zImage的entry point

mkimge的參數”-e”決定kernel entry point address, 由於zImage是一個 raw binary file, 所以zImage dump在memory時起始位置所擺內容即是the lowest ELF section of vmlinux (arch/arm/boot/compressed/vmliux) , .text 為該section, link script "arch/arm/boot/compressed/vmlinux.lds"決定vmlinux內的sections排列順序: (1)vmlinux的起始位置是0, (2).text起始位置是0, (3)the first byte of .text 所放的指令.start 即是vmlinux的entry point (_start), 該link script (有關.text部份) 如下所示：

OUTPUT_ARCH(arm)
ENTRY(_start) //the entry point
SECTIONS
{
. = 0; //The current ouput location counter is moved to "0".
_text = .; //The current ouput location counter is assigned to _text (the starting address of .text)

.text : {
_start = .; // The current ouput location counter is assigned to _start
*(.start) // Include all input .start sections
*(.text)
*(.text.*)
*(.fixup)
*(.gnu.warning)
*(.rodata)
*(.rodata.*)
*(.glue_7)
*(.glue_7t)
*(.piggydata)
. = ALIGN(4);
}

_etext = .; // "_text + SIZEOF(.text)" is assigned to _etext
....
}

所以vmlinux(zImage)的 entry point 為.start (arch/arm/boot/compressed/head.S定義該section), 因為vmlinux(zImage)的 entry point在 address 0, head.S 需要做relocation 動作讓vmlinux(zImage)可以被載入到任意的physical memory address執行。需不需要做relocate由以下動作做判斷：

adr r0, LC0 @r0=offset between LC0 and _start (program entry pointer), 若zImage載入到physical memory address 0x20008040執行那_start address就是0x20008040。

ldmia r0, {r1, r2, r3, r4, r5, r6, ip, sp}
subs r0, r0, r1 @ calculate the delta offset. 所以目前r0=_start (program entry pointer) address

@ if delta is zero, we are
beq not_relocated @ running at the address we
@ were linked at.

/*
* We're running at a different address. We need to fix
* up various pointers:
* r5 - zImage base address
* r6 - GOT start
* ip - GOT end
*/
add r5, r5, r0
add r6, r6, r0
add ip, ip, r0

....

LC0: .word LC0 @ r1
.word __bss_start @ r2
.word _end @ r3
.word zreladdr @ r4
.word _start @ r5
.word _got_start @ r6
.word _got_end @ ip
.word user_stack+4096 @ sp


在arch/arm/mach-xxx/Makefile.boot 定義”zreladdr-y”值, 該值是piggy.gz decompress 後的Image (kernel proper without debug symbols)的entry point address ,該entry point address與zImage的 entry point address無關。

arch/arm/boot/compressed/head.S的工作是(1) relocate zImage base address ,GOT table address and GOT table entries, (2)enable MMU for using I/D cache for quickly decompressing piggy.gz (kernel proper ) , (3)copy the decomprssed codes to the memory space of address ”zreladdr-y”(in r4)(4) flush I/D cache and disable MMU, (5)restore r0=0, r1=machine id, and r3=ATAG pointer , (6) mov pc, r4 。