/
ENTRY(_main)
/*
* Set up initial C runtime environment and call board_init_f(0).
*/
#if defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_STACK)
ldr sp, =(CONFIG_SPL_STACK)
#else
ldr sp, =(CONFIG_SYS_INIT_SP_ADDR)
#endif
bic sp, sp, #7 /* 8-byte alignment for ABI compliance */
mov r2, sp
sub sp, sp, #GD_SIZE /* allocate one GD above SP */
bic sp, sp, #7 /* 8-byte alignment for ABI compliance */
mov r9, sp /* GD is above SP */
mov r1, sp
mov r0, #0
clr_gd:
cmp r1, r2 /* while not at end of GD */
strlo r0, [r1] /* clear 32-bit GD word */
addlo r1, r1, #4 /* move to next */
blo clr_gd
#if defined(CONFIG_SYS_MALLOC_F_LEN)
sub sp, sp, #CONFIG_SYS_MALLOC_F_LEN
str sp, [r9, #GD_MALLOC_BASE]
#endif
/* mov r0, #0 not needed due to above code */
bl board_init_f
#if ! defined(CONFIG_SPL_BUILD)
/*
* Set up intermediate environment (new sp and gd) and call
* relocate_code(addr_moni). Trick here is that we'll return
* 'here' but relocated.
*/
ldr sp, [r9, #GD_START_ADDR_SP] /* sp = gd->start_addr_sp */
bic sp, sp, #7 /* 8-byte alignment for ABI compliance */
ldr r9, [r9, #GD_BD] /* r9 = gd->bd */
sub r9, r9, #GD_SIZE /* new GD is below bd */
adr lr, here
ldr r0, [r9, #GD_RELOC_OFF] /* r0 = gd->reloc_off */
add lr, lr, r0
ldr r0, [r9, #GD_RELOCADDR] /* r0 = gd->relocaddr */
/*
* void relocate_code(addr_moni)
*
* This function relocates the monitor code.
*
* NOTE:
* To prevent the code below from containing references with an R_ARM_ABS32
* relocation record type, we never refer to linker-defined symbols directly.
* Instead, we declare literals which contain their relative location with
* respect to relocate_code, and at run time, add relocate_code back to them.
*/
b relocate_code /* #add-symbol-file /home/wind/workspace/opensrc/u-boot/u-boot relocation-address = 0xbff60000 */
here:
/*
* now relocate vectors
*/
bl relocate_vectors
/* Set up final (full) environment */
bl c_runtime_cpu_setup /* we still call old routine here */
#endif
#if !defined(CONFIG_SPL_BUILD) || defined(CONFIG_SPL_FRAMEWORK)
# ifdef CONFIG_SPL_BUILD
/* Use a DRAM stack for the rest of SPL, if requested */
bl spl_relocate_stack_gd
cmp r0, #0
movne sp, r0
# endif
ldr r0, =__bss_start /* this is auto-relocated! */
#ifdef CONFIG_USE_ARCH_MEMSET
ldr r3, =__bss_end /* this is auto-relocated! */
mov r1, #0x00000000 /* prepare zero to clear BSS */
subs r2, r3, r0 /* r2 = memset len */
bl memset
#else
ldr r1, =__bss_end /* this is auto-relocated! */
mov r2, #0x00000000 /* prepare zero to clear BSS */
clbss_l:cmp r0, r1 /* while not at end of BSS */
strlo r2, [r0] /* clear 32-bit BSS word */
addlo r0, r0, #4 /* move to next */
blo clbss_l
#endif
#if ! defined(CONFIG_SPL_BUILD)
bl coloured_LED_init
bl red_led_on
#endif
/* call board_init_r(gd_t *id, ulong dest_addr) */
mov r0, r9 /* gd_t */
ldr r1, [r9, #GD_RELOCADDR] /* dest_addr */
/* call board_init_r */
ldr pc, =board_init_r /* this is auto-relocated! */
/* we should not return here. */
#endif
ENDPROC(_main)
0x60803644 [u-boot] board_init_f(): common/board_f.c, line 966
if (initcall_run_list(init_sequence_f))
timer console arch dram i2c spi
static init_fnc_t init_sequence_f[] = {
