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#include "gdt.h"
#include "mem.h"
struct GDT_REGISTER GDTR = { 0, 0 };
void gdt_memcpy(){
GDTR.limit=8*4; // Each entry is 8 bytes and 4 entries
GDTR.base=0x800;
GDT_ENTRY cs_desc;
cs_desc.base=0;
cs_desc.limit=0xFFFFF;
cs_desc.flags=GDT_SZ|GDT_GR;
cs_desc.access=GDT_PR|GDT_PRVL_0|GDT_S|GDT_EXEC|GDT_RW;
GDT_ENTRY ds_desc;
ds_desc.base=0;
ds_desc.limit=0xFFFFF;
ds_desc.flags=GDT_SZ|GDT_GR;
ds_desc.access=GDT_PR|GDT_PRVL_0|GDT_S|GDT_RW;
GDT_ENTRY ss_desc;
ss_desc.base=0;
ss_desc.limit=0;
ss_desc.flags=GDT_SZ|GDT_GR;
ss_desc.access=GDT_PR|GDT_PRVL_0|GDT_S|GDT_RW|GDT_DC;
// Write GDT descriptors into memory
gdt_write_entry((GDT_ENTRY){0,0,0,0},GDTR.base); // First one must be null
gdt_write_entry(cs_desc, GDTR.base+8); // Each entry is 64 bits (8 bytes)
gdt_write_entry(ds_desc, GDTR.base+8*2);
gdt_write_entry(ss_desc, GDTR.base+8*3);
}
void gdt_write_entry(GDT_ENTRY entry, u32 addr){
int descriptor[2];
// First row of the descriptor
descriptor[0]=(entry.limit & 0xFFFF)|(entry.base << 16);
// Format second row of the descriptor
u16 base=(entry.base >> 16) & 0xFF;
u16 access=entry.access & 0xFF;
u16 limit=(entry.limit >> 16) & 0xF; // Remember: limits it is on 20 bits so 4 last bits
u8 flags=entry.flags & 0xF;
u8 base2=entry.base >> 24; // Take the last 8 bits
descriptor[1]=base|access<<8|limit<<16|flags<<20|base2<<24;
// Copy descriptor into memory
memcpy(descriptor,(void*)addr,8);
}
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