Poncho OS Tutorials

GDT.asm

[bits 64]

loadGDT:
	lgdt [rdi] ; "rdi" register will contain parameter passed to loadGDT when called from C.

	; Upload kernelData segment.
	mov ax, 0x10
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	mov ss, ax
	pop rdi ; Pop return address of loadGDT function.

	; Upload kernelCode segment.
	mov rax, 0x08
	push rax
	push rdi
	retfq

GLOBAL loadGDT

GDT.cpp

    __attribute__((aligned(0x1000)))
    GlobalDescriptorTable defaultGDT = {
        {0, 0, 0, 0x00, 0x00, 0}, // null
        {0, 0, 0, 0x9a, 0xa0, 0}, // kernelCode
        {0, 0, 0, 0x92, 0xa0, 0}, // kernelData
        {0, 0, 0, 0x9a, 0xa0, 0}, // userCode
        {0, 0, 0, 0x92, 0xa0, 0}, // userData
    };

GDT.h

#pragma once

#include <Common/Types.h>

struct GDTDescriptor {
	UInt16 size;
	UInt64 offset;
} __attribute__((packed));

struct GDTEntry {
	UInt16 limit0;
	UInt16 base0;
	UInt8 base1;
	UInt8 accessByte;
	UInt8 limit1Flags;
	UInt8 base2;
}__attribute__((packed));

struct GlobalDescriptorTable {
	GDTEntry null; // 0x00
	GDTEntry kernelCode; // 0x08
	GDTEntry kernelData; // 0x10
	GDTEntry userNull;
	GDTEntry userCode;
	GDTEntry userData;
}__attribute__((packed)) __attribute__((aligned(0x1000)));

extern GlobalDescriptorTable defaultGDT;
extern "C" void loadGDT(GDTDescriptor *gdtDescriptor);

KernelUtility.cpp

KernelInformation initializeKernel(BootInformation *bootInformation)
{
    GDTDescriptor gdtDescriptor;
    gdtDescriptor.size = (sizeof(GlobalDescriptorTable) - 1);
    gdtDescriptor.offset = (UInt64)&defaultGDT;
    loadGDT(&gdtDescriptor);

    prepareMemory(bootInformation);

    return kernelInformation;
}