#ifdef CONFIG_SANDBOX
setup_ram_buf,
#endif
setup_mon_len,
#ifdef CONFIG_OF_CONTROL
fdtdec_setup,
#endif
#if defined(CONFIG_X86) && defined(CONFIG_HAVE_FSP)
x86_fsp_init,
#endif
#ifdef CONFIG_TRACE
trace_early_init,
#endif
initf_malloc,
#if defined(CONFIG_MPC85xx) || defined(CONFIG_MPC86xx)
/* TODO: can this go into arch_cpu_init()? */
probecpu,
#endif
arch_cpu_init, /* basic arch cpu dependent setup */
mark_bootstage,
initf_dm,
arch_cpu_init_dm,
#if defined(CONFIG_BOARD_EARLY_INIT_F)
board_early_init_f,
#endif
/* TODO: can any of this go into arch_cpu_init()? */
#if defined(CONFIG_PPC) && !defined(CONFIG_8xx_CPUCLK_DEFAULT)
get_clocks, /* get CPU and bus clocks (etc.) */
#if defined(CONFIG_TQM8xxL) && !defined(CONFIG_TQM866M) \
&& !defined(CONFIG_TQM885D)
adjust_sdram_tbs_8xx,
#endif
/* TODO: can we rename this to timer_init()? */
init_timebase,
#endif
#if defined(CONFIG_ARM) || defined(CONFIG_MIPS) || defined(CONFIG_BLACKFIN)
timer_init, /* initialize timer */
#endif
#ifdef CONFIG_SYS_ALLOC_DPRAM
#if !defined(CONFIG_CPM2)
dpram_init,
#endif
#endif
#if defined(CONFIG_BOARD_POSTCLK_INIT)
board_postclk_init,
#endif
#ifdef CONFIG_FSL_ESDHC
get_clocks,
#endif
#ifdef CONFIG_M68K
get_clocks,
#endif
env_init, /* initialize environment */
#if defined(CONFIG_8xx_CPUCLK_DEFAULT)
/* get CPU and bus clocks according to the environment variable */
get_clocks_866,
/* adjust sdram refresh rate according to the new clock */
sdram_adjust_866,
init_timebase,
#endif
init_baud_rate, /* initialze baudrate settings */
serial_init, /* serial communications setup */
console_init_f, /* stage 1 init of console */
#ifdef CONFIG_SANDBOX
sandbox_early_getopt_check,
#endif
#ifdef CONFIG_OF_CONTROL
fdtdec_prepare_fdt,
#endif
display_options, /* say that we are here */
display_text_info, /* show debugging info if required */
#if defined(CONFIG_MPC8260)
prt_8260_rsr,
prt_8260_clks,
#endif /* CONFIG_MPC8260 */
#if defined(CONFIG_MPC83xx)
prt_83xx_rsr,
#endif
#if defined(CONFIG_PPC) || defined(CONFIG_M68K)
checkcpu,
#endif
print_cpuinfo, /* display cpu info (and speed) */
#if defined(CONFIG_MPC5xxx)
prt_mpc5xxx_clks,
#endif /* CONFIG_MPC5xxx */
#if defined(CONFIG_DISPLAY_BOARDINFO)
show_board_info,
#endif
INIT_FUNC_WATCHDOG_INIT
#if defined(CONFIG_MISC_INIT_F)
misc_init_f,
#endif
INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_HARD_I2C) || defined(CONFIG_SYS_I2C)
init_func_i2c,
#endif
#if defined(CONFIG_HARD_SPI)
init_func_spi,
#endif
announce_dram_init,
/* TODO: unify all these dram functions? */
#if defined(CONFIG_ARM) || defined(CONFIG_X86) || defined(CONFIG_MICROBLAZE) || defined(CONFIG_AVR32)
dram_init, /* configure available RAM banks */
#endif
#if defined(CONFIG_MIPS) || defined(CONFIG_PPC) || defined(CONFIG_M68K)
init_func_ram,
#endif
#ifdef CONFIG_POST
post_init_f,
#endif
INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_SYS_DRAM_TEST)
testdram,
#endif /* CONFIG_SYS_DRAM_TEST */
INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_POST
init_post,
#endif
INIT_FUNC_WATCHDOG_RESET
/*
* Now that we have DRAM mapped and working, we can
* relocate the code and continue running from DRAM.
