问题
I'm learning a bit of OS development from OSDev.org. I have a kernel and I'm trying to boot in GRUB Legacy (0.97) using qemu. However, when I type kernel 200+9, I get the message
[Multiboot-elf, <0x100000:0x80:0x4008>(bad), entry=0x10000c]
This is what I expect except for the (bad) part. If I type boot now GRUB just hangs.
I think the numbers 0x100000, 0x44, 0x4008 stand for the .text segment start address, the .bss start address, and the .bss section size, respectively. I think this because running objdump -h on the kernel image gives this output:
kernel.bin: file format elf32-i386
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000044 00100000 00100000 00001000 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .bss 00004008 00100044 00100044 00001044 2**2
ALLOC
So you can see that the numbers I mentioned almost match up. The issue is that instead of 100044, the start of .bss is just 44. And I think this is the reason why GRUB is saying bad. I can't have a section below 1 MB in memory (low memory). But objdump is telling me my sections are above that threshold, so I don't know what's wrong. Anyway, I'll paste my code below, it's relatively short. Although my question is probably very basic if you've done OS dev before, so the code might be extraneous.
;loader.s - contains the multiboot header for grub and calls the main kernel method
global loader ; making entry point visible to linker
global magic ; we will use this in kmain
global mbd ; we will use this in kmain
extern kmain ; kmain is defined in kmain.cpp
; setting up the Multiboot header - see GRUB docs for details
MODULEALIGN equ 1<<0 ; align loaded modules on page boundaries
MEMINFO equ 1<<1 ; provide memory map
FLAGS equ 0x03;MODULEALIGN | MEMINFO ; this is the Multiboot 'flag' field
MAGIC equ 0x1BADB002 ; 'magic number' lets bootloader find the header
CHECKSUM equ -(MAGIC + FLAGS) ; checksum required
section .text
loader:
align 4
dd MAGIC
dd FLAGS
dd CHECKSUM
; reserve initial kernel stack space
STACKSIZE equ 0x4000 ; that's 16k.
mov esp, stack + STACKSIZE ; set up the stack
mov [magic], eax ; Multiboot magic number
mov [mbd], ebx ; Multiboot info structure
call kmain ; call kernel proper
cli
.hang:
hlt ; halt machine should kernel return
jmp .hang
section .bss
align 4
stack: resb STACKSIZE ; reserve 16k stack on a doubleword boundary
magic: resd 1
mbd: resd 1
.
// kernel.c - Contains the main kernel method
void kmain() {
extern unsigned int magic;
if (magic != 0x2BADB002) {
// Something went wrong
}
volatile unsigned char *videoram = (unsigned char *) 0xB800;
videoram[0] = 65;
videoram[1] = 0x07;
}
Below is my custom linker script:
ENTRY (loader)
SECTIONS {
. = 0x00100000;
.text ALIGN (0x1000) : {
*(.text)
}
.rodata ALIGN (0x1000) :
{
*(.rodata*)
}
.data ALIGN (0x1000) :
{
*(.data)
}
.bss :
{
sbss = .;
*(COMMON)
*(.bss)
ebss = .;
}
/DISCARD/ : {
*(.eh_frame)
*(.comment)
}
}
And finally, I build the kernel with the following lines:
nasm -f elf -o loader.o loader.s
gcc -c -o kernel.o kernel.c
ld -T linker.ld -o kernel.bin loader.o kernel.o
cat stage1 stage2 pad kernel.bin > floppy.img
Where stage1 and stage2 are file from GRUB Legacy and pad is any 750 byte file (So stage1+stage2+pad have a file size of 102400 bytes, or 200 blocks, which is why I boot with kernel 200+9).
Finally, I run the kernel in qemu:
qemu-system-x86_64 -fda floppy.img
回答1:
+1 for nice question with all the details, thank you.
At least on my machine the generated kernel.bin comes out as 4869 bytes, which only fits in 10 sectors not 9. Also, the VGA text memory is at 0xb8000 not 0xb800 (one more zero - 0xb800 is the real mode segment, has to be multiplied by 16). With those little adjustments it works fine here.
来源：https://stackoverflow.com/questions/14311761/simple-kernel-wont-boot-in-grub