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129 lines
3.1 KiB
C
129 lines
3.1 KiB
C
#ifndef _GDT_H
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#define _GDT_H
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#include <kernel.h>
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// Privilege levels
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#define DPL_KERNEL 0
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#define DPL_USER 3
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/*
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* Segment selector explanation (why kcode is 0x08?)
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*
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* Each segment selector must be 16 bits. These bits
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* have the following layout: (example is 0x08)
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* 0000000000001 0 00 (lsb)
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* | | '- Privilege level
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* | '--- Table indicator
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* '------ Index in table
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*
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* Bits 0-1 are the privilege level. x86 has four,
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* where 00 is the highest and 11 is the lowest. Only
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* these two are widely used. 00 being kernel space &
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* 11 being userspace. Bit 3 is the table indicator.
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* 0 means the GDT and 1 means the LDT. This will most
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* likely be 0. Bits 4-15 are the index. This is just
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* which descriptor in the table is used.
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*
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* SO, for the kernel code segment:
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* Privilege is 00 for kernel space, table indicator
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* s 0 for the GDT and the index is one, because it
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* is the second entry in our GDT (after null desc.)
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*/
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#define SEGMENT_SEL(_idx, _tab, _dpl) (\
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(((_dpl) ) & 3) | \
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(((_tab) << 2) & 4) | \
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(((_idx) << 3) ) \
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)
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/*
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* Entries:
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* 0 Null desc.
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* 1 Kernel code desc.
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* 2 Kernel data desc.
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* 3 User code desc.
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* 4 User data desc.
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* 5 Task State Segment
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*/
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#define GDT_ENTRIES 6
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#define GDT_NULL SEGMENT_SEL(0, 0, 0)
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#define GDT_KERNEL_CODE SEGMENT_SEL(1, 0, DPL_KERNEL)
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#define GDT_KERNEL_DATA SEGMENT_SEL(2, 0, DPL_KERNEL)
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#define GDT_USER_CODE SEGMENT_SEL(3, 0, DPL_USER)
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#define GDT_USER_DATA SEGMENT_SEL(4, 0, DPL_USER)
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#define GDT_TSS SEGMENT_SEL(5, 0, DPL_KERNEL)
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#define GDT_DESC(_lim, _base, _access, _flags) (struct gdt_entry) { \
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.base_low = (_base) & 0xFFFF, \
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.base_mid = ((_base) >> 16) & 0xFF, \
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.base_high = ((_base) >> 24) & 0xFF, \
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.limit = (_lim) & 0xFFFF, \
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.flags = ((_flags) & 0xF0) | (((_lim) >> 16) & 0x0F), \
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.access = (_access) \
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}
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/*
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* GDT descriptor/entry explanation
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*
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* 0-15 low 16 bits of limit
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* 15-31 low 16 bits of base
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* 32-39 mid 8 bits of base
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* 40-47 access byte
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* 40 Accessed
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* 41 Read/Write
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* 42 Direction/Conforming
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* 43 Executable
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* 44 Descriptor type
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* 45-46 DPL
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* 47 Present
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* 48-51 high 4 bits of limit
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* 52-55 flags
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* 52 Reserved
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* 53 Long-mode
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* 54 Size (0 - 16-bit, 1 - 32-bit)
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* 55 Granularity (0 - limit in bytes, 1 - limit in pages)
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* 56-63 high 8 bits of base
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*/
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struct gdt_entry {
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uint16_t limit;
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uint16_t base_low;
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uint8_t base_mid;
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uint8_t access;
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uint8_t flags; // low 4 bits are 4 msbs of limit, high 4 bits are flags
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uint8_t base_high;
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} __packed;
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struct gdt_pointer {
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uint16_t limit;
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uint32_t base;
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} __packed;
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struct tss_pointer {
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uint16_t prev_task, __reserved0;
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uint32_t esp0;
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uint16_t ss0, __reserved1;
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uint32_t esp1;
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uint16_t ss1, __reserved2;
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uint32_t esp2;
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uint16_t ss2, __reserved3;
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uint32_t cr3;
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uint32_t eip;
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uint32_t eflags;
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uint32_t eax, ecx, edx, ebx, esp, ebp, esi, edi;
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uint16_t es, __reserved4;
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uint16_t cs, __reserved5;
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uint16_t ss, __reserved6;
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uint16_t ds, __reserved7;
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uint16_t fs, __reserved8;
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uint16_t gs, __reserved9;
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uint16_t ldtr;
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uint32_t __reserved10;
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uint16_t iopb;
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uint32_t ssp;
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} __packed;
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void tss_esp0(uint32_t esp0);
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int gdt_init(void);
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#endif
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