*
* Reserve memory at end of RAM for (top down in that order):
* - area that won't get touched by U-Boot and Linux (optional)
* - kernel log buffer
* - protected RAM
* - LCD framebuffer
* - monitor code
* - board info struct
*/
setup_dest_addr,
#if defined(CONFIG_BLACKFIN) || defined(CONFIG_NIOS2)
/* Blackfin u-boot monitor should be on top of the ram */
reserve_uboot,
#endif
#if defined(CONFIG_LOGBUFFER) && !defined(CONFIG_ALT_LB_ADDR)
reserve_logbuffer,
#endif
#ifdef CONFIG_PRAM
reserve_pram,
#endif
reserve_round_4k,
#if !(defined(CONFIG_SYS_ICACHE_OFF) && defined(CONFIG_SYS_DCACHE_OFF)) && \
defined(CONFIG_ARM)
reserve_mmu,
#endif
#ifdef CONFIG_LCD
reserve_lcd,
#endif
reserve_trace,
/* TODO: Why the dependency on CONFIG_8xx? */
#if defined(CONFIG_VIDEO) && (!defined(CONFIG_PPC) || defined(CONFIG_8xx)) && \
!defined(CONFIG_ARM) && !defined(CONFIG_X86) && \
!defined(CONFIG_BLACKFIN) && !defined(CONFIG_M68K)
reserve_video,
#endif
#if !defined(CONFIG_BLACKFIN) && !defined(CONFIG_NIOS2)
reserve_uboot,
#endif
#ifndef CONFIG_SPL_BUILD
reserve_malloc,
reserve_board,
#endif
setup_machine,
reserve_global_data,
reserve_fdt,
reserve_arch,
reserve_stacks,
setup_dram_config,
show_dram_config,
#if defined(CONFIG_PPC) || defined(CONFIG_M68K)
setup_board_part1,
INIT_FUNC_WATCHDOG_RESET
setup_board_part2,
#endif
display_new_sp,
#ifdef CONFIG_SYS_EXTBDINFO
setup_board_extra,
#endif
INIT_FUNC_WATCHDOG_RESET
reloc_fdt,
setup_reloc,
#if defined(CONFIG_X86) || defined(CONFIG_ARC)
copy_uboot_to_ram,
clear_bss,
do_elf_reloc_fixups,
#endif
#if !defined(CONFIG_ARM) && !defined(CONFIG_SANDBOX)
jump_to_copy, for PPC
#endif
NULL,
};
/* ARM calls relocate_code from its crt0.S, here for powerpc and intel and mips ....*/
#if !defined(CONFIG_ARM) && !defined(CONFIG_SANDBOX)
static int jump_to_copy(void)
{
if (gd->flags & GD_FLG_SKIP_RELOC)
return 0;
/*
* x86 is special, but in a nice way. It uses a trampoline which
* enables the dcache if possible.
*
* For now, other archs use relocate_code(), which is implemented
* similarly for all archs. When we do generic relocation, hopefully
* we can make all archs enable the dcache prior to relocation.
*/
#if defined(CONFIG_X86) || defined(CONFIG_ARC)
/*
* SDRAM and console are now initialised. The final stack can now
* be setup in SDRAM. Code execution will continue in Flash, but
* with the stack in SDRAM and Global Data in temporary memory
* (CPU cache)
*/
arch_setup_gd(gd->new_gd);
board_init_f_r_trampoline(gd->start_addr_sp);
#else
relocate_code(gd->start_addr_sp, gd->new_gd, gd->relocaddr);
#endif
return 0;
}
#endif
继续执行main函数
0x6081cdfc [u-boot] initcall_run_list(): lib/initcall.c, line 13
void board_init_r(gd_t *new_gd, ulong dest_addr)
{
#ifdef CONFIG_NEEDS_MANUAL_RELOC
int i;
#endif
#ifdef CONFIG_AVR32
mmu_init_r(dest_addr);
#endif
#if !defined(CONFIG_X86) && !defined(CONFIG_ARM) && !defined(CONFIG_ARM64)
gd = new_gd;
#endif
#ifdef CONFIG_NEEDS_MANUAL_RELOC
for (i = 0; i < ARRAY_SIZE(init_sequence_r); i++)
init_sequence_r[i] += gd->reloc_off;
#endif
if (initcall_run_list(init_sequence_r))
hang();
/* NOTREACHED - run_main_loop() does not return */
hang();
}
/*
* Over time we hope to remove these functions with code fragments and
* stub funtcions, and instead call the relevant function directly.
*
* We also hope to remove most of the driver-related init and do it if/when
* the driver is later used.
*
* TODO: perhaps reset the watchdog in the initcall function after each call?
*/
init_fnc_t init_sequence_r[] = {
initr_trace,
initr_reloc,
/* TODO: could x86/PPC have this also perhaps? */
#ifdef CONFIG_ARM
initr_caches,
#endif
initr_reloc_global_data,
#if defined(CONFIG_SYS_INIT_RAM_LOCK) && defined(CONFIG_E500)
initr_unlock_ram_in_cache,
#endif
initr_barrier,
initr_malloc,
#ifdef CONFIG_SYS_NONCACHED_MEMORY
initr_noncached,
#endif
bootstage_relocate,
#ifdef CONFIG_DM
initr_dm,
#endif
#ifdef CONFIG_ARM
board_init, /* Setup chipselects */
#endif
/*
* TODO: printing of the clock inforamtion of the board is now
* implemented as part of bdinfo command. Currently only support for
* davinci SOC's is added. Remove this check once all the board
* implement this.
*/
#ifdef CONFIG_CLOCKS
set_cpu_clk_info, /* Setup clock information */
#endif
stdio_init_tables,
initr_serial,
initr_announce,
INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_NEEDS_MANUAL_RELOC
initr_manual_reloc_cmdtable,
#endif
#if defined(CONFIG_PPC) || defined(CONFIG_M68K)
initr_trap,
#endif
#ifdef CONFIG_ADDR_MAP
initr_addr_map,
#endif
#if defined(CONFIG_BOARD_EARLY_INIT_R)
board_early_init_r,
#endif
INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_LOGBUFFER
initr_logbuffer,
#endif
#ifdef CONFIG_POST
initr_post_backlog,
#endif
INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_SYS_DELAYED_ICACHE
initr_icache_enable,
#endif
#if defined(CONFIG_PCI) && defined(CONFIG_SYS_EARLY_PCI_INIT)
/*
* Do early PCI configuration _before_ the flash gets initialised,
* because PCU ressources are crucial for flash access on some boards.
