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<b>Gavare's eXperimental Emulator:</b></font><br>
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<a href="./">Back to the index</a>

<p><br>
<h2>Technical details</h2>

<p>This page describes some of the internals of GXemul.

<p>
<ul>
  <li><a href="#speed">Speed and emulation modes</a>
  <li><a href="#net">Networking</a>
  <li><a href="#devices">Emulation of hardware devices</a>
</ul>


<p><br>
<a name="speed"></a>
<h3>Speed and emulation modes</h3>

So, how fast is GXemul? There is no short answer to this. There is 
especially no answer to the question <b>What is the slowdown factor?</b>, 
because the host architecture and emulated architecture can usually not be 
compared just like that.

<p>Performance depends on several factors, including (but not limited to)  
host architecture, target architecture, host clock speed, which compiler
and compiler flags were used to build the emulator, what the workload is,
what additional runtime flags are given to the emulator, and so on.

<p>Devices are generally not timing-accurate: for example, if an emulated
operating system tries to read a block from disk, from its point of view
the read was instantaneous (no waiting). So 1 MIPS in an emulated OS might
have taken more than one million instructions on a real machine.

<p>Also, if the emulator says it has executed 1 million instructions, and 
the CPU family in question was capable of scalar execution (i.e. one cycle 
per instruction), it might still have taken more than 1 million cycles on 
a real machine because of cache misses and similar micro-architectural 
penalties that are not simulated by GXemul.

<p>Because of these issues, it is in my opinion best to measure
performance as the actual (real-world) time it takes to perform a task
with the emulator, e.g.:

<ul>
  <li>"How long does it take to install NetBSD onto a disk image?"
  <li>"How long does it take to compile XYZ inside NetBSD
        in the emulator?".
</ul>

<p>So, how fast is it? :-)&nbsp;&nbsp;&nbsp;Answer: it varies.


<p><br>
<a name="net"></a>
<h3>Networking</h3>

<font color="#ff0000">NOTE/TODO: This section is very old and a bit
out of date.</font>

<p>Running an entire operating system under emulation is very interesting
in itself, but for several reasons, running a modern OS without access to
TCP/IP networking is a bit akward. Hence, I feel the need to implement
TCP/IP (networking) support in the emulator.

<p>
As far as I have understood it, there seems to be two different ways to go:

<ol>
  <li>Forward ethernet packets from the emulated ethernet controller to
        the host machine's ethernet controller, and capture incoming 
        packets on the host's controller, giving them back to the
        emulated OS. Characteristics are:
        <ul>
          <li>Requires <i>direct</i> access to the host's NIC, which
                means on most platforms that the emulator cannot be
                run as a normal user!
          <li>Reduced portability, as not every host operating system
                uses the same programming interface for dealing with
                hardware ethernet controllers directly.
          <li>When run on a switched network, it might be problematic to
                connect from the emulated OS to the OS running on the
                host, as packets sent out on the host's NIC are not
                received by itself. (?)
          <li>All specific networking protocols will be handled by the
                physical network.
        </ul>
  <p>
  or
  <p>
  <li>Whenever the emulated ethernet controller wishes to send a packet,
        the emulator looks at the packet and creates a response. Packets
        that can have an immediate response never go outside the emulator,
        other packet types have to be converted into suitable other
        connection types (UDP, TCP, etc). Characteristics:
        <ul>
          <li>Each packet type sent out on the emulated NIC must be handled.
                This means that I have to do a lot of coding.
                (I like this, because it gives me an opportunity to
                learn about networking protocols.)
          <li>By not relying on access to the host's NIC directly,
                portability is maintained. (It would be sad if the networking
                portion of a portable emulator isn't as portable as the
                rest of the emulator.)
          <li>The emulator can be run as a normal user process, does
                not require root privilegies.
          <li>Connecting from the emulated OS to the host's OS should
                not be problematic.
          <li>The emulated OS will experience the network just as a single
                machine behind a NAT gateway/firewall would. The emulated
                OS is thus automatically protected from the outside world.
        </ul>
</ol>

<p>
Some emulators/simulators use the first approach, while others use the 
second. I think that SIMH and QEMU are examples of emulators using the 
first and second approach, respectively.

