Save and restore disk number

src/x86/header.S

@@ -5,6 +5,11 @@
 #define STACK_BASE  0x2000
 
 .code16
+.global disk_number
+.global restore_disk_number
+
+.section .data
+disk_number: .byte 0
 
 .section .header
     ljmp $0, $init
@@ -21,4 +26,11 @@ init:
     mov %ax, %fs
     mov %ax, %gs
     mov $STACK_BASE, %sp
+    mov %dl, disk_number
     ljmp $0, $_start
+
+.section .text
+restore_disk_number:
+    mov $disk_number, %bx
+    mov (%bx), %dl
+    ret

src/x86/stage1.S

@@ -18,13 +18,8 @@ parenth: .string " ("
 of: .string " of "
 to_address: .string ") to address "
 
-.section .data
-disk: .byte 0
-
 .section .text
 _start:
-    mov %dl, disk
-
     mov $hello, %si
     call print_str
 
@@ -107,6 +102,7 @@ read_loop:
     jmp read_loop
 
 finish:
+    call restore_disk_number
     ljmp $0, $STAGE2_BASE
     ljmp $0, $hang