*/
initr_pci,
#endif
#ifdef CONFIG_WINBOND_83C553
initr_w83c553f,
#endif
#ifdef CONFIG_ARCH_EARLY_INIT_R
arch_early_init_r,
#endif
power_init_board,
#ifndef CONFIG_SYS_NO_FLASH
initr_flash,
#endif
INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_PPC) || defined(CONFIG_M68K)
/* initialize higher level parts of CPU like time base and timers */
cpu_init_r,
#endif
#ifdef CONFIG_PPC
initr_spi,
#endif
#if defined(CONFIG_X86) && defined(CONFIG_SPI)
init_func_spi,
#endif
#ifdef CONFIG_CMD_NAND
initr_nand,
#endif
#ifdef CONFIG_CMD_ONENAND
initr_onenand,
#endif
#ifdef CONFIG_GENERIC_MMC
initr_mmc,
#endif
#ifdef CONFIG_HAS_DATAFLASH
initr_dataflash,
#endif
initr_env,
#ifdef CONFIG_SYS_BOOTPARAMS_LEN
initr_malloc_bootparams,
#endif
INIT_FUNC_WATCHDOG_RESET
initr_secondary_cpu,
#ifdef CONFIG_SC3
initr_sc3_read_eeprom,
#endif
#if defined(CONFIG_ID_EEPROM) || defined(CONFIG_SYS_I2C_MAC_OFFSET)
mac_read_from_eeprom,
#endif
INIT_FUNC_WATCHDOG_RESET
#if defined(CONFIG_PCI) && !defined(CONFIG_SYS_EARLY_PCI_INIT)
/*
* Do pci configuration
*/
initr_pci,
#endif
stdio_add_devices,
initr_jumptable,
#ifdef CONFIG_API
initr_api,
#endif
console_init_r, /* fully init console as a device */
#ifdef CONFIG_DISPLAY_BOARDINFO_LATE
show_board_info,
#endif
#ifdef CONFIG_ARCH_MISC_INIT
arch_misc_init, /* miscellaneous arch-dependent init */
#endif
#ifdef CONFIG_MISC_INIT_R
misc_init_r, /* miscellaneous platform-dependent init */
#endif
INIT_FUNC_WATCHDOG_RESET
#ifdef CONFIG_CMD_KGDB
initr_kgdb,
#endif
interrupt_init,
#if defined(CONFIG_ARM) || defined(CONFIG_AVR32)
initr_enable_interrupts,
#endif
#if defined(CONFIG_X86) || defined(CONFIG_MICROBLAZE) || defined(CONFIG_AVR32) \
|| defined(CONFIG_M68K)
timer_init, /* initialize timer */
#endif
#if defined(CONFIG_STATUS_LED) && defined(STATUS_LED_BOOT)
initr_status_led,
#endif
/* PPC has a udelay(20) here dating from 2002. Why? */
#ifdef CONFIG_CMD_NET
initr_ethaddr,
#endif
#ifdef CONFIG_BOARD_LATE_INIT
board_late_init,
#endif
#ifdef CONFIG_CMD_SCSI
INIT_FUNC_WATCHDOG_RESET
initr_scsi,
#endif
#ifdef CONFIG_CMD_DOC
INIT_FUNC_WATCHDOG_RESET
initr_doc,
#endif
#ifdef CONFIG_BITBANGMII
initr_bbmii,
#endif
#ifdef CONFIG_CMD_NET
INIT_FUNC_WATCHDOG_RESET
initr_net,
#endif
#ifdef CONFIG_POST
initr_post,
#endif
#if defined(CONFIG_CMD_PCMCIA) && !defined(CONFIG_CMD_IDE)
initr_pcmcia,
#endif
#if defined(CONFIG_CMD_IDE)
initr_ide,
#endif
#ifdef CONFIG_LAST_STAGE_INIT
INIT_FUNC_WATCHDOG_RESET
/*
* Some parts can be only initialized if all others (like
* Interrupts) are up and running (i.e. the PC-style ISA
* keyboard).