<p>
Since I have choosen the second kind of implementation, I have to write 
support explicitly for any kind of network protocol that should be
supported. As of 2004-07-09, the following has been implemented and seems 
to work under at least NetBSD/pmax and OpenBSD/pmax under DECstation 5000/200
emulation (-E dec -e 3max):

<p>
<ul>
  <li>ARP requests sent out from the emulated NIC are interpreted,
        and converted to ARP responses. (This is used by the emulated OS
        to find out the MAC address of the gateway.)
  <li>ICMP echo requests (that is the kind of packet produced by the
        <b><tt>ping</tt></b> program) are interpreted and converted to ICMP echo
        replies, <i>regardless of the IP address</i>. This means that
        running ping from within the emulated OS will <i>always</i>
        receive a response. The ping packets never leave the emulated
        environment.
  <li>UDP packets are interpreted and passed along to the outside world.
        If the emulator receives an UDP packet from the outside world, it
        is converted into an UDP packet for the emulated OS. (This is not
        implemented very well yet, but seems to be enough for nameserver
        lookups, tftp file transfers, and NFS mounts using UDP.)
  <li>TCP packets are interpreted one at a time, similar to how UDP 
        packets are handled (but more state is kept for each connection).
        <font color="#ff0000">NOTE: Much of the TCP handling code is very
        ugly and hardcoded.</font>
<!--
  <li>RARP is not implemented yet. (I haven't needed it so far.)
-->
</ul>

<p>
The gateway machine, which is the only "other" machine that the emulated 
OS sees on its emulated network, works as a NAT-style firewall/gateway. It 
usually has a fixed IPv4 address of <tt>10.0.0.254</tt>. An OS running in 
the emulator would usually have an address of the form <tt>10.x.x.x</tt>;
a typical choice would be <tt>10.0.0.1</tt>.

<p>
Inside emulated NetBSD/pmax or OpenBSD/pmax, running the following 
commands should configure the emulated NIC:
<pre>
        # <b>ifconfig le0 10.0.0.1</b>
        # <b>route add default 10.0.0.254</b>
        add net default: gateway 10.0.0.254
</pre>

<p>
If you want nameserver lookups to work, you need a valid /etc/resolv.conf 
as well:
<pre>
        # <b>echo nameserver 129.16.1.3 > /etc/resolv.conf</b>
</pre>
(But replace <tt>129.16.1.3</tt> with the actual real-world IP address of 
your nearest nameserver.)

<p>
Now, host lookups should work:
<pre>
        # <b>host -a www.netbsd.org</b>
        Trying null domain
        rcode = 0 (Success), ancount=2
        The following answer is not authoritative:
        The following answer is not verified as authentic by the server:
        www.netbsd.org  86400 IN        AAAA    2001:4f8:4:7:290:27ff:feab:19a7
        www.netbsd.org  86400 IN        A       204.152.184.116
        For authoritative answers, see:
        netbsd.org      83627 IN        NS      uucp-gw-2.pa.dec.com
        netbsd.org      83627 IN        NS      ns.netbsd.org
        netbsd.org      83627 IN        NS      adns1.berkeley.edu
        netbsd.org      83627 IN        NS      adns2.berkeley.edu
        netbsd.org      83627 IN        NS      uucp-gw-1.pa.dec.com
        Additional information:
        ns.netbsd.org   83627 IN        A       204.152.184.164
        uucp-gw-1.pa.dec.com    172799 IN       A       204.123.2.18
        uucp-gw-2.pa.dec.com    172799 IN       A       204.123.2.19
</pre>

<p>
At this point, UDP and TCP should (mostly) work.

