diff --git a/kernel/lang/kio.cc b/kernel/lang/kio.cc
index e558c96..3579f82 100644
--- a/kernel/lang/kio.cc
+++ b/kernel/lang/kio.cc
@@ -24,7 +24,7 @@ void kio_bootstrap()
     cursor_y = 9;
 }
 
-void kprintf(char * fmt, ...)
+void kprintf(const char * fmt, ...)
 {
     va_list args;
     va_start(args, fmt);
@@ -34,7 +34,7 @@ void kprintf(char * fmt, ...)
     va_end(args);
 }
 
-void kvprintf(char * fmt, va_list args)
+void kvprintf(const char * fmt, va_list args)
 {
     char tmpbuf[25];
     for ( ; *fmt; fmt++)
@@ -127,7 +127,7 @@ void kputc(char c)
     }
 }
 
-void kputs(char * s)
+void kputs(const char * s)
 {
     while (*s)
     {
diff --git a/kernel/lang/kio.h b/kernel/lang/kio.h
index 58b4e15..925b407 100644
--- a/kernel/lang/kio.h
+++ b/kernel/lang/kio.h
@@ -10,13 +10,13 @@ extern "C" {
 
 void kio_bootstrap();
 
-void kprintf(char * fmt, ...);
+void kprintf(const char * fmt, ...);
 
-void kvprintf(char * fmt, va_list args);
+void kvprintf(const char * fmt, va_list args);
 
 void kputc(char c);
 
-void kputs(char * s);
+void kputs(const char * s);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/kernel/mm/mm.cc b/kernel/mm/mm.cc
index c8a088a..625472a 100644
--- a/kernel/mm/mm.cc
+++ b/kernel/mm/mm.cc
@@ -3,6 +3,7 @@
 
 #include "mm.h"
 #include "boot/k_early_panic.h"
+#include "lang/string.h"
 
 #define MM_MAX_MMAP_ENTRIES 64
 
@@ -13,6 +14,7 @@ static mm_mem_range_t mm_mmap_entries[MM_MAX_MMAP_ENTRIES];
 static int mm_mmap_num_entries = 0;
 static int mm_num_free_pages = 0;
 u32_t * mm_free_page_ptr = NULL;
+gdtr_t mm_gdtr;
 
 /**************************************************************************
  * This function is run in segmented memory before paging is in effect.   *
@@ -141,6 +143,15 @@ void mm_bootstrap()
         mm_map(page_base, page_base - KERNEL_OFFSET, 0, 1);
     }
 
+    /* set up the global descriptor table */
+    u32_t gdt_base = mm_page_alloc();
+    u64_t * gdt = (u64_t *) ((u32_t) gdt_base + (u32_t) KERNEL_OFFSET);
+    memsetd((u32_t *) gdt, 0, PAGE_SIZE / 4);
+    mm_gdtr.length = PAGE_SIZE - 1;
+    mm_gdtr.phys_addr = gdt_base;
+    gdt[1] = MAKE_DESCRIPTOR(0, 0xFFFFF, 1, 0, 1, 1);
+    gdt[2] = MAKE_DESCRIPTOR(0, 0xFFFFF, 1, 0, 1, 0);
+
     /* set the page directory base register */
     set_cr3(page_directory);
 
diff --git a/kernel/mm/mm.h b/kernel/mm/mm.h
index daa0eda..cfc28b0 100644
--- a/kernel/mm/mm.h
+++ b/kernel/mm/mm.h
@@ -26,8 +26,32 @@ int mm_map(u32_t virtual_address, u32_t physical_address,
 
 u32_t mm_page_alloc();
 
+typedef struct
+{
+    u16_t   length;
+    u32_t   phys_addr;
+} __attribute__ ((packed)) gdtr_t;
+
 #define set_cr3(address) \
     __asm__ __volatile__ ("movl %0, %%cr3" : : "r" (address));
 
+/* http://courses.ece.illinois.edu/ece391/references/descriptors.pdf */
+/* granularity: 0: limit in bytes; 1: limit in pages */
+/* dpl: 0: system mode; 3: user mode */
+/* normal: 0: TSS/LDT/call gate; 1: code/data segment */
+/* code_seg: 0: data segment; 1: code segment */
+#define MAKE_DESCRIPTOR(base, limit, granularity, dpl, normal, code_seg)       \
+        (u64_t) ( ( (((u64_t) base) & 0xFF000000ull) << 32 ) /* base 31:24 */  \
+                | ( (((u64_t) granularity) & 0x1ull) << 55 ) /* granularity */ \
+                | ( ( (u64_t) 0x1ull) << 54 )                /* 32-bit */      \
+                | ( (((u64_t) limit) & 0xF0000ull) << 32 )   /* limit 19:16 */ \
+                | ( ( (u64_t) 0x1ull) << 47 )                /* present */     \
+                | ( (((u64_t) dpl) & 0x3ull) << 45 )         /* dpl */         \
+                | ( (((u64_t) normal) & 0x1ull) << 44 )      /* normal */      \
+                | ( (((u64_t) code_seg) & 0x1ull) << 43 )    /* code seg */    \
+                | ( ( (u64_t) 0x2ull) << 40 )                /* ? */           \
+                | ( (((u64_t) base) & 0x00FFFFFFull) << 16)  /* base 23:00 */  \
+                | ( (((u64_t) limit) & 0x0000FFFFull) ) )    /* limit 15:00 */
+
 #endif