Add dnomem device (see PR #45)

The dnomem device represents a memory region where any
reads or writes terminates with simulation failure.

The device is meant as a debugging aid to ensure our code is
not accessing non-existent memory.

doc/reference/devices.rst

@@ -344,6 +344,82 @@ All attempts to write into read-only memory are silently ignored.
 
 
 
+No-access memory ``dnomem``
+---------------------------
+
+The ``dnomem`` device represents a memory region where any reads or writes
+terminates with simulation failure.
+
+The device is meant as a debugging aid to ensure our code is not accessing
+non-existent memory.
+
+The device operates in three modes. In the default ``warn`` mode, attempts
+to access are printed to console but the simulation continues.
+In ``break`` mode, the simulation is switched to interactive mode after each
+access.
+And in ``halt`` mode, the simulation is immediately terminated.
+
+It is possible to dump registers of the CPU that caused the violation
+automatically by setting ``rd yes``.
+
+
+Initialization parameters: ``address`` ``size``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``address``
+   Starting physical address of the memory block.
+``size``
+   Memory size.
+
+Commands
+^^^^^^^^
+
+``help [cmd]``
+   Print a help on the command specified or a list of available commands.
+``info``
+   Print the device information (block address, size and mode)
+``mode``
+   Set device mode (either ``warn``, ``halt`` or ``break``).
+``rd``
+   Whether to dump registers on violation (``yes`` or ``no``).
+
+
+Examples
+^^^^^^^^
+
+In the following example, the ``add`` command creates a protected memory
+region ``nomem`` starting at the physical address 0x08000000.
+The size of the region is set to 4 KB.
+
+.. code:: msim
+
+   [msim] add dnomem nomem 0x08000000 0x1000
+   [msim]
+
+All attempts to read/write into this memory are reported.
+
+.. code:: asm
+
+    la $a0, 0x88000004
+    lw $a1, 0($a0)
+    /* <msim> Alert: Ignoring READ (at 0x008000004, 0x4 inside nomem). */
+
+If we configure the device to ``mode`` ``halt``, the simulation is halted upon
+reaching the ``lw`` instruction. By using ``nomem rd yes``, registers are
+dumped before the machine is halted.
+
+.. code:: msim
+
+   [msim] nomem rd yes
+   [msim] nomem mode halt
+   [msim]
+   # la $a0, 0x88000004
+   # lw $a1, 0($a0)
+   # <msim> Alert: Halting after forbidden READ (at 0x008000004, 0x4 inside nomem).
+
+
+
+
 Character output device ``dprinter``
 ------------------------------------
 

src/Makefile.in

@@ -51,6 +51,7 @@ SOURCES = \
 	device/drvcpu.c  \
 	device/dcycle.c \
 	device/dkeyboard.c \
+	device/dnomem.c \
 	device/dorder.c \
 	device/dprinter.c \
 	device/dtime.c \

src/device/device.c

@@ -22,6 +22,7 @@
 #include "drvcpu.h"
 #include "dcycle.h"
 #include "dkeyboard.h"
+#include "dnomem.h"
 #include "dorder.h"
 #include "ddisk.h"
 #include "dprinter.h"
@@ -34,7 +35,7 @@
 #include "../fault.h"
 
 /** Count of device types */
-#define DEVICE_TYPE_COUNT  10
+#define DEVICE_TYPE_COUNT  11
 
 /* Implemented peripheral list */
 const device_type_t *device_types[DEVICE_TYPE_COUNT] = {
@@ -46,6 +47,7 @@ const device_type_t *device_types[DEVICE_TYPE_COUNT] = {
 	&dprinter,
 	&dorder,
 	&dkeyboard,
+	&dnomem,
 	&ddisk,
 	&dtime
 };