<p>
Here is an example of how to configure a server machine and an emulated 
client machine for sharing files via NFS:

<p>
(This is very useful if you want to share entire directory trees
between the emulated environment and another machine. These instruction
will work for FreeBSD, if you are running something else, use your
imagination to modify them.)

<p>
<ul>
  <li>On the server, add a line to your /etc/exports file, exporting
        the files you wish to use in the emulator:<pre>
        <b>/tftpboot -mapall=nobody -ro 123.11.22.33</b>
</pre>
        where 123.11.22.33 is the IP address of the machine running the
        emulator process, as seen from the outside world.
  <p>
  <li>Then start up the programs needed to serve NFS via UDP. Note the
        -n argument to mountd. This is needed to tell mountd to accept
        connections from unprivileged ports (because the emulator does
        not need to run as root).<pre>
        # <b>portmap</b>
        # <b>nfsd -u</b>       &lt;--- u for UDP
        # <b>mountd -n</b>
</pre>
  <li>In the guest OS in the emulator, once you have ethernet and IPv4
        configured so that you can use UDP, mounting the filesystem
        should now be possible:  (this example is for NetBSD/pmax
        or OpenBSD/pmax)<pre>
        # <b>mount -o ro,-r=1024,-w=1024,-U,-3 my.server.com:/tftpboot /mnt</b>
    or
        # <b>mount my.server.com:/tftpboot /mnt</b>
</pre>
        If you don't supply the read and write sizes, there is a risk
        that the default values are too large. The emulator currently
        does not handle fragmentation/defragmentation of <i>outgoing</i>
        packets, so going above the ethernet frame size (1518) is a very
        bad idea. Incoming packets (reading from nfs) should work, though,
        for example during an NFS install.
</ul>

The example above uses read-only mounts. That is enough for things like
letting NetBSD/pmax or OpenBSD/pmax install via NFS, without the need for
a CDROM ISO image. You can use a read-write mount if you wish to share
files in both directions, but then you should be aware of the 
fragmentation issue mentioned above.


<p><br>
<a name="devices"></a>
<h3>Emulation of hardware devices</h3>

Each file called <tt>dev_*.c</tt> in the <tt>src/device/</tt> directory is
responsible for one hardware device. These are used from
<tt>src/machines/machine_*.c</tt>, when initializing which hardware a particular
machine model will be using, or when adding devices to a machine using the
<tt>device()</tt> command in configuration files.

<p>(I'll be using the name "<tt>foo</tt>" as the name of the device in all
these examples.  This is pseudo code, it might need some modification to
actually compile and run.)

<p>Each device should have the following:

<p>
<ul>
  <li>A <tt>devinit</tt> function in <tt>src/devices/dev_foo.c</tt>. It
        would typically look something like this:
<pre>
        DEVINIT(foo)
        {
                struct foo_data *d = malloc(sizeof(struct foo_data));

                if (d == NULL) {
                        fprintf(stderr, "out of memory\n");
                        exit(1);
                }
                memset(d, 0, sizeof(struct foo_data));

                /*
                 *  Set up stuff here, for example fill d with useful
                 *  data. devinit contains settings like address, irq_nr,
                 *  and other things.
                 *
                 *  ...
                 */
        
                memory_device_register(devinit->machine->memory, devinit->name,
                    devinit->addr, DEV_FOO_LENGTH,
                    dev_foo_access, (void *)d, DM_DEFAULT, NULL);
        
                /*  This should only be here if the device
                    has a tick function:  */
                machine_add_tickfunction(machine, dev_foo_tick, d,
                    FOO_TICKSHIFT);

                /*  Return 1 if the device was successfully added.  */
                return 1;       
        }       
</pre><br>

        <p><tt>DEVINIT(foo)</tt> is defined as <tt>int devinit_foo(struct devinit *devinit)</tt>,
        and the <tt>devinit</tt> argument contains everything that the device driver's
        initialization function needs.