*/
last_stage_init,
#endif
#ifdef CONFIG_CMD_BEDBUG
INIT_FUNC_WATCHDOG_RESET
initr_bedbug,
#endif
#if defined(CONFIG_PRAM) || defined(CONFIG_LOGBUFFER)
initr_mem,
#endif
#ifdef CONFIG_PS2KBD
initr_kbd,
#endif
run_main_loop,
};
让 simics 停在 run_main_loop处：
static int run_main_loop(void)
{
#ifdef CONFIG_SANDBOX
sandbox_main_loop_init();
#endif
/* main_loop() can return to retry autoboot, if so just run it again */
for (;;)
main_loop();
return 0;
}
/* We come here after U-Boot is initialised and ready to process commands */
void main_loop(void)
{
const char *s;
bootstage_mark_name(BOOTSTAGE_ID_MAIN_LOOP, "main_loop");
#ifndef CONFIG_SYS_GENERIC_BOARD
puts("Warning: Your board does not use generic board. Please read\n");
puts("doc/README.generic-board and take action. Boards not\n");
puts("upgraded by the late 2014 may break or be removed.\n");
#endif
modem_init();
#ifdef CONFIG_VERSION_VARIABLE
setenv("ver", version_string); /* set version variable */
#endif /* CONFIG_VERSION_VARIABLE */
cli_init();
run_preboot_environment_command();
#if defined(CONFIG_UPDATE_TFTP)
update_tftp(0UL);
#endif /* CONFIG_UPDATE_TFTP */
s = bootdelay_process();
if (cli_process_fdt(&s))
cli_secure_boot_cmd(s);
autoboot_command(s);
cli_loop();
}
void autoboot_command(const char *s)
{
debug("### main_loop: bootcmd=\"%s\"\n", s ? s : "<UNDEFINED>");
if (stored_bootdelay != -1 && s && !abortboot(stored_bootdelay)) {
#if defined(CONFIG_AUTOBOOT_KEYED) && !defined(CONFIG_AUTOBOOT_KEYED_CTRLC)
int prev = disable_ctrlc(1); /* disable Control C checking */
#endif
run_command_list(s, -1, 0);
#if defined(CONFIG_AUTOBOOT_KEYED) && !defined(CONFIG_AUTOBOOT_KEYED_CTRLC)
disable_ctrlc(prev); /* restore Control C checking */
#endif
}
#ifdef CONFIG_MENUKEY
if (menukey == CONFIG_MENUKEY) {
s = getenv("menucmd");
if (s)
run_command_list(s, -1, 0);
}
#endif /* CONFIG_MENUKEY */
}
void cli_loop(void)
{
#ifdef CONFIG_SYS_HUSH_PARSER
parse_file_outer();
/* This point is never reached */
for (;;);
#else
cli_simple_loop();
#endif /*CONFIG_SYS_HUSH_PARSER*/
}
After press boot buttom
/*******************************************************************/
/* bootm - boot application image from image in memory */
/*******************************************************************/
int do_bootm(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
{
#ifdef CONFIG_NEEDS_MANUAL_RELOC
static int relocated = 0;
if (!relocated) {
int i;
/* relocate names of sub-command table */
for (i = 0; i < ARRAY_SIZE(cmd_bootm_sub); i++)
cmd_bootm_sub[i].name += gd->reloc_off;
relocated = 1;
}
#endif
/* determine if we have a sub command */
argc--; argv++;
if (argc > 0) {
char *endp;
simple_strtoul(argv[0], &endp, 16);
/* endp pointing to NULL means that argv[0] was just a
* valid number, pass it along to the normal bootm processing
*
* If endp is ':' or '#' assume a FIT identifier so pass
* along for normal processing.
*
* Right now we assume the first arg should never be '-'
*/
if ((*endp != 0) && (*endp != ':') && (*endp != '#'))
return do_bootm_subcommand(cmdtp, flag, argc, argv);
}
return do_bootm_states(cmdtp, flag, argc, argv, BOOTM_STATE_START |
BOOTM_STATE_FINDOS | BOOTM_STATE_FINDOTHER |
BOOTM_STATE_LOADOS |
#if defined(CONFIG_PPC) || defined(CONFIG_MIPS)
BOOTM_STATE_OS_CMDLINE |
#endif
BOOTM_STATE_OS_PREP | BOOTM_STATE_OS_FAKE_GO |
BOOTM_STATE_OS_GO, &images, 1);
}
/**
* Execute selected states of the bootm command.
*
* Note the arguments to this state must be the first argument, Any 'bootm'
* or sub-command arguments must have already been taken.
*
* Note that if states contains more than one flag it MUST contain
* BOOTM_STATE_START, since this handles and consumes the command line args.
*
* Also note that aside from boot_os_fn functions and bootm_load_os no other
* functions we store the return value of in 'ret' may use a negative return
* value, without special handling.
*
* @param cmdtp Pointer to bootm command table entry
* @param flag Command flags (CMD_FLAG_...)
* @param argc Number of subcommand arguments (0 = no arguments)
* @param argv Arguments
* @param states Mask containing states to run (BOOTM_STATE_...)
* @param images Image header information
* @param boot_progress 1 to show boot progress, 0 to not do this
* @return 0 if ok, something else on error. Some errors will cause this
* function to perform a reboot! If states contains BOOTM_STATE_OS_GO
* then the intent is to boot an OS, so this function will not return
* unless the image type is standalone.
*/
int do_bootm_states(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[],
int states, bootm_headers_t *images, int boot_progress)
{
boot_os_fn *boot_fn;
ulong iflag = 0;
int ret = 0, need_boot_fn;
images->state |= states;
/*
* Work through the states and see how far we get. We stop on
* any error.
