diff --git a/Makefile b/Makefile
index 733507b..70d5b06 100644
--- a/Makefile
+++ b/Makefile
@@ -46,7 +46,7 @@ kernel.bin: kernel.o console.o drivers/vga.o drivers/keyboard.o \
 	$(CC) -m32 -ffreestanding -Wall -Werror -c -g $< -o $@
 
 %.o: %.S
-	$(AS) --32 -g $^ -o $@
+	$(CC) -m32 -ffreestanding -c -g $^ -o $@
 
 mbr.bin: mbr.o
 	$(LD) -m elf_i386 -Ttext=0x7c00 --oformat=binary $^ -o $@
diff --git a/cpu/gdt.h b/cpu/gdt.h
index 578fcc8..ca1acba 100644
--- a/cpu/gdt.h
+++ b/cpu/gdt.h
@@ -12,6 +12,11 @@
 #define SEG_UDATA 4
 #define SEG_TSS   5
 
+#define SEG_ASM(type,base,lim)                                  \
+        .word (((lim) >> 12) & 0xffff), ((base) & 0xffff);      \
+        .byte (((base) >> 16) & 0xff), (0x90 | (type)),         \
+                (0xC0 | (((lim) >> 28) & 0xf)), (((base) >> 24) & 0xff)
+
 #define USER_BASE 0x400000 // 4 MB
 
 #ifndef __ASSEMBLER__
diff --git a/mbr.S b/mbr.S
index a214220..34fa7c5 100644
--- a/mbr.S
+++ b/mbr.S
@@ -1,3 +1,5 @@
+#include "cpu/gdt.h"
+
     .code16
     .global _start
 _start:
@@ -102,12 +104,12 @@ switch_to_32bit:
     mov %cr0, %eax
     or $1, %eax             // 3. enable protected mode
     mov %eax, %cr0
-    ljmp $CODE_SEG, $init_32bit // 4. far jump
+    ljmp $SEG_KCODE << 3, $init_32bit // 4. far jump
 
 
 .code32
 init_32bit:
-    mov $DATA_SEG, %ax      // 5. update segment registers
+    mov $SEG_KDATA << 3, %ax      // 5. update segment registers
     mov %ax, %ds
     mov %ax, %ss
     mov %ax, %es
@@ -155,25 +157,8 @@ sectors_per_track:
     .balign 4
 gdt_start:
     .quad 0x0 // null descriptor
-
-// code segment descriptor
-gdt_code:
-    .word 0xffff    // segment length, bits 0-15
-    .word 0x0       // segment base, bits 0-15
-    .byte 0x0       // segment base, bits 16-23
-    .byte 0b10011010 // flags (8 bits)
-    .byte 0b11001111 // flags (4 bits) + segment length, bits 16-19
-    .byte 0x0       // segment base, bits 24-31
-
-// data segment descriptor
-gdt_data:
-    .word 0xffff    // segment length, bits 0-15
-    .word 0x0       // segment base, bits 0-15
-    .byte 0x0       // segment base, bits 16-23
-    .byte 0b10010010 // flags (8 bits)
-    .byte 0b11001111 // flags (4 bits) + segment length, bits 16-19
-    .byte 0x0       // segment base, bits 24-31
-
+    SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff)   # code seg
+    SEG_ASM(STA_W, 0x0, 0xffffffff)         # data seg
 gdt_end:
 
 // GDT descriptor
@@ -181,8 +166,5 @@ gdt_descriptor:
     .word gdt_end - gdt_start - 1 // size (16 bit)
     .int  gdt_start               // address (32 bit)
 
-.equ CODE_SEG, gdt_code - gdt_start
-.equ DATA_SEG, gdt_data - gdt_start
-
     . = _start + 510     # pad to 510 bytes
     .byte 0x55, 0xaa     # boot sector magic value