52 lines
1.5 KiB
C
52 lines
1.5 KiB
C
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#include "gdt.h"
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#include "mem.h"
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struct GDT_REGISTER GDTR = { 0, 0 };
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void gdt_memcpy(){
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GDTR.limit=8*4; // Each entry is 8 bytes and 4 entries
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GDTR.base=0x800;
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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((GDT_ENTRY){0,0,0,0},GDTR.base); // First one must be null
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gdt_write_entry(cs_desc, GDTR.base+8); // Each entry is 64 bits (8 bytes)
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gdt_write_entry(ds_desc, GDTR.base+8*2);
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gdt_write_entry(ss_desc, GDTR.base+8*3);
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}
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void gdt_write_entry(GDT_ENTRY entry, u32 addr){
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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*)addr,8);
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}
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