Improve register hygiene

kernel/bpb.asm

@@ -1,16 +1,31 @@
 ; IN	DL	drive number
 ;	BP	ptr to bpb_size'd buffer
 bpb_load:
+  push ax
+  push dx
+
   mov [bp+(bpb_size-1)], dl
 
+  ; load first sector
   xor dx, dx
   xor ax, ax
   call drive_read
 
+  push cx
+  push si
+  push di
+
+  ; copy BPB to BP
   mov si, (diskbuf+0x0B)
   mov di, bp
   mov cx, (bpb_size-1)
   rep movsb
 
+  pop di
+  pop si
+  pop cx
+
+  pop dx
+  pop ax
   ret