*/
if (states & BOOTM_STATE_START)
ret =bootm_start(cmdtp, flag, argc, argv);
if (!ret && (states & BOOTM_STATE_FINDOS))
ret =bootm_find_os(cmdtp, flag, argc, argv);
if (!ret && (states & BOOTM_STATE_FINDOTHER)) {
ret = bootm_find_other(cmdtp, flag, argc, argv);
argc = 0; /* consume the args */
}
/* Load the OS */
if (!ret && (states & BOOTM_STATE_LOADOS)) {
ulong load_end;
iflag = bootm_disable_interrupts();
ret =bootm_load_os(images, &load_end, 0);
if (ret == 0)
lmb_reserve(&images->lmb, images->os.load,
(load_end - images->os.load));
else if (ret && ret != BOOTM_ERR_OVERLAP)
goto err;
else if (ret == BOOTM_ERR_OVERLAP)
ret = 0;
#if defined(CONFIG_SILENT_CONSOLE) && !defined(CONFIG_SILENT_U_BOOT_ONLY)
if (images->os.os == IH_OS_LINUX)
fixup_silent_linux();
#endif
}
/* Relocate the ramdisk */
#ifdef CONFIG_SYS_BOOT_RAMDISK_HIGH
if (!ret && (states & BOOTM_STATE_RAMDISK)) {
ulong rd_len = images->rd_end - images->rd_start;
ret = boot_ramdisk_high(&images->lmb, images->rd_start,
rd_len, &images->initrd_start, &images->initrd_end);
if (!ret) {
setenv_hex("initrd_start", images->initrd_start);
setenv_hex("initrd_end", images->initrd_end);
}
}
#endif
#if defined(CONFIG_OF_LIBFDT) && defined(CONFIG_LMB)
if (!ret && (states & BOOTM_STATE_FDT)) {
boot_fdt_add_mem_rsv_regions(&images->lmb, images->ft_addr);
ret = boot_relocate_fdt(&images->lmb, &images->ft_addr,
&images->ft_len);
}
#endif
/* From now on, we need the OS boot function */
if (ret)
return ret;
boot_fn = bootm_os_get_boot_func(images->os.os);
need_boot_fn = states & (BOOTM_STATE_OS_CMDLINE |
BOOTM_STATE_OS_BD_T | BOOTM_STATE_OS_PREP |
BOOTM_STATE_OS_FAKE_GO | BOOTM_STATE_OS_GO);
if (boot_fn == NULL && need_boot_fn) {
if (iflag)
enable_interrupts();
printf("ERROR: booting os '%s' (%d) is not supported\n",
genimg_get_os_name(images->os.os), images->os.os);
bootstage_error(BOOTSTAGE_ID_CHECK_BOOT_OS);
return 1;
}
/* Call various other states that are not generally used */
if (!ret && (states & BOOTM_STATE_OS_CMDLINE))
ret = boot_fn(BOOTM_STATE_OS_CMDLINE, argc, argv, images);
if (!ret && (states & BOOTM_STATE_OS_BD_T))
ret = boot_fn(BOOTM_STATE_OS_BD_T, argc, argv, images);
if (!ret && (states & BOOTM_STATE_OS_PREP))
ret = boot_fn(BOOTM_STATE_OS_PREP, argc, argv, images);
#ifdef CONFIG_TRACE
/* Pretend to run the OS, then run a user command */
if (!ret && (states & BOOTM_STATE_OS_FAKE_GO)) {
char *cmd_list = getenv("fakegocmd");
ret = boot_selected_os(argc, argv, BOOTM_STATE_OS_FAKE_GO,
images, boot_fn);
if (!ret && cmd_list)
ret = run_command_list(cmd_list, -1, flag);
}
#endif
/* Check for unsupported subcommand. */
if (ret) {
puts("subcommand not supported\n");
return ret;
}
/* Now run the OS! We hope this doesn't return */
if (!ret && (states & BOOTM_STATE_OS_GO))
ret = boot_selected_os(argc, argv, BOOTM_STATE_OS_GO,
images, boot_fn);
/* Deal with any fallout */
err:
if (iflag)
enable_interrupts();
if (ret == BOOTM_ERR_UNIMPLEMENTED)
bootstage_error(BOOTSTAGE_ID_DECOMP_UNIMPL);
else if (ret == BOOTM_ERR_RESET)
do_reset(cmdtp, flag, argc, argv);
return ret;
}uboot向kernel的传参机制——bootm与tags
http://blog.csdn.net/skyflying2012/article/details/35787971
/* Main Entry point for arm bootm implementation
*
* Modeled after the powerpc implementation
* DIFFERENCE: Instead of calling prep and go at the end
* they are called if subcommand is equal 0.
