From 4843406bf3537de4a43d7a154d44cd847970a7fe Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Mon, 24 Aug 2026 16:24:42 +0200
Subject: [PATCH] feat(cpu): add GDT and TSS for ring 0 and ring 3 segments

---
 include/gdt.h | 15 ++++++++++++++
 include/tss.h |  9 +++++++++
 src/gdt.c     | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++
 src/tss.c     | 38 +++++++++++++++++++++++++++++++++++
 4 files changed, 117 insertions(+)
 create mode 100644 include/gdt.h
 create mode 100644 include/tss.h
 create mode 100644 src/gdt.c
 create mode 100644 src/tss.c

diff --git a/include/gdt.h b/include/gdt.h
new file mode 100644
index 0000000..0a87bcd
--- /dev/null
+++ b/include/gdt.h
@@ -0,0 +1,15 @@
+#ifndef FROSTBYTE_GDT_H
+#define FROSTBYTE_GDT_H
+
+#include "types.h"
+
+#define kernelCodeSelector 0x08
+#define kernelDataSelector 0x10
+#define userCodeSelector   0x18
+#define userDataSelector   0x20
+#define tssSelector        0x28
+
+void gdtInit(void);
+void gdtInstallTss(uint64 base, uint32 limit);
+
+#endif
diff --git a/include/tss.h b/include/tss.h
new file mode 100644
index 0000000..a668a50
--- /dev/null
+++ b/include/tss.h
@@ -0,0 +1,9 @@
+#ifndef FROSTBYTE_TSS_H
+#define FROSTBYTE_TSS_H
+
+#include "types.h"
+
+void tssInit(uint64 rsp0);
+void tssSetRsp0(uint64 rsp0);
+
+#endif
diff --git a/src/gdt.c b/src/gdt.c
new file mode 100644
index 0000000..642bf2d
--- /dev/null
+++ b/src/gdt.c
@@ -0,0 +1,55 @@
+#include "gdt.h"
+#include "types.h"
+
+typedef struct GdtPointer {
+    uint16 limit;
+    uint64 base;
+} __attribute__((packed)) GdtPointer;
+
+static uint64 gdtEntries[7];
+static GdtPointer gdtPointer;
+
+void gdtInit(void)
+{
+    gdtEntries[0] = 0x0000000000000000ull;
+    gdtEntries[1] = 0x00AF9A000000FFFFull;
+    gdtEntries[2] = 0x00CF92000000FFFFull;
+    gdtEntries[3] = 0x00AFFA000000FFFFull;
+    gdtEntries[4] = 0x00CFF2000000FFFFull;
+    gdtEntries[5] = 0x0000000000000000ull;
+    gdtEntries[6] = 0x0000000000000000ull;
+
+    gdtPointer.limit = (uint16)(sizeof(gdtEntries) - 1);
+    gdtPointer.base = (uint64)&gdtEntries;
+
+    __asm__ volatile(
+        "lgdt %0\n"
+        "pushq $0x08\n"
+        "leaq 1f(%%rip), %%rax\n"
+        "pushq %%rax\n"
+        "lretq\n"
+        "1:\n"
+        "movw $0x10, %%ax\n"
+        "movw %%ax, %%ds\n"
+        "movw %%ax, %%es\n"
+        "movw %%ax, %%ss\n"
+        "movw %%ax, %%fs\n"
+        "movw %%ax, %%gs\n"
+        :
+        : "m"(gdtPointer)
+        : "rax", "memory");
+}
+
+void gdtInstallTss(uint64 base, uint32 limit)
+{
+    uint64 low = 0;
+    low |= (uint64)(limit & 0xFFFF);
+    low |= (base & 0xFFFFull) << 16;
+    low |= ((base >> 16) & 0xFFull) << 32;
+    low |= (uint64)0x89 << 40;
+    low |= (uint64)((limit >> 16) & 0xF) << 48;
+    low |= ((base >> 24) & 0xFFull) << 56;
+
+    gdtEntries[5] = low;
+    gdtEntries[6] = (base >> 32) & 0xFFFFFFFFull;
+}
diff --git a/src/tss.c b/src/tss.c
new file mode 100644
index 0000000..10952f9
--- /dev/null
+++ b/src/tss.c
@@ -0,0 +1,38 @@
+#include "tss.h"
+#include "gdt.h"
+#include "string.h"
+
+typedef struct Tss {
+    uint32 reserved0;
+    uint64 rsp0;
+    uint64 rsp1;
+    uint64 rsp2;
+    uint64 reserved1;
+    uint64 ist1;
+    uint64 ist2;
+    uint64 ist3;
+    uint64 ist4;
+    uint64 ist5;
+    uint64 ist6;
+    uint64 ist7;
+    uint64 reserved2;
+    uint16 reserved3;
+    uint16 iomapBase;
+} __attribute__((packed)) Tss;
+
+static Tss tss;
+
+void tssInit(uint64 rsp0)
+{
+    memset(&tss, 0, sizeof(tss));
+    tss.rsp0 = rsp0;
+    tss.iomapBase = sizeof(Tss);
+
+    gdtInstallTss((uint64)&tss, sizeof(Tss) - 1);
+    __asm__ volatile("ltr %w0" : : "r"((uint16)tssSelector));
+}
+
+void tssSetRsp0(uint64 rsp0)
+{
+    tss.rsp0 = rsp0;
+}