Re: Loading boot image retrieved from CD-ROM

[Bean]

On Sun, Jun 24, 2007 at 12:31:14AM -0400, Alex Roman wrote:
> Hello,
> 
> As you may recall, I'm working on the Google Summer of Code project to
> enable GRUB2 to boot off CDs.
> 
> I can now read boot images from CDs using int 13h BIOS calls :) And
> I've confirmed I'm actually reading the ISOLINUX image because I can
> perform some manual disassembly and it matches what I see in the
> ISOLINUX source code :)
> 
> The problem is now booting them. From what I've gathered from the
> specs, I'm supposed to load the bootable image at address 0x7c00,
> clear the segments (-code segment) and perform a long jump (to set the
> code segment) at address 0x7c00 using segment 0x7c0 and offset 0x0.
> 
> Now, I can't read the bootable image directly at 0x7c00, so what I do
> is I read it in GRUB_MEMORY_MACHINE_SCRATCH_ADDR + a small offset I
> use for some structures. Then I have a function in startup.S which
> will copy from that address, into 0x7c00 and then start executing the
> image.
> 
> The problem is that QEMU freezes when I call the asm function. Was
> wondering if anyone could take a quick look at the function to see if
> I'm doing anything wrong, or if someone could suggest how to best
> debug this issue...
> 
> Thanks in advance!
> 
> Here is the function:
> 
> --start--
> /*
> *  void grub_eltorito_boot (int drive, void *buf, int size)
> *
> *  This starts an eltorito boot image from 0:7c00h
> */
> 
> FUNCTION(grub_eltorito_boot)
>       pushl   %eax
>    pushl   %edx
>    pushl   %ecx
> 
>       call    EXT_C(grub_dl_unload_all)
> 
>       /* set up to pass boot drive */
>       popl    %edx
> 
>    /* address in %ebx */
>    popl    %ebx
> 
>    /* count in %ecx */
>    popl    %ecx
> 
>    /* must move image to 0x07c0:0 */
>    movw    $0x07c0,  %ax
>    movw    %ax, %es
>    xorw    %ax, %ax
>    movw    %ax, %di
> 
>    pushl   %ebx
> 
>    /* offset to move from in  %si */
>    andw    $0xf, %bx
>    movw    %bx, %si
> 
>    /* segment to move from in %ds */
>    popl    %ebx
>    shrl    $4, %ebx
>    movw    %bx, %ds
> 
>    /* move image */
>    rep
>    movsb
> 
> 
>       /* Turn off Gate A20 */
>       xorl    %eax, %eax
>       call    EXT_C(grub_gate_a20)
>       
>       call    prot_to_real
>       .code16
>    xorw    %ax, %ax
>    movw    %ax, %ds
>    movw    %ax, %es
>    movw    %ax, %fs
>    movw    %ax, %gs
>    movw    %ax, %ss
>    ljmp    $0x07c0,$0
>       .code32
> --end--

Maybe the image overwrites code in statrt.S after it's copied to 0x7c00. To
verify, you can print the address of function like grub_eltorito_boot,
grub_gate_a20 and prot_to_real, see if they're within range of 0x7c00 to
0x7c00 + image length.

-- 
Bean <address@hidden>