  <p>
  <li>At the top of <tt>dev_foo.c</tt>, the <tt>foo_data</tt> struct
        should be defined.
<pre>
        struct foo_data {
                int     irq_nr;
                /*  ...  */
        }
</pre><br>
        (There is an exception to this rule; ugly hacks which allow
        code in <tt>src/machine.c</tt> to use some structures makes it
        necessary to place the <tt>struct foo_data</tt> in
        <tt>src/include/devices.h</tt> instead of in <tt>dev_foo.c</tt>
        itself. This is useful for example for interrupt controllers.)
  <p>
  <li>If <tt>foo</tt> has a tick function (that is, something that needs to be
        run at regular intervals) then <tt>FOO_TICKSHIFT</tt> and a tick 
        function need to be defined as well:
<pre>
        #define FOO_TICKSHIFT           14

        void dev_foo_tick(struct cpu *cpu, void *extra)
        {
                struct foo_data *d = (struct foo_data *) extra;

                if (.....)
                        cpu_interrupt(cpu, d->irq_nr);
                else
                        cpu_interrupt_ack(cpu, d->irq_nr);
        }
</pre><br>

  <li>Does this device belong to a standard bus?
        <ul>
          <li>If this device should be detectable as a PCI device, then
                glue code should be added to
                <tt>src/devices/bus_pci.c</tt>.
          <li>If this is a legacy ISA device which should be usable by
                any machine which has an ISA bus, then the device should
                be added to <tt>src/devices/bus_isa.c</tt>.
        </ul>
  <p>
  <li>And last but not least, the device should have an access function.
        The access function is called whenever there is a load or store
        to an address which is in the device' memory mapped region. To
        simplify things a little, a macro <tt>DEVICE_ACCESS(x)</tt>
        is expanded into<pre>
        int dev_x_access(struct cpu *cpu, struct memory *mem,
            uint64_t relative_addr, unsigned char *data, size_t len,
            int writeflag, void *extra)
</pre>  The access function can look like this:
<pre>
        DEVICE_ACCESS(foo)
        {
                struct foo_data *d = extra;
                uint64_t idata = 0, odata = 0;

                idata = memory_readmax64(cpu, data, len);
                switch (relative_addr) {
                /* .... */
                }

                if (writeflag == MEM_READ)
                        memory_writemax64(cpu, data, len, odata);

                /*  Perhaps interrupts need to be asserted or
                    deasserted:  */
                dev_foo_tick(cpu, extra);

                /*  Return successfully.  */
                return 1;
        }
</pre><br>
</ul>

<p>
The return value of the access function has until 2004-07-02 been a 
true/false value; 1 for success, or 0 for device access failure. A device 
access failure (on MIPS) will result in a DBE exception.

<p>
Some devices are converted to support arbitrary memory latency
values. The return value is the number of cycles that the read or 
write access took. A value of 1 means one cycle, a value of 10 means 10 
cycles. Negative values are used for device access failures, and the 
absolute value of the value is then the number of cycles; a value of -5 
means that the access failed, and took 5 cycles.

<p>
To be compatible with pre-20040702 devices, a return value of 0 is treated 
by the caller (in <tt>src/memory_rw.c</tt>) as a value of -1.


