1 files changed, 169 insertions, 0 deletions
diff --git a/src/gdt.cpp b/src/gdt.cpp
new file mode 100644
index 0000000..24d974b
--- /dev/null
+++ b/src/gdt.cpp
@@ -0,0 +1,169 @@
+#include <array>
+#include <cstdint>
+
+struct gdt_entry_bits {
+    std::uint32_t limit_low              : 16;
+    std::uint32_t base_low               : 24;
+    std::uint32_t accessed               :  1;
+    std::uint32_t read_write             :  1; // readable for code, writable for data
+    std::uint32_t conforming_expand_down :  1; // conforming for code, expand down for data
+    std::uint32_t code                   :  1; // 1 for code, 0 for data
+    std::uint32_t code_data_segment      :  1; // should be 1 for everything but TSS and LDT
+    std::uint32_t DPL                    :  2; // privilege level
+    std::uint32_t present                :  1;
+    std::uint32_t limit_high             :  4;
+    std::uint32_t available              :  1; // only used in software; has no effect on hardware
+    std::uint32_t long_mode              :  1;
+    std::uint32_t big                    :  1; // 32-bit opcodes for code, uint32_t stack for data
+    std::uint32_t gran                   :  1; // 1 to use 4k page addressing, 0 for byte addressing
+    std::uint32_t base_high              :  8;
+} __attribute__((packed));
+
+struct TSSEntry
+{
+    std::uint32_t prevTSS;
+    std::uint32_t esp0;
+    std::uint32_t ss0;
+    std::uint32_t unused[23] = {};
+} __attribute__((packed));
+
+static TSSEntry tss = {
+    .prevTSS = 0,
+    .esp0 = 0,
+    .ss0 = 0x10
+};
+
+static const std::array<gdt_entry_bits, 6> gdt {{
+    {},
+    /* kernel_code = */ {
+        .limit_low = 0xFFFF,
+        .base_low = 0x0000,
+        .accessed = 0,
+        .read_write = 1,
+        .conforming_expand_down = 0,
+        .code = 1,
+        .code_data_segment = 1,
+        .DPL = 0,
+        .present = 1,
+        .limit_high = 0xF,
+        .available = 0,
+        .long_mode = 0,
+        .big = 1,
+        .gran = 1,
+        .base_high = 0x00
+    },
+    /* kernel_data = */ {
+        .limit_low = 0xFFFF,
+        .base_low = 0x0000,
+        .accessed = 0,
+        .read_write = 1,
+        .conforming_expand_down = 0,
+        .code = 0,
+        .code_data_segment = 1,
+        .DPL = 0,
+        .present = 1,
+        .limit_high = 0xF,
+        .available = 0,
+        .long_mode = 0,
+        .big = 1,
+        .gran = 1,
+        .base_high = 0x00
+    },
+    /* user_code = */ {
+        .limit_low = 0xFFFF,
+        .base_low = 0x0000,
+        .accessed = 0,
+        .read_write = 1,
+        .conforming_expand_down = 0,
+        .code = 1,
+        .code_data_segment = 1,
+        .DPL = 3,
+        .present = 1,
+        .limit_high = 0xF,
+        .available = 0,
+        .long_mode = 0,
+        .big = 1,
+        .gran = 1,
+        .base_high = 0x00
+    },
+    /* user_data = */ {
+        .limit_low = 0xFFFF,
+        .base_low = 0x0000,
+        .accessed = 0,
+        .read_write = 1,
+        .conforming_expand_down = 0,
+        .code = 0,
+        .code_data_segment = 1,
+        .DPL = 3,
+        .present = 1,
+        .limit_high = 0xF,
+        .available = 0,
+        .long_mode = 0,
+        .big = 1,
+        .gran = 1,
+        .base_high = 0x00
+    },
+    /* tss = */ {
+        .limit_low = sizeof(TSSEntry),
+        .base_low = (std::uint32_t)&tss & 0xFFFFFF,
+        .accessed = 1,
+        .read_write = 0,
+        .conforming_expand_down = 0,
+        .code = 1,
+        .code_data_segment = 0,
+        .DPL = 0,
+        .present = 1,
+        .limit_high = 0,
+        .available = 0,
+        .long_mode = 0,
+        .big = 0,
+        .gran = 0,
+        .base_high = (std::uint32_t)&tss >> 24
+    }
+}};
+
+void gdt_initialize()
+{
+    auto gdtr = reinterpret_cast<std::uint64_t>(gdt.data());
+    gdtr <<= 16;
+    gdtr |= gdt.size() * sizeof(gdt[0]);
+
+    asm volatile(R"(
+        lgdt %0
+        pushl $0x8
+        push $.setcs
+        ljmp *(%%esp)
+    .setcs:
+        add $8, %%esp
+        mov $0x10, %%eax
+        mov %%eax, %%ds
+        mov %%eax, %%es
+        mov %%eax, %%fs
+        mov %%eax, %%gs
+        mov %%eax, %%ss
+
+        mov $0x28, %%ax
+        ltr %%ax
+    )" :: "m"(gdtr));
+}
+
+void enter_user_mode(void (*func)())
+{
+    asm volatile("mov %%esp, %0" : "=r" (tss.esp0));
+
+    asm volatile(R"(
+        mov $0x23, %%ax
+        mov %%ax, %%ds
+        mov %%ax, %%es
+        mov %%ax, %%fs
+        mov %%ax, %%gs
+        mov %%esp, %%eax
+        push $0x23
+        push %%esp
+        pushf
+        push $0x1b
+        push %0
+        iret
+    )" :: "b"(func));
+}
+