66 lines
2.0 KiB
C
66 lines
2.0 KiB
C
#ifndef GDT_H
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#define GDT_H
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#include <stdint.h>
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#define GDT_NB_ENTRIES 3
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#define GDT_SIZE (GDT_NB_ENTRIES * sizeof(struct gdt_entry))
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#define KERNEL_CS_INDEX 1
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#define KERNEL_DS_INDEX 2
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struct gdt_entry
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{
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uint16_t limit_1 : 16; // 16 lsbs of the segment limit
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uint16_t base_1 : 16; // 16 lsbs of the segment base address
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uint8_t base_2 : 8; // 8 next bits of the segment base address
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/* Access Byte */
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uint8_t accessed : 1; // set to 0, is set to 1 by CPU when segment is
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// accessed
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uint8_t rw : 1; // read/write bit, 1 to grant read acces on code
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// segments or write access on data segments
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uint8_t dc : 1; // Direction/Conforming bit
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uint8_t ex : 1; // Executable bit
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uint8_t desc_type : 1; // Descriptor type: 1 for data or code segmentm
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// 0 for system segments (like TSS)
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uint8_t desc_priv : 2; // Descriptor privilege
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uint8_t present : 1; // Always set to 1
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uint8_t limit_2 : 4; // 4 msbs of the segment limit
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/* Flags */
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uint8_t available : 1; // Available for use by system software
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uint8_t l : 1; // 64-bit code segment
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uint8_t db : 1; // size, 0 for 16 bits, 1 for 32 bits
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uint8_t granularity : 1; // limit unit, 0 for bytes, 1 for 4KiB (1 page)
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uint8_t base_3 : 8; // 8 msbs of the segment base
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} __attribute__ ((packed));
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struct gdt
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{
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struct gdt_entry entries[GDT_NB_ENTRIES];
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};
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struct gdt_r
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{
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uint16_t limit; // total size of the GDT - 1 in bytes
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uint32_t addr; // base address of the DGT
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} __attribute__ ((packed));
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struct segment_selector
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{
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uint8_t rpl : 2; // request privilege level
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uint8_t table_indicator : 1; // 0 for GDT, 1 for LDT
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uint16_t index : 13; // index of the entry in the table
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} __attribute__ ((packed));
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void init_gdt(void);
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#endif /* !GDT_H */
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