diff --git a/doc/mdk_tut.texi b/doc/mdk_tut.texi
index e2d3a15..9ff6f86 100644
--- a/doc/mdk_tut.texi
+++ b/doc/mdk_tut.texi
@@ -684,14 +684,17 @@ OPCODE = 34, MOD = I/O unit.
 @noindent
 In all the above instructions, the @samp{MOD} subfile must be in the
 range 0-20, since it denotes the operation's target device. The
-@samp{IOC} instruction only makes sense for tape devices (@samp{MOD} =
-0-7 or 20): it shifts the read/write pointer by the number of words
-given by @samp{M} (if it equals zero, the tape is rewound)@footnote{In
-Knuth's original definition, there are other control operations
-available, but they do not make sense when implementing the block
-devices as disk files (as we do in @sc{mdk} simulator). For the same
-reason, @sc{mdk} devices are always ready, since all input-output
-operations are performed using synchronous system calls.}.
+@samp{IOC} instruction makes sense for magnetic tape devices (@samp{MOD} =
+0-7): it shifts the read/write pointer by the number of blocks
+given by @samp{M} (if it equals zero, the tape is rewound), paper tape
+devices (@samp{MOD} = 20): @samp{M} should be 0, the tape is rewound,
+and disk/drum devices (@samp{MOD} = 8-15): it moves the read/write
+pointer to the block specified in rX and @samp{M} should be 0@footnote{In
+Knuth's original definition, there are other control operations available,
+but they do not make sense when implementing the devices as disk files (as
+we do in @sc{mdk} simulator). For the same reason, @sc{mdk} devices are
+always ready, since all input-output operations are performed using
+synchronous system calls.}.
 
 
 @node Conversion operators, Shift operators, Input-output operators, MIX instruction set
diff --git a/mixgtk/mixgtk_device.c b/mixgtk/mixgtk_device.c
index d3ce934..0cae037 100644
--- a/mixgtk/mixgtk_device.c
+++ b/mixgtk/mixgtk_device.c
@@ -272,9 +272,9 @@ read_ (mix_device_t *dev, mix_word_t *block)
 }
 
 static gboolean
-ioc_ (mix_device_t *dev, mix_short_t cmd)
+ioc_ (mix_device_t *dev, mix_short_t cmd, mix_word_t val)
 {
-  return (DEF_DEV_VTABLE_->ioc)(dev, cmd);
+  return (DEF_DEV_VTABLE_->ioc)(dev, cmd, val);
 }
 
 static gboolean
diff --git a/mixlib/mix_device.c b/mixlib/mix_device.c
index 5d3927c..e63119b 100644
--- a/mixlib/mix_device.c
+++ b/mixlib/mix_device.c
@@ -148,11 +148,11 @@ mix_device_read (mix_device_t *dev, mix_word_t *block)
 }
 
 gboolean
-mix_device_ioc (mix_device_t *dev, mix_short_t arg)
+mix_device_ioc (mix_device_t *dev, mix_short_t arg, mix_word_t val)
 {
   g_return_val_if_fail (dev != NULL, FALSE);
   g_assert (dev->vtable != NULL);
-  return (dev->vtable->ioc) (dev, arg);
+  return (dev->vtable->ioc) (dev, arg, val);
 }
 
 gboolean
diff --git a/mixlib/mix_device.h b/mixlib/mix_device.h
index 1273f18..c0e03ab 100644
--- a/mixlib/mix_device.h
+++ b/mixlib/mix_device.h
@@ -148,7 +148,7 @@ mix_device_read (mix_device_t *dev, mix_word_t *block);
    >0 - skip forward the given number of blocks
 */
 extern gboolean
-mix_device_ioc (mix_device_t *dev, mix_short_t arg);
+mix_device_ioc (mix_device_t *dev, mix_short_t arg, mix_word_t val);
 
 /*
   Check if a device is busy
diff --git a/mixlib/xmix_device.c b/mixlib/xmix_device.c
index 8d4084c..6fb085f 100644
--- a/mixlib/xmix_device.c
+++ b/mixlib/xmix_device.c
@@ -143,14 +143,20 @@ read_ (mix_device_t *dev, mix_word_t *block)
 }
 
 static gboolean
-ioc_ (mix_device_t *dev, mix_short_t arg)
+ioc_ (mix_device_t *dev, mix_short_t arg, mix_word_t val)
 {
   int m;
   FILE *file;
+  long diskblock;
 
   m = mix_short_magnitude(arg);
   if (mix_short_is_negative(arg)) m = -m;
   m *= sizeof (mix_word_t) * SIZES_[dev->type];
+
+  diskblock = mix_word_magnitude(val);
+  if (mix_word_is_negative(val)) diskblock = -diskblock;
+  diskblock *= sizeof (mix_word_t) * SIZES_[dev->type];
+
   file = mix_io_to_FILE (GET_CHANNEL_(dev));
 
   if (dev->type >= mix_dev_TAPE_0 && dev->type <= mix_dev_TAPE_7)
@@ -160,12 +166,13 @@ ioc_ (mix_device_t *dev, mix_short_t arg)
     }
   if (dev->type >= mix_dev_DISK_0 && dev->type <= mix_dev_DISK_7)
     {
-      if (m == 0) return FALSE;
-      // position disk
+      // move read/write head to block
+      if (m != 0 || diskblock < 0) return FALSE;
+      else fseek (file, diskblock, SEEK_SET);
     }
   if (dev->type == mix_dev_PAPER_TAPE)
     {
-      if (m == 0) return FALSE;
+      if (m != 0) return FALSE;
       rewind (file);
     }
   return TRUE;
diff --git a/mixlib/xmix_device.h b/mixlib/xmix_device.h
index 13950c9..4de321d 100644
--- a/mixlib/xmix_device.h
+++ b/mixlib/xmix_device.h
@@ -32,7 +32,7 @@ extern gchar *DEV_DIR_;
 /* table of overridable device operations */
 typedef gboolean (*mix_dev_write_func_t) (mix_device_t *, const mix_word_t *);
 typedef gboolean (*mix_dev_read_func_t) (mix_device_t *, mix_word_t *);
-typedef gboolean (*mix_dev_ioc_func_t) (mix_device_t *, mix_short_t);
+typedef gboolean (*mix_dev_ioc_func_t) (mix_device_t *, mix_short_t, mix_word_t);
 typedef gboolean (*mix_dev_busy_func_t) (const mix_device_t *);
 typedef void (*mix_dev_destroy_t) (mix_device_t *);
 
diff --git a/mixlib/xmix_vm.c b/mixlib/xmix_vm.c
index 68a8106..1b8d80a 100644
--- a/mixlib/xmix_vm.c
+++ b/mixlib/xmix_vm.c
@@ -303,6 +303,7 @@ ioc_handler_ (mix_vm_t *vm, const mix_ins_t *ins)
 {
   mix_address_t addr;
   mix_device_t *dev;
+  mix_word_t val;
 
   g_assert (ins->opcode == mix_opIOC);
 
@@ -312,7 +313,8 @@ ioc_handler_ (mix_vm_t *vm, const mix_ins_t *ins)
   dev = get_dev_ (vm, ins->fspec);
   fail_if_not_ (vm, dev != NULL, MIX_VM_ERROR_BAD_DEVICE_NO);
 
-  fail_if_not_ (vm, mix_device_ioc (dev, addr), MIX_VM_ERROR_DEV_CTL);
+  val = get_rX_ (vm);
+  fail_if_not_ (vm, mix_device_ioc (dev, addr, val), MIX_VM_ERROR_DEV_CTL);
 
   inc_loc_ (vm);
   return TRUE;
