Add routine to backup IVT

kernel/intr.asm

@@ -1,3 +1,25 @@
+; out	ds:si	copy of interrupt table
+ivt_backup:
+  push ax
+  push cx
+  push di ; backup original value
+  push ds
+  mov ax, 0x80
+  call malloc
+  push di
+  mov cx, 0x80
+  mov ax, cs
+  mov es, ax
+  mov si, 0x0000
+  mov ds, si
+  rep movsb
+  pop si ; this was di after malloc
+  pop ds
+  pop di ; restore original di
+  pop cx
+  pop ax
+  ret
+
 ; set item in interrupt vector table
 ; in:	bx	interrupt number
 ;	ds:dx	new handler address