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1	<html><head><title>Gavare's eXperimental Emulator:   Technical details</title>
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7	<b>Gavare's eXperimental Emulator:</b></font><br>
8	<font color="#000000" size="6"><b>Technical details</b>
9	</font></td></tr></table></td></tr></table><p>
10
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41
42
43
44	<a href="./">Back to the index</a>
45
46	<p><br>
47	<h2>Technical details</h2>
48
49	<p>This page describes some of the internals of GXemul.
50
51	<p>
52	<ul>
53	<li><a href="#speed">Speed and emulation modes</a>
54	<li><a href="#net">Networking</a>
55	<li><a href="#devices">Emulation of hardware devices</a>
56	</ul>
57
58
59
60
61
62
63	<p><br>
64	<a name="speed"></a>
65	<h3>Speed and emulation modes</h3>
66
67	So, how fast is GXemul? There is no short answer to this. There is
68	especially no answer to the question <b>What is the slowdown factor?</b>,
69	because the host architecture and emulated architecture can usually not be
70	compared just like that.
71
72	<p>Performance depends on several factors, including (but not limited to)
73	host architecture, target architecture, host clock speed, which compiler
74	and compiler flags were used to build the emulator, what the workload is,
75	what additional runtime flags are given to the emulator, and so on.
76
77	<p>Devices are generally not timing-accurate: for example, if an emulated
78	operating system tries to read a block from disk, from its point of view
79	the read was instantaneous (no waiting). So 1 MIPS in an emulated OS might
80	have taken more than one million instructions on a real machine.
81
82	<p>Also, if the emulator says it has executed 1 million instructions, and
83	the CPU family in question was capable of scalar execution (i.e. one cycle
84	per instruction), it might still have taken more than 1 million cycles on
85	a real machine because of cache misses and similar micro-architectural
86	penalties that are not simulated by GXemul.
87
88	<p>Because of these issues, it is in my opinion best to measure
89	performance as the actual (real-world) time it takes to perform a task
90	with the emulator, e.g.:
91
92	<ul>
93	<li>"How long does it take to install NetBSD onto a disk image?"
94	<li>"How long does it take to compile XYZ inside NetBSD
95	in the emulator?".
96	</ul>
97
98	<p>So, how fast is it? :-)   Answer: it varies.
99
100
101
102
103
104
105
106	<p><br>
107	<a name="net"></a>
108	<h3>Networking</h3>
109
110	<font color="#ff0000">NOTE/TODO: This section is very old and a bit
111	out of date.</font>
112
113	<p>Running an entire operating system under emulation is very interesting
114	in itself, but for several reasons, running a modern OS without access to
115	TCP/IP networking is a bit akward. Hence, I feel the need to implement
116	TCP/IP (networking) support in the emulator.
117
118	<p>
119	As far as I have understood it, there seems to be two different ways to go:
120
121	<ol>
122	<li>Forward ethernet packets from the emulated ethernet controller to
123	the host machine's ethernet controller, and capture incoming
124	packets on the host's controller, giving them back to the
125	emulated OS. Characteristics are:
126	<ul>
127	<li>Requires <i>direct</i> access to the host's NIC, which
128	means on most platforms that the emulator cannot be
129	run as a normal user!
130	<li>Reduced portability, as not every host operating system
131	uses the same programming interface for dealing with
132	hardware ethernet controllers directly.
133	<li>When run on a switched network, it might be problematic to
134	connect from the emulated OS to the OS running on the
135	host, as packets sent out on the host's NIC are not
136	received by itself. (?)
137	<li>All specific networking protocols will be handled by the
138	physical network.
139	</ul>
140	<p>
141	or
142	<p>
143	<li>Whenever the emulated ethernet controller wishes to send a packet,
