kernel has gdt now and loads segment register and we can allocate pages

1 files changed, 48 insertions, 0 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
new file mode 100644
index 0000000..0d3a404
--- /dev/null
+++ b/kernel/cpu.c
@@ -0,0 +1,48 @@
+#include "cpu.h"
+
+#include <stdint.h>
+
+#define KERNEL_CODE_DESC 0x00af9a000000ffff
+#define KERNEL_DATA_DESC 0x00cf92000000ffff 
+
+#define KERNEL_CODE_SEL 0x8
+#define KERNEL_DATA_SEL 0x10
+
+struct gdtr64
+{
+    uint16_t limit;
+    uint64_t base;
+} __attribute__((packed));
+
+static uint64_t gdt[16];
+
+void cpu_init(void)
+{
+    gdt[0] = 0;
+    gdt[1] = KERNEL_CODE_DESC;
+    gdt[2] = KERNEL_DATA_DESC;
+
+    struct gdtr64 gdtr = {sizeof(gdt) - 1, (uint64_t)&gdt};
+
+    __asm__ volatile
+    (   "pushf\n"
+        "mov %%rsp, %%rcx\n"
+        "cli\n"
+        "lgdt %0\n"
+        "movw %w2, %%ds\n"
+        "movw %w2, %%es\n"
+        "movw %w2, %%fs\n"
+        "movw %w2, %%gs\n"
+        "push %q2\n"
+        "push %%rcx\n"
+        "pushf\n"
+        "push %q1\n"
+        "lea .flush(%%rip), %%rax\n"
+        "push %%rax\n"
+        "iretq\n"
+        ".flush:\n"
+       "popf\n"
+        :: "m"(gdtr), "a"(KERNEL_CODE_SEL), "d"(KERNEL_DATA_SEL)
+    );
+}
+