JZ2440程序如何从steppingstone跳到SDRAM中执行

由于steppingstone因素，保存在nand flash的执行指令的前4k会搬到SRAM上执行。此时CPU是从SRAM取指令执行。
但是由于整个执行指令的代码会大于4k，因此前4k执行指令的部分功能是把所有代码搬到内存中，再由CPU从内存中读取指令。

.text
.global _start
_start:

    bl disable_watch_dog
    bl memsetup
    bl copy_to_sdram
    ldr pc, =on_sdram

on_sdram:
    ldr sp, =0x34000000
    bl main

halt_loop:
    b halt_loop

第9行就是跳转取指令的地方。由从SRAM取指令转为从内存取指令。

sdram_elf: file format elf32-littlearm

Disassembly of section .text:

30000000 <_start>:
30000000:    eb000005     bl    3000001c <disable_watch_dog>
30000004:    eb000008     bl    3000002c <memsetup>
30000008:    eb000010     bl    30000050 <copy_to_sdram>
3000000c:    e59ff090     ldr    pc, [pc, #144]    ; 300000a4 <mem_cfg_val+0x34>

30000010 <on_sdram>:
30000010:    e3a0d30d     mov    sp, #872415232    ; 0x34000000
30000014:    eb000032     bl    300000e4 <main>

30000018 <halt_loop>:
30000018:    eafffffe     b    30000018 <halt_loop>

3000001c <disable_watch_dog>:
3000001c:    e3a01453     mov    r1, #1392508928    ; 0x53000000
30000020:    e3a02000     mov    r2, #0    ; 0x0
30000024:    e5812000     str    r2, [r1]
30000028:    e1a0f00e     mov    pc, lr

3000002c <memsetup>:
3000002c:    e3a01312     mov    r1, #1207959552    ; 0x48000000
30000030:    e28f2038     add    r2, pc, #56    ; 0x38
30000034:    e1a00000     nop            (mov r0,r0)
30000038:    e2813034     add    r3, r1, #52    ; 0x34
3000003c:    e4924004     ldr    r4, [r2], #4
30000040:    e4814004     str    r4, [r1], #4
30000044:    e1510003     cmp    r1, r3
30000048:    1afffffb     bne    3000003c <memsetup+0x10>
3000004c:    e1a0f00e     mov    pc, lr

30000050 <copy_to_sdram>:
30000050:    e3a01000     mov    r1, #0    ; 0x0
30000054:    e3a02203     mov    r2, #805306368    ; 0x30000000
30000058:    e3a03a01     mov    r3, #4096    ; 0x1000
3000005c:    e4914004     ldr    r4, [r1], #4
30000060:    e4824004     str    r4, [r2], #4
30000064:    e1510003     cmp    r1, r3
30000068:    1afffffb     bne    3000005c <copy_to_sdram+0xc>
3000006c:    e1a0f00e     mov    pc, lr

30000070 <mem_cfg_val>:
30000070:    22011110     andcs    r1, r1, #4    ; 0x4
30000074:    00000700     andeq    r0, r0, r0, lsl #14
30000078:    00000700     andeq    r0, r0, r0, lsl #14
3000007c:    00000700     andeq    r0, r0, r0, lsl #14
30000080:    00000700     andeq    r0, r0, r0, lsl #14
30000084:    00000700     andeq    r0, r0, r0, lsl #14
30000088:    00000700     andeq    r0, r0, r0, lsl #14
3000008c:    00018005     andeq    r8, r1, r5
30000090:    00018005     andeq    r8, r1, r5
30000094:    008c07a3     addeq    r0, ip, r3, lsr #15
30000098:    000000b1     streqh    r0, [r0], -r1
3000009c:    00000030     andeq    r0, r0, r0, lsr r0
300000a0:    00000030     andeq    r0, r0, r0, lsr r0
300000a4:    30000010     andcc    r0, r0, r0, lsl r0
300000a8:    e1a00000     nop            (mov r0,r0)
300000ac:    e1a00000     nop            (mov r0,r0)

第9行ldr pc, =on_sdram 对应的反汇编是：
3000000c: e59ff090 ldr pc, [pc, #144] ; 300000a4
最左边是链接地址，pc=3000000c+8，[pc, #144]=[3000000c+4+144]=[300000a4],取300000a4地址的值复制给PC
300000a4: 30000010 andcc r0, r0, r0, lsl r0
PC=[300000a4]=30000010，就是标记on_sdram开始的地址
注意：链接地址从30000000是有在链接时候决定的：
arm-linux-ld -Ttext 0x30000000 sdram.o leds.o -o sdram_elf