144	the emulator looks at the packet and creates a response. Packets
145	that can have an immediate response never go outside the emulator,
146	other packet types have to be converted into suitable other
147	connection types (UDP, TCP, etc). Characteristics:
148	<ul>
149	<li>Each packet type sent out on the emulated NIC must be handled.
150	This means that I have to do a lot of coding.
151	(I like this, because it gives me an opportunity to
152	learn about networking protocols.)
153	<li>By not relying on access to the host's NIC directly,
154	portability is maintained. (It would be sad if the networking
155	portion of a portable emulator isn't as portable as the
156	rest of the emulator.)
157	<li>The emulator can be run as a normal user process, does
158	not require root privilegies.
159	<li>Connecting from the emulated OS to the host's OS should
160	not be problematic.
161	<li>The emulated OS will experience the network just as a single
162	machine behind a NAT gateway/firewall would. The emulated
163	OS is thus automatically protected from the outside world.
164	</ul>
165	</ol>
166
167	<p>
168	Some emulators/simulators use the first approach, while others use the
169	second. I think that SIMH and QEMU are examples of emulators using the
170	first and second approach, respectively.
171
172	<p>
173	Since I have choosen the second kind of implementation, I have to write
174	support explicitly for any kind of network protocol that should be
175	supported. As of 2004-07-09, the following has been implemented and seems
176	to work under at least NetBSD/pmax and OpenBSD/pmax under DECstation 5000/200
177	emulation (-E dec -e 3max):
178
179	<p>
180	<ul>
181	<li>ARP requests sent out from the emulated NIC are interpreted,
182	and converted to ARP responses. (This is used by the emulated OS
183	to find out the MAC address of the gateway.)
184	<li>ICMP echo requests (that is the kind of packet produced by the
185	<b><tt>ping</tt></b> program) are interpreted and converted to ICMP echo
186	replies, <i>regardless of the IP address</i>. This means that
187	running ping from within the emulated OS will <i>always</i>
188	receive a response. The ping packets never leave the emulated
189	environment.
190	<li>UDP packets are interpreted and passed along to the outside world.
191	If the emulator receives an UDP packet from the outside world, it
192	is converted into an UDP packet for the emulated OS. (This is not
193	implemented very well yet, but seems to be enough for nameserver
194	lookups, tftp file transfers, and NFS mounts using UDP.)
195	<li>TCP packets are interpreted one at a time, similar to how UDP
196	packets are handled (but more state is kept for each connection).
197	<font color="#ff0000">NOTE: Much of the TCP handling code is very
198	ugly and hardcoded.</font>
199	<!--
200	<li>RARP is not implemented yet. (I haven't needed it so far.)
201	-->
202	</ul>
203
204	<p>
205	The gateway machine, which is the only "other" machine that the emulated
206	OS sees on its emulated network, works as a NAT-style firewall/gateway. It
207	usually has a fixed IPv4 address of <tt>10.0.0.254</tt>. An OS running in
208	the emulator would usually have an address of the form <tt>10.x.x.x</tt>;
209	a typical choice would be <tt>10.0.0.1</tt>.
210
211	<p>
212	Inside emulated NetBSD/pmax or OpenBSD/pmax, running the following
213	commands should configure the emulated NIC:
214	<pre>
215	# <b>ifconfig le0 10.0.0.1</b>
216	# <b>route add default 10.0.0.254</b>
217	add net default: gateway 10.0.0.254
218	</pre>
219
220	<p>
221	If you want nameserver lookups to work, you need a valid /etc/resolv.conf
222	as well:
223	<pre>
224	# <b>echo nameserver 129.16.1.3 > /etc/resolv.conf</b>
225	</pre>
226	(But replace <tt>129.16.1.3</tt> with the actual real-world IP address of
227	your nearest nameserver.)
228
229	<p>
230	Now, host lookups should work:
231	<pre>
232	# <b>host -a www.netbsd.org</b>
233	Trying null domain
234	rcode = 0 (Success), ancount=2