*/
int do_bootm_linux(int flag, int argc, char * const argv[],
bootm_headers_t *images)
{
/* No need for those on ARM */
if (flag & BOOTM_STATE_OS_BD_T || flag & BOOTM_STATE_OS_CMDLINE)
return -1;
if (flag & BOOTM_STATE_OS_PREP) {
boot_prep_linux(images);
return 0;
}
if (flag & (BOOTM_STATE_OS_GO | BOOTM_STATE_OS_FAKE_GO)) {
boot_jump_linux(images, flag);
return 0;
}
boot_prep_linux(images);
boot_jump_linux(images, flag);
return 0;
}
/* Subcommand: GO */
static void boot_jump_linux(bootm_headers_t *images, int flag)
{
#ifdef CONFIG_ARM64
void (*kernel_entry)(void *fdt_addr, void *res0, void *res1,
void *res2);
int fake = (flag & BOOTM_STATE_OS_FAKE_GO);
kernel_entry = (void (*)(void *fdt_addr, void *res0, void *res1,
void *res2))images->ep;
debug("## Transferring control to Linux (at address %lx)...\n",
(ulong) kernel_entry);
bootstage_mark(BOOTSTAGE_ID_RUN_OS);
announce_and_cleanup(fake);
if (!fake) {
do_nonsec_virt_switch();
kernel_entry(images->ft_addr, NULL, NULL, NULL);
}
#else
unsigned long machid = gd->bd->bi_arch_number;
char *s;
void (*kernel_entry)(int zero, int arch, uint params);
unsigned long r2;
int fake = (flag & BOOTM_STATE_OS_FAKE_GO);
kernel_entry = (void (*)(int, int, uint))images->ep;
s = getenv("machid");
if (s) {
strict_strtoul(s, 16, &machid);
printf("Using machid 0x%lx from environment\n", machid);
}
debug("## Transferring control to Linux (at address %08lx)" \
"...\n", (ulong) kernel_entry);
bootstage_mark(BOOTSTAGE_ID_RUN_OS);
announce_and_cleanup(fake);
if (IMAGE_ENABLE_OF_LIBFDT && images->ft_len)
r2 = (unsigned long)images->ft_addr;
else
r2 = gd->bd->bi_boot_params;
if (!fake) {
#if defined(CONFIG_ARMV7_NONSEC) || defined(CONFIG_ARMV7_VIRT)
if (armv7_boot_nonsec()) {
armv7_init_nonsec();
secure_ram_addr(_do_nonsec_entry)(kernel_entry,
0, machid, r2);
} else
#endif
kernel_entry(0, machid, r2);
}
#endif
}add-symbol-file /home/wind/workspace/opensrc/freescale/fsl-linux/vmlinux relocation-address = 0x80008000
The code following is the first part of u-boot to define interrupt vector table, and my question is how every line will be used.
I understand the first 2 lines which is the starting point and the first instruction to implement: reset, and we define reset below.
But when will we use these instructions below? According to System.map, every instruction has a fixed address, so _fiq is at 0x0000001C, when we want to execute fiq, we will copy this address into pc and then execute,right? But in which way can we jump to this instruction: ldr pc, _fiq? It’s realised by hardware or software?
ARM的位置无关程序设计
ARM处理器支持位置无关的程序设计，这种程序加载到存储器的任意地址空间都可以正常运 行，其设计方法在嵌入式应用系统开发中具有重要的作用。尤其在裸机状态下开发Bootloader程序及进行内核初始化设计；利用位置无关的程序设计方法 还可以在具体应用中用于构建高效率动态链接库，因而了解位置无关的程序设计方法，有助于开发人员设计出结构简单、清晰的应用程序。
