OSDev.org

Posted: **Sat Sep 02, 2006 10:44 am**

Hi everybody, I'm tring to boot a very simple kernel with grub but everytime I do it grub will complain that the kernel can't be loaded bellow 1mb of memory. I know that, that can happen under two situations when you actually load the kernel under the 1mb mark or when a very high memmory location is specified which "overlaps" and give the same error (#7).
Bellow is my asm file and the linker script:

asm:

Code: Select all

[BITS 32]
global start
start:
    mov esp, _sys_stack     ; This points the stack to our new stack area
    jmp stublet

; This part MUST be 4byte aligned, so we solve that issue using 'ALIGN 4'
ALIGN 4
mboot:
    ; Multiboot macros to make a few lines later more readable
    MULTIBOOT_PAGE_ALIGN        equ 1<<0
    MULTIBOOT_MEMORY_INFO       equ 1<<1
    MULTIBOOT_HEADER_MAGIC      equ 0x1BADB002
    MULTIBOOT_HEADER_FLAGS      equ MULTIBOOT_PAGE_ALIGN | MULTIBOOT_MEMORY_INFO
    MULTIBOOT_CHECKSUM  equ -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)
    EXTERN code, bss, end

    ; This is the GRUB Multiboot header. A boot signature
    dd MULTIBOOT_HEADER_MAGIC
    dd MULTIBOOT_HEADER_FLAGS
    dd MULTIBOOT_CHECKSUM

    ; AOUT kludge - must be physical addresses. Make a note of these:
    ; The linker script fills in the data for these ones!
    dd mboot
    dd code
    dd bss
    dd end
    dd start


; This is an endless loop here. Make a note of this: Later on, we
; will insert an 'extern _main', followed by 'call _main', right
; before the 'jmp $'.
stublet:

        extern main
        call main
        jmp $

;this loads our gdt here
global install_gdt
extern gdt

install_gdt:
    lgdt [gdt]
    mov ax, 0x10
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax
    mov ss, ax
    jmp 0x08:loadcs
loadcs:
   ret
; Here is the definition of our BSS section. Right now, we'll use
; it just to store the stack. Remember that a stack actually grows
; downwards, so we declare the size of the data before declaring
; the identifier '_sys_stack'
SECTION .bss
    resb 0x4000               ; This reserves 16KBs of memory here
_sys_stack:

linker_elf.ld:

Code: Select all

OUTPUT_FORMAT("elf32-i386")
ENTRY(start)
virt = 0xC0000000; /* 3 gig */
phys = 0x100000; /* 1 meg */
SECTIONS
{   .text virt : AT(phys)
    {   code = .;
        *(.text)
        . = ALIGN(4096);
    }
    .data :  AT(phys + (data - code))
    {   data = .;
         *(.data)
         . = ALIGN(4096);
    }
    .bss :  AT(phys + (bss - code))
    {   bss = .;
        *(.bss)
        *(COMMON)
        . = ALIGN(4096);
    }
    end = .;
}

the assembly file is taken from "Bran's Kernel Development Tutorial" from osdever.net and the linker script from
http://www.openbg.net/sto/os/xml/grub.html.

Btw, I'm working on a Linux machine and generating an ELF kernel, this is however working fine in Windows XP with DJGPP and a plaing bin image kernel. Any thoughts on what is the problem?

<pype:edit> "code" tag is preferred to "bold" for code snippets</pype>

Posted: **Tue Sep 05, 2006 12:07 am**

Try:

OUTPUT_FORMAT("elf32-i386")
ENTRY(start)
phys = 0x100000; /* 1 meg */
SECTIONS
{
.data phys :
{ data = .;
*(.data)
. = ALIGN(4096);
}
.text . :
{ code = .;
*(.text)
. = ALIGN(4096);
}
.bss . :
{ bss = .;
*(.bss)
. = ALIGN(4096);
}
.rodata . :
{ rodata = .;
*(.rodata)
. = ALIGN(4096);
}
end = .;
}

You should not need the a.out kludge if the target is elf32-i386. GRUB should read the ELF headers, for the physical memory location.

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