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#include "gdt.h"
#include "mem.h"
struct GDT_REGISTER GDTR = { 0, 0 };
GDT_TSS TSS;
void gdt_memcpy(){
GDTR.limit=8*GDT_MAX_ENTRIES; // Each entry is 8 bytes and 8 entries
GDTR.base=0x800;
gdt_write_entry((GDT_ENTRY){0,0,0,0},0); // First one must be null
// ----- Kernel Segments
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(cs_desc, 1);
gdt_write_entry(ds_desc, 2);
gdt_write_entry(ss_desc, 3);
// ----- User Segments
GDT_ENTRY ucs_desc;
ucs_desc.base=0; // With paging will take care of allocating the right address for each task
ucs_desc.limit=0xFFFFF;
ucs_desc.flags=GDT_SZ|GDT_GR;
ucs_desc.access=GDT_PR|GDT_PRVL_3|GDT_S|GDT_EXEC|GDT_RW|GDT_DC;
GDT_ENTRY uds_desc;
uds_desc.base=0;
uds_desc.limit=0xFFFFF;
uds_desc.flags=GDT_SZ|GDT_GR;
uds_desc.access=GDT_PR|GDT_PRVL_3|GDT_S|GDT_RW;
GDT_ENTRY uss_desc;
uss_desc.base=0; // Not used in stack descriptor
uss_desc.limit=0x64; // Define how much entry it can contains
uss_desc.flags=GDT_SZ|GDT_GR;
uss_desc.access=GDT_PR|GDT_PRVL_3|GDT_S|GDT_RW|GDT_DC;
// Write GDT descriptors into memory
gdt_write_entry(ucs_desc, 4); // Each entry is 64 bits (8 bytes)
gdt_write_entry(uds_desc, 5);
gdt_write_entry(uss_desc, 6);
// Init TSS segment
TSS.t_reserved=0;
TSS.io_map=0;
GDT_ENTRY tss_desc;
tss_desc.base=(u32)&TSS; // Not used in stack descriptor
tss_desc.limit=0x68; // Define how much bytes it occupies
tss_desc.flags=0;
tss_desc.access=0x89 | GDT_PRVL_3; // Note that 0x89 is specific to TSS!
gdt_write_entry(tss_desc, 7);
}
void gdt_write_entry(GDT_ENTRY entry, u32 id){
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*)GDTR.base+8*id,8); // Each entry is 64 bits (8 bytes)
}
int gdt_user_ds_base(){
char *addr=(char*)GDTR.base+48;
int *base0_15=(int*)addr+2;
int *base16_31=(int*)addr+7;
int base0_15_content=*base0_15 & 0xFFFF;
int base16_21_content=*base16_31 & 0xFFFF;
return(base16_21_content<<16 & base0_15_content);
}
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