91 lines
2.6 KiB
C
91 lines
2.6 KiB
C
#include "gdt.h"
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#include "mem.h"
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struct GDT_REGISTER GDTR = { 0, 0 };
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GDT_TSS TSS;
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void gdt_memcpy(){
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GDTR.limit=8*8; // Each entry is 8 bytes and 8 entries
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GDTR.base=0x800;
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gdt_write_entry((GDT_ENTRY){0,0,0,0},0); // First one must be null
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// ----- Kernel Segments
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GDT_ENTRY cs_desc;
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cs_desc.base=0;
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cs_desc.limit=0xFFFFF;
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cs_desc.flags=GDT_SZ|GDT_GR;
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cs_desc.access=GDT_PR|GDT_PRVL_0|GDT_S|GDT_EXEC|GDT_RW;
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GDT_ENTRY ds_desc;
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ds_desc.base=0;
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ds_desc.limit=0xFFFFF;
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ds_desc.flags=GDT_SZ|GDT_GR;
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ds_desc.access=GDT_PR|GDT_PRVL_0|GDT_S|GDT_RW;
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GDT_ENTRY ss_desc;
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ss_desc.base=0;
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ss_desc.limit=0;
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ss_desc.flags=GDT_SZ|GDT_GR;
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ss_desc.access=GDT_PR|GDT_PRVL_0|GDT_S|GDT_RW|GDT_DC;
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// Write GDT descriptors into memory
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gdt_write_entry(cs_desc, 1);
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gdt_write_entry(ds_desc, 2);
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gdt_write_entry(ss_desc, 3);
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// ----- User Segments
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GDT_ENTRY ucs_desc;
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ucs_desc.base=0x300000;
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ucs_desc.limit=0xFFFFF;
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ucs_desc.flags=GDT_SZ|GDT_GR;
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ucs_desc.access=GDT_PR|GDT_PRVL_3|GDT_S|GDT_EXEC|GDT_RW|GDT_DC;
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GDT_ENTRY uds_desc;
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uds_desc.base=0x300000;
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uds_desc.limit=0xFFFFF;
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uds_desc.flags=GDT_SZ|GDT_GR;
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uds_desc.access=GDT_PR|GDT_PRVL_3|GDT_S|GDT_RW;
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GDT_ENTRY uss_desc;
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uss_desc.base=0; // Not used in stack descriptor
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uss_desc.limit=0x64; // Define how much entry it can contains
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uss_desc.flags=GDT_SZ|GDT_GR;
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uss_desc.access=GDT_PR|GDT_PRVL_3|GDT_S|GDT_RW|GDT_DC;
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// Write GDT descriptors into memory
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gdt_write_entry(ucs_desc, 4); // Each entry is 64 bits (8 bytes)
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gdt_write_entry(uds_desc, 5);
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gdt_write_entry(uss_desc, 6);
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// Init TSS segment
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TSS.t_reserved=0;
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TSS.io_map=0;
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TSS.ss0=0x18;
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TSS.esp0=0x50000;
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GDT_ENTRY tss_desc;
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tss_desc.base=(u32)&TSS; // Not used in stack descriptor
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tss_desc.limit=0x68; // Define how much entry it can contains
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tss_desc.flags=0;
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tss_desc.access=0x89 | GDT_PRVL_3; // Note that 0x89 is specific to TSS!
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gdt_write_entry(tss_desc, 7);
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}
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void gdt_write_entry(GDT_ENTRY entry, u32 id){
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int descriptor[2];
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// First row of the descriptor
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descriptor[0]=(entry.limit & 0xFFFF)|(entry.base << 16);
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// Format second row of the descriptor
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u16 base=(entry.base >> 16) & 0xFF;
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u16 access=entry.access & 0xFF;
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u16 limit=(entry.limit >> 16) & 0xF; // Remember: limits it is on 20 bits so 4 last bits
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u8 flags=entry.flags & 0xF;
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u8 base2=entry.base >> 24; // Take the last 8 bits
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descriptor[1]=base|access<<8|limit<<16|flags<<20|base2<<24;
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// Copy descriptor into memory
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memcpy(descriptor,(void*)GDTR.base+8*id,8); // Each entry is 64 bits (8 bytes)
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}
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