#include /* * Global Descriptor Table Entry */ struct gdt_entry { /* Limits */ unsigned short limit_low; /* Segment address */ unsigned short base_low; unsigned char base_middle; /* Access modes */ unsigned char access; unsigned char granularity; unsigned char base_high; } __attribute__((packed)); /* * GDT pointer */ struct gdt_ptr { unsigned short limit; unsigned int base; } __attribute__((packed)); struct gdt_entry gdt[3]; struct gdt_ptr gp; /* * (ASM) gdt_flush * Reloads the segment registers */ extern void gdt_flush(); /* * gdt_set_gate * Set a GDT descriptor */ void gdt_set_gate( int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran ) { /* Base Address */ gdt[num].base_low = (base & 0xFFFF); gdt[num].base_middle = (base >> 16) & 0xFF; gdt[num].base_high = (base >> 24) & 0xFF; /* Limits */ gdt[num].limit_low = (limit & 0xFFFF); gdt[num].granularity = (limit >> 16) & 0X0F; /* Granularity */ gdt[num].granularity |= (gran & 0xF0); /* Access flags */ gdt[num].access = access; } /* * gdt_install * Install the kernel's GDTs */ void gdt_install() { /* GDT pointer and limits */ gp.limit = (sizeof(struct gdt_entry) * 3) - 1; gp.base = (unsigned int)&gdt; /* NULL */ gdt_set_gate(0, 0, 0, 0, 0); /* Code segment */ gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF); /* Data segment */ gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF); /* Go go go */ gdt_flush(); }