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#include "gdt.h"
#include <stdlib.h>
using seg = GDT::SegmentDescriptor;
static_assert(sizeof(GDT::Pointer) == 6);
constexpr uint32_t null_sz = 0;
constexpr uint32_t kseg_sz = 64 * 1024 * 1024 + 0xfff;
static GDT::SegmentDescriptor segments[256]{
[GDT::null0] = seg::make<null_sz>(0, {}),
[GDT::kcode] = seg::make<kseg_sz>(0, {.r_w = true, .exe = true, .segment = true, .present = true}),
[GDT::kdata] = seg::make<kseg_sz>(0, {.r_w = true, .segment = true, .present = true}),
};
GDT::GDT() {
Pointer gdtr{.limit = sizeof(segments) - 1, .base = reinterpret_cast<uint32_t>(segments)};
asm volatile("lgdt %0" : : "p"(gdtr));
// flush the segment registers
const auto cs = descriptor(kcode);
const auto ds = descriptor(kdata);
asm volatile(
"jmp %0,$.reload_cs\n\t" // far jump takes cs
".reload_cs:\n\t"
" mov %1, %%ds\n\t" // ds
" mov %1, %%es\n\t"
" mov %1, %%fs\n\t"
" mov %1, %%gs\n\t"
" mov %1, %%ss\n\t" ::"i"(cs),
"r"(ds));
printk("GDT installed at ", uhex{gdtr.base}, '\n');
}
uint16_t GDT::descriptor(GDT::SegmentMap i) const {
return static_cast<uint16_t>(reinterpret_cast<const uint32_t>(&segments[i]) -
reinterpret_cast<const uint32_t>(&segments));
}
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