235	The following answer is not authoritative:
236	The following answer is not verified as authentic by the server:
237	www.netbsd.org 86400 IN AAAA 2001:4f8:4:7:290:27ff:feab:19a7
238	www.netbsd.org 86400 IN A 204.152.184.116
239	For authoritative answers, see:
240	netbsd.org 83627 IN NS uucp-gw-2.pa.dec.com
241	netbsd.org 83627 IN NS ns.netbsd.org
242	netbsd.org 83627 IN NS adns1.berkeley.edu
243	netbsd.org 83627 IN NS adns2.berkeley.edu
244	netbsd.org 83627 IN NS uucp-gw-1.pa.dec.com
245	Additional information:
246	ns.netbsd.org 83627 IN A 204.152.184.164
247	uucp-gw-1.pa.dec.com 172799 IN A 204.123.2.18
248	uucp-gw-2.pa.dec.com 172799 IN A 204.123.2.19
249	</pre>
250
251	<p>
252	At this point, UDP and TCP should (mostly) work.
253
254	<p>
255	Here is an example of how to configure a server machine and an emulated
256	client machine for sharing files via NFS:
257
258	<p>
259	(This is very useful if you want to share entire directory trees
260	between the emulated environment and another machine. These instruction
261	will work for FreeBSD, if you are running something else, use your
262	imagination to modify them.)
263
264	<p>
265	<ul>
266	<li>On the server, add a line to your /etc/exports file, exporting
267	the files you wish to use in the emulator:<pre>
268	<b>/tftpboot -mapall=nobody -ro 123.11.22.33</b>
269	</pre>
270	where 123.11.22.33 is the IP address of the machine running the
271	emulator process, as seen from the outside world.
272	<p>
273	<li>Then start up the programs needed to serve NFS via UDP. Note the
274	-n argument to mountd. This is needed to tell mountd to accept
275	connections from unprivileged ports (because the emulator does
276	not need to run as root).<pre>
277	# <b>portmap</b>
278	# <b>nfsd -u</b> <--- u for UDP
279	# <b>mountd -n</b>
280	</pre>
281	<li>In the guest OS in the emulator, once you have ethernet and IPv4
282	configured so that you can use UDP, mounting the filesystem
283	should now be possible: (this example is for NetBSD/pmax
284	or OpenBSD/pmax)<pre>
285	# <b>mount -o ro,-r=1024,-w=1024,-U,-3 my.server.com:/tftpboot /mnt</b>
286	or
287	# <b>mount my.server.com:/tftpboot /mnt</b>
288	</pre>
289	If you don't supply the read and write sizes, there is a risk
290	that the default values are too large. The emulator currently
291	does not handle fragmentation/defragmentation of <i>outgoing</i>
292	packets, so going above the ethernet frame size (1518) is a very
293	bad idea. Incoming packets (reading from nfs) should work, though,
294	for example during an NFS install.
295	</ul>
296
297	The example above uses read-only mounts. That is enough for things like
298	letting NetBSD/pmax or OpenBSD/pmax install via NFS, without the need for
299	a CDROM ISO image. You can use a read-write mount if you wish to share
300	files in both directions, but then you should be aware of the
301	fragmentation issue mentioned above.
302
303
304
305
306
307
308
309	<p><br>
310	<a name="devices"></a>
311	<h3>Emulation of hardware devices</h3>
312
313	Each file called <tt>dev_*.c</tt> in the <tt>src/device/</tt> directory is
314	responsible for one hardware device. These are used from
315	<tt>src/machines/machine_*.c</tt>, when initializing which hardware a particular
316	machine model will be using, or when adding devices to a machine using the
317	<tt>device()</tt> command in configuration files.
318
319	<p>(I'll be using the name "<tt>foo</tt>" as the name of the device in all
320	these examples. This is pseudo code, it might need some modification to
321	actually compile and run.)
322
323	<p>Each device should have the following:
324
325	<p>
326	<ul>
327	<li>A <tt>devinit</tt> function in <tt>src/devices/dev_foo.c</tt>. It
328	would typically look something like this:
329	<pre>
330	DEVINIT(foo)
331	{
332	struct foo_data *d = malloc(sizeof(struct foo_data));
333
334	if (d == NULL) {
335	fprintf(stderr, "out of memory\n");
336	exit(1);
337	}
338	memset(d, 0, sizeof(struct foo_data));
339
340	/*