应用程序必须经过编译、汇编和链接后才变成可执行文件，在链接时，要对所有目标文件进行重 定位(relocation)，建立符号引用规则，同时为变量、函数等分配运行地址。当程序执行时，系统必须把代码加载到链接时所指定的地址空间，以保证 程序在执行过程中对变量、函数等符号的正确引用，使程序正常运行。在具有操作系统的系统中，重定位过程由操作系统自动完成。
在设计Bootloader程序时，必须在裸机环境中进行，这时Bootloader映像 文件的运行地址必须由程序员设定。通常情况下，将Bootloader程序下载到ROM的0x0地址进行启动，而在大多数应用系统中，为了快速启动，首先 将Bootloader程序拷贝到SDRAM中再运行。一般情况下，这两者的地址并不相同，程序在SDRAM中的地址重定位过程必须由程序员完成。实际 上，由于Bootloader是系统上电后要执行的第一段程序，Bootloader程序的拷贝和在这之前的所有工作都必须由其自身来完成，而这些指令都 是在ROM中执行的。也就是说，这些代码即使不在链接时所指定的运行时地址空间，也可以正确执行。这就是位置无关代码，它是一段加载到任意地址空间都能正 常执行的特殊代码。
位置无关代码常用于以下场合：
程序在运行期间动态加载到内存；
程序在不同场合与不同程序组合后加载到内存(如共享的动态链接库)；
在运行期间不同地址相互之间的映射（如Bootloader程序）。
虽然在用GCC编译时，使用-fPIC选项可为C语言产生位置无关代码，但这并不能修正程序设计中固有的位置相关性缺陷。特别是汇编语言代码，必须由程序员遵循一定的程序设计准则，才能保证程序的位置无关性。
ARM程序的位置无关可执行文件PIE（PositionIndependent Executable）包括位置无关代码PIC和位置无关数据PID（PositionIndependent Data）两部分。
PID主要针对可读写数据段（.data段），其中保存已赋初值的全局变量。为实现其位置 无关性，通常使用寄存器R9作为静态基址寄存器，使其指向该可读写段的首地址，并使用相对于基址寄存器的偏移量来对该段的变量进行寻址。这种方法常用于为 可重入程序的多个实例产生多个独立的数据段。在程序设计中，一般不必考虑可读写段的位置无关性，这主要是因为可读写数据主要分配在SDRAM中。
PIC包括程序中的代码和只读数据（.text段），为保证程序能在ROM和SDRAM空间都能正确运行（如裸机状态下的Bootloader程序），必须采用位置无关代码程序设计。
PIC遵循只读段位置无关ROPI（ReadOnly Position Independence）的ATPCS（ARMThumb Procedure Call Standard）的程序设计规范：
（1） 程序设计规范1
引用同一ROPI段或相对位置固定的另一ROPI段中的符号时，必须是基于PC的符号引用，即使用相对于当前PC的偏移量来实现跳转或进行常量访问。
① 位置无关的程序跳转。在ARM汇编程序中，使用相对跳转指令B/BL实现程序跳转。指令中所跳转的目标地址用基于当前PC的偏移量来表示，与链接时分配给地址标号的绝对地址值无关，因而代码可以在任何位置进行跳转，实现位置无关性。
另外，还可使用ADR或ADRL伪指令将地址标号值读取到PC中实现程序跳转。这是因为ADR或ADRL等伪指令会被编译器替换为对基于PC的地址值进行操作，但这种方式所能读取的地址范围较小，并且会因地址值是否为字对齐而异。
但在ARM程序中，使用LDR等指令直接将地址标号值读取到PC中实现程序跳转不是位置无关的。例如： LDR PC, =main
上面的伪指令编译后的结果为： LDR PC, [PC, OFFSET_TO_LPOOL]
LPOOL
DCD main
可见，虽然LDR是把基于PC的一个存储单元LPOOL的内容加载到PC中，但该存储单元中保存的却是链接时所决定的main函数入口的绝对地址，所以main函数实际所在的段不是位置无关。
② 位置无关的常量访问。在应用程序中，经常要读写相关寄存器以完成必要的硬件初始化。为增强程序的可读性，利用EQU伪指令对一些常量进行赋值，但在访问过程中，必须实现位置无关性。下面以PXA270的GPIO初始化介绍位置无关的常量访问方法。
GPIO_BASE EQU 0x40e00000; GPIO基址寄存器地址
GPDR0 EQU 0x00c; 相对于GPIO基址寄存器的偏移量
init_GPDR0 EQU 0xfffbfe00; 寄存器GPDR0初值
LDR R1, =GPIO_BASE
LDR R0, =init_GPDR0
STR R0, [R1, #GPDR0]
上述汇编代码段经编译后的结果为：
LDR R1, [PC, OFFSET_TO_GPIO_BASE]
LDR R0, [PC, OFFSET_TO_init_GPDR0]
STR R0, [R1, #0xc]
GPIO_BASE
DCD 0x40e00000
GPDR0
DCD 0x00c
init_GPDR0
DCD 0xfffbfe00
可见，LDR伪指令实际上使用基于PC的偏移量来对符号常量GPIO_BASE和 init_GPDR0进行引用，因而是位置无关的。由此可以得出如下结论：使用LDR伪指令将一个常量读取到非PC的其他通用寄存器中可实现位置无关的常 量访问；但将一个地址值读取到PC中进行程序跳转时，跳转目标则是位置相关的。
（2） 程序设计规范2
其他被ROPI段中的代码引用的必须是绝对地址，或者是基于可读写位置无关(RWPI)段的静态基址寄存器的可写数据。
使用绝对地址只能引用被重定位到特定位置的代码段中的符号，通过在位置无关代码中引入绝对 地址，可以让程序跳转到指定位置。例如，假设Bootloader的阶段1将其自身代码拷贝到链接时所指定的SDRAM地址空间后，当要跳转到阶段2的C 程序入口时，可以使用指令“LDR PC, =main”跳转到程序在SDRAM中的main函数入口地址开始执行。这是因为程序在编译链接时给main函数分派绝对地址，系统通过将main函数的 绝对地址直接赋给PC实现程序跳转。如果使用相对跳转指令“B main”，那么只会跳转到启动ROM内部的main函数入口（这种情况反而不能写 B main了，但要写之前是不是之前 写一句 ldr pc，= xx也可以？by imjacob）。