Guide to Make Your Own Operating System: Bootstrap Yourself To Write An Operating System From Scratch by Marshall Sam

Author:Marshall, Sam [Marshall, Sam]
Language: eng
Format: epub
Published: 2020-10-09T16:00:00+00:00

And the C structure:

struct gdtdesc {

u16 lim0_15;

u16 base0_15;

u8 base16_23;

u8 acces;

u8 lim16_19 :4 ;

u8 other :4 ;

u8 base24_31;

} __attribute__ ((packed));

How to define our GDT table?

We need now to define our GDT in memory and finally load it using the GDTR registry.

We are going to store our GDT at the address:

# define GDTBASE 0x00000800

The function init_gdt_desc in x86.cc initialize a gdt segment descriptor.

void init_gdt_desc(u32 base, u32 limite, u8 acces, u8 other, struct gdtdesc * desc)

{

desc -> lim0_15 = (limite & 0xffff );

desc -> base0_15 = (base & 0xffff );

desc -> base16_23 = (base & 0xff0000 ) >> 16 ;

desc -> acces = acces;

desc -> lim16_19 = (limite & 0xf0000 ) >> 16 ;

desc -> other = (other & 0xf );

desc -> base24_31 = (base & 0xff000000 ) >> 24 ;

return ;

}

And the function init_gdt initialize the GDT, some parts of the below function will be explained later and are used for multitasking.

void init_gdt( void )

{

default_tss.debug_flag = 0x00 ;

default_tss.io_map = 0x00 ;

default_tss.esp0 = 0x1FFF0 ;

default_tss.ss0 = 0x18 ;

​

/* initialize gdt segments */

init_gdt_desc( 0x0 , 0x0 , 0x0 , 0x0 , & kgdt[ 0 ]);

init_gdt_desc( 0x0 , 0xFFFFF , 0x9B , 0x0D , & kgdt[ 1 ]); /* code */

init_gdt_desc( 0x0 , 0xFFFFF , 0x93 , 0x0D , & kgdt[ 2 ]); /* data */

init_gdt_desc( 0x0 , 0x0 , 0x97 , 0x0D , & kgdt[ 3 ]); /* stack */

​

init_gdt_desc( 0x0 , 0xFFFFF , 0xFF , 0x0D , & kgdt[ 4 ]); /* ucode */

init_gdt_desc( 0x0 , 0xFFFFF , 0xF3 , 0x0D , & kgdt[ 5 ]); /* udata */

init_gdt_desc( 0x0 , 0x0 , 0xF7 , 0x0D , & kgdt[ 6 ]); /* ustack */

​

init_gdt_desc((u32) & default_tss, 0x67 , 0xE9 , 0x00 , & kgdt[ 7 ]); /* descripteur de tss */

​

/* initialize the gdtr structure */

kgdtr.limite = GDTSIZE * 8 ;

kgdtr.base = GDTBASE;

​

/* copy the gdtr to its memory area */

memcpy(( char * ) kgdtr.base, ( char * ) kgdt, kgdtr.limite);

​

/* load the gdtr registry */

asm ( "lgdtl (kgdtr)" );

​

/* initiliaz the segments */

asm ( " movw $0x10, %ax

movw %ax, %ds

movw %ax, %es

movw %ax, %fs

movw %ax, %gs

ljmp $0x08, $next

next:
" );

}

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