341	* Set up stuff here, for example fill d with useful
342	* data. devinit contains settings like address, irq_nr,
343	* and other things.
344	*
345	* ...
346	*/
347
348	memory_device_register(devinit->machine->memory, devinit->name,
349	devinit->addr, DEV_FOO_LENGTH,
350	dev_foo_access, (void *)d, DM_DEFAULT, NULL);
351
352	/* This should only be here if the device
353	has a tick function: */
354	machine_add_tickfunction(machine, dev_foo_tick, d,
355	FOO_TICKSHIFT);
356
357	/* Return 1 if the device was successfully added. */
358	return 1;
359	}
360	</pre><br>
361
362	<p><tt>DEVINIT(foo)</tt> is defined as <tt>int devinit_foo(struct devinit *devinit)</tt>,
363	and the <tt>devinit</tt> argument contains everything that the device driver's
364	initialization function needs.
365
366	<p>
367	<li>At the top of <tt>dev_foo.c</tt>, the <tt>foo_data</tt> struct
368	should be defined.
369	<pre>
370	struct foo_data {
371	int irq_nr;
372	/* ... */
373	}
374	</pre><br>
375	(There is an exception to this rule; ugly hacks which allow
376	code in <tt>src/machine.c</tt> to use some structures makes it
377	necessary to place the <tt>struct foo_data</tt> in
378	<tt>src/include/devices.h</tt> instead of in <tt>dev_foo.c</tt>
379	itself. This is useful for example for interrupt controllers.)
380	<p>
381	<li>If <tt>foo</tt> has a tick function (that is, something that needs to be
382	run at regular intervals) then <tt>FOO_TICKSHIFT</tt> and a tick
383	function need to be defined as well:
384	<pre>
385	#define FOO_TICKSHIFT 14
386
387	void dev_foo_tick(struct cpu cpu, void extra)
388	{
389	struct foo_data d = (struct foo_data ) extra;
390
391	if (.....)
392	cpu_interrupt(cpu, d->irq_nr);
393	else
394	cpu_interrupt_ack(cpu, d->irq_nr);
395	}
396	</pre><br>
397
398	<li>Does this device belong to a standard bus?
399	<ul>
400	<li>If this device should be detectable as a PCI device, then
401	glue code should be added to
402	<tt>src/devices/bus_pci.c</tt>.
403	<li>If this is a legacy ISA device which should be usable by
404	any machine which has an ISA bus, then the device should
405	be added to <tt>src/devices/bus_isa.c</tt>.
406	</ul>
407	<p>
408	<li>And last but not least, the device should have an access function.
409	The access function is called whenever there is a load or store
410	to an address which is in the device' memory mapped region. To
411	simplify things a little, a macro <tt>DEVICE_ACCESS(x)</tt>
412	is expanded into<pre>
413	int dev_x_access(struct cpu cpu, struct memory mem,
414	uint64_t relative_addr, unsigned char *data, size_t len,
415	int writeflag, void *extra)
416	</pre> The access function can look like this:
417	<pre>
418	DEVICE_ACCESS(foo)
419	{
420	struct foo_data *d = extra;
421	uint64_t idata = 0, odata = 0;
422
423	idata = memory_readmax64(cpu, data, len);
424	switch (relative_addr) {
425	/* .... */
426	}
427
428	if (writeflag == MEM_READ)
429	memory_writemax64(cpu, data, len, odata);
430
431	/* Perhaps interrupts need to be asserted or
432	deasserted: */
433	dev_foo_tick(cpu, extra);
434
435	/* Return successfully. */
436	return 1;
437	}
438	</pre><br>
439	</ul>
440
441	<p>
442	The return value of the access function has until 2004-07-02 been a
443	true/false value; 1 for success, or 0 for device access failure. A device
444	access failure (on MIPS) will result in a DBE exception.
445
446	<p>
447	Some devices are converted to support arbitrary memory latency
448	values. The return value is the number of cycles that the read or
449	write access took. A value of 1 means one cycle, a value of 10 means 10
450	cycles. Negative values are used for device access failures, and the
451	absolute value of the value is then the number of cycles; a value of -5
452	means that the access failed, and took 5 cycles.
453
454	<p>
455	To be compatible with pre-20040702 devices, a return value of 0 is treated
456	by the caller (in <tt>src/memory_rw.c</tt>) as a value of -1.
457
458
459
460
461
462
463	</body>
464	</html>