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Gujin: Difference between revisions
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{{You}}
Gujin is a GPLed [[bootloader]] for the PC.
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Same for interrupts, Gujin switches to protected mode but does not re-enable interrupts, that is the job of the kernel being booted to set-up the Interrupt Decriptor Table and handle each interrupts, because BIOS will not help the kernel any more.
Gujin can also relocate the ELF file if it contains relocation information, see option [http://sourceware.org/binutils/docs-2.19/ld/Options.html#index-g_t_002d_002demit_002drelocs-79 --emit-relocs] of the "ld" linker of the "
Because most applications will want to collect BIOS informations before the switch to protected mode, Gujin can call in real-mode a function of the ELF file, and if this function returns zero - continue the loading process - else display an error message (for instance: "trying to execute a 64 bits application on a processor without this feature!").
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----
{{FirstPerson|section=1}}
So enough text description, let's try it!
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Then, we reformat that dedicated USB stick as a bootable FAT filesystem, erasing all its content: first go in "root" by typing "su" or "sudo" (distribution dependant), then get the device name of you USB stick (let's say it is /dev/sdg) (sometimes /dev/mmcblk0 for SD cards), and type:
<syntaxhighlight lang="bash">
./gujin /dev/sdg --disk=BIOS:0x00,auto
</syntaxhighlight>
Depending on the size of the USB stick, that will have created either a FAT16 or a FAT32 (or even a FAT12) filesystem, but that point is not really important.
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To check that this filesystem is correctly created, you can type:
<syntaxhighlight lang="bash">
/sbin/fsck.vfat /dev/sdg
</syntaxhighlight>
With the Gujin installer parameters we used, that would have created a "superfloppy" format on our USB stick, that is currently the format most PC will be able to understand and boot from.
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Then, we want to generate this ELF kernel - let's try to generate a "hello world":
Create a file with that content:
<syntaxhighlight lang="c">
const char msg1[] = "Hello protected-mode text world! please reboot ...";
#define STACKSIZE 64 * 1024
static unsigned stack[STACKSIZE / 4] __attribute__ ((aligned(32)));
void _start (void)
{
/* We are flat non-paged memory and interrupt disabled */
asm (" mov %0,%%esp " : : "i" (&stack[STACKSIZE / 4]));
volatile unsigned short *video_array = (volatile unsigned short *)0xB8000;
unsigned cpt1;
video_array += 10 * 80; /* few empty lines */
/* We want blue background color and lightgray foreground, so 0x1700: */
for (cpt1 = 0; cpt1 < sizeof(msg1) - 1; cpt1++)
video_array[cpt1] = 0x1700 + msg1[cpt1];
while (1)
continue;
}
</syntaxhighlight>
Then compile it like (you may need to add "-fno-stack-protector" too, distribution dependant):
<syntaxhighlight lang="bash">
gcc -m32 -Wall -O2 -s -static -nostartfiles -nodefaultlibs -Wl,-Ttext=0x110000 hello.c -o hello.elf
</syntaxhighlight>
and compress it by:
<syntaxhighlight lang="bash">
gzip -9 hello.elf -c > hello.kgz
</syntaxhighlight>
You just need to copy that hello.kgz file into the USB stick, and reboot with this USB stick, you will get a menu with "hello.kgz" displayed.
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Just create a file which [http://www.mirrorservice.org/sites/download.sourceforge.net/pub/sourceforge/g/project/gu/gujin/gujin/2.8.2/gujin-2.8.2.tar.gz/gujin/hello_bios.c?extract=true contains]:
<syntaxhighlight lang="c">
/* hello_bios.c - public domain - no support whatsoever */
/* Constant entry block, max 64 KB code+data+stack, do not touch: */
asm (
" .code16gcc \n"
" .global
"_start: \n"
// "int $3 # if debugging with borland td.exe, started by tiny.exe \n"
" .previous \n"
);
/* For information about asm() construct, see GCC manual:
[http://gcc.gnu.org/onlinedocs/gcc-4.5.0/gcc/Extended-Asm.html http://gcc.gnu.org/onlinedocs/gcc-4.5.0/gcc/Extended-Asm.html]
For information about BIOS services, see
[http://stanislavs.org/helppc/ http://stanislavs.org/helppc/]
*/
enum standard_color {
red, magenta, brown, lightgray,
darkgray, lightblue, lightgreen, lightcyan,
lightred, lightmagenta, yellow, white
};
/* This assumes 16 colors modes, text or graphic - i.e. not 256 colors: */
static inline void
VGA_writestring (const char *string, unsigned char row, unsigned char col)
{
unsigned short page_attribute = 16 * blue + lightgray, strlen_string;
const char *endstring = string;
/* We cannot rely on libc to provide strlen(): */
while
strlen_string = endstring - string;
asm (
" xchgl %%ebp,%4 \n"
" int $0x10 # writestring \n"
" xchgl %%ebp,%4 \n"
: :
"c" (strlen_string), "d" (((unsigned short)row << 8) | col),
"r" (string) /* in fact %es:%bp, but "B" (string) doesn't work */
);
}
static inline void BIOS_wait (unsigned nb_microsecond)
{
unsigned short status;
/* Use
is not used, the code has to be inserted (status is the result): */
asm volatile (" int $0x15 "
: "=a" (status)
: "a" (0x8600), "d" (nb_microsecond & 0xFFFF), "c" (nb_microsecond >> 16)
);
}
int main (void)
{
VGA_writestring ("Hello BIOS world! Please wait...\r\n", 0, 0);
BIOS_wait (3 * 1000 * 1000); /* 3 seconds before returning to Gujin*/
// return 0x80000000; /* to return to Gujin menu */
return 0xA0000000; /* to return to Gujin menu, do not display "press a key to continue" */
}
</syntaxhighlight>
And then compile it by:
<syntaxhighlight lang="bash">
gcc -m32 -Wall -Os -s -static -nostartfiles -nodefaultlibs -Wa,-R -Wl,--section-start,.init=0 hello_bios.c -o hello_bios.elf
</syntaxhighlight>
and compress it:
<syntaxhighlight lang="bash">
gzip -9 hello_bios.elf -c > hello_bios.kgz
</syntaxhighlight>
you can copy this hello_bios.kgz to the USB stick and run it, I hope the different functions of the source file are obvious.
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