94 lines
No EOL
3.2 KiB
C
94 lines
No EOL
3.2 KiB
C
#include "gdt.h"
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#include <stdint.h>
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typedef struct
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{
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uint16_t LimitLow; // limit (bits 0-15)
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uint16_t BaseLow; // base (bits 0-15)
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uint8_t BaseMiddle; // base (bits 16-23)
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uint8_t Access; // access
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uint8_t FlagsLimitHi; // limit (bits 16-19) | flags
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uint8_t BaseHigh; // base (bits 24-31)
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} __attribute__((packed)) GDTEntry;
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typedef struct
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{
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uint16_t Limit; // sizeof(gdt) - 1
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GDTEntry* Ptr; // address of GDT
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} __attribute__((packed)) GDTDescriptor;
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typedef enum
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{
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GDT_ACCESS_CODE_READABLE = 0x02,
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GDT_ACCESS_DATA_WRITEABLE = 0x02,
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GDT_ACCESS_CODE_CONFORMING = 0x04,
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GDT_ACCESS_DATA_DIRECTION_NORMAL = 0x00,
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GDT_ACCESS_DATA_DIRECTION_DOWN = 0x04,
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GDT_ACCESS_DATA_SEGMENT = 0x10,
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GDT_ACCESS_CODE_SEGMENT = 0x18,
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GDT_ACCESS_DESCRIPTOR_TSS = 0x00,
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GDT_ACCESS_RING0 = 0x00,
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GDT_ACCESS_RING1 = 0x20,
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GDT_ACCESS_RING2 = 0x40,
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GDT_ACCESS_RING3 = 0x60,
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GDT_ACCESS_PRESENT = 0x80,
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} GDT_ACCESS;
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typedef enum
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{
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GDT_FLAG_64BIT = 0x20,
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GDT_FLAG_32BIT = 0x40,
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GDT_FLAG_16BIT = 0x00,
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GDT_FLAG_GRANULARITY_1B = 0x00,
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GDT_FLAG_GRANULARITY_4K = 0x80,
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} GDT_FLAGS;
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// Helper macros
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#define GDT_LIMIT_LOW(limit) (limit & 0xFFFF)
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#define GDT_BASE_LOW(base) (base & 0xFFFF)
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#define GDT_BASE_MIDDLE(base) ((base >> 16) & 0xFF)
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#define GDT_FLAGS_LIMIT_HI(limit, flags) (((limit >> 16) & 0xF) | (flags & 0xF0))
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#define GDT_BASE_HIGH(base) ((base >> 24) & 0xFF)
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#define GDT_ENTRY(base, limit, access, flags) { \
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GDT_LIMIT_LOW(limit), \
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GDT_BASE_LOW(base), \
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GDT_BASE_MIDDLE(base), \
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access, \
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GDT_FLAGS_LIMIT_HI(limit, flags), \
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GDT_BASE_HIGH(base) \
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}
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GDTEntry g_GDT[] = {
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// NULL descriptor
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GDT_ENTRY(0, 0, 0, 0),
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// Kernel 32-bit code segment
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GDT_ENTRY(0,
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0xFFFFF,
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GDT_ACCESS_PRESENT | GDT_ACCESS_RING0 | GDT_ACCESS_CODE_SEGMENT | GDT_ACCESS_CODE_READABLE,
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GDT_FLAG_32BIT | GDT_FLAG_GRANULARITY_4K),
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// Kernel 32-bit data segment
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GDT_ENTRY(0,
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0xFFFFF,
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GDT_ACCESS_PRESENT | GDT_ACCESS_RING0 | GDT_ACCESS_DATA_SEGMENT | GDT_ACCESS_DATA_WRITEABLE,
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GDT_FLAG_32BIT | GDT_FLAG_GRANULARITY_4K),
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};
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GDTDescriptor g_GDTDescriptor = { sizeof(g_GDT) - 1, g_GDT};
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void __attribute__((cdecl)) i686_GDT_Load(GDTDescriptor* descriptor, uint16_t codeSegment, uint16_t dataSegment);
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void i686_GDT_Initialize()
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{
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i686_GDT_Load(&g_GDTDescriptor, i686_GDT_CODE_SEGMENT, i686_GDT_DATA_SEGMENT);
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} |