Move to ring 3

src/kernel/gdt.c

@@ -0,0 +1,54 @@
+#include "gdt.h"
+#include "libc.h"
+#include "str.h"
+
+inline uint16_t gdt_segment_selector(uint16_t index, uint16_t table_indicator, uint16_t priv_level)
+{
+    return (index            << 3)
+         | ((table_indicator << 2) & 0b100)
+         | ((priv_level      << 0) & 0b011);
+}
+
+struct gdt_table_entry gdt_encode_entry(struct gdt_entry_content content)
+{
+    if (content.limit > GDT_LIMIT_MAX)
+        panic(str_attach("GDT cannot encode limits larger than " CSTR(GDT_LIMIT_MAX)));
+
+    struct gdt_table_entry entry;
+
+    entry.data[GDT_SDESC_LIMIT_0] = (content.limit >>  0) & 0xFF;
+    entry.data[GDT_SDESC_LIMIT_1] = (content.limit >>  8) & 0xFF;
+    entry.data[GDT_SDESC_LIMIT_2] = (content.limit >> 16) & 0xFF;
+
+    entry.data[GDT_SDESC_BASE_0] = (content.base >>  0) & 0xFF;
+    entry.data[GDT_SDESC_BASE_1] = (content.base >>  8) & 0xFF;
+    entry.data[GDT_SDESC_BASE_2] = (content.base >> 16) & 0xFF;
+    entry.data[GDT_SDESC_BASE_3] = (content.base >> 24) & 0xFF;
+
+    entry.data[GDT_SDESC_ACCESSBYTE] = content.access_byte;
+
+    entry.data[GDT_SDESC_FLAGS] |= (content.flags << 4);
+
+    return entry;
+}
+
+void gdt_set(uint16_t size, const struct gdt_table_entry base[], uint32_t offset)
+{
+    base += offset;
+
+    /* the lgdt instruction requires a packed alignment */
+    struct __attribute__((packed)) gdtr  {
+        uint16_t size;
+        uint32_t base;
+    } gdt = {
+        .size = size,
+        .base = (uint32_t)base,
+    };
+
+    __asm__ volatile (
+        "lgdt %[gdt]"
+        :
+        : [gdt] "m"(gdt)
+    );
+}
+