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-revision 4 by dpavlin,
Mon Oct  8 16:18:00 2007 UTC
+revision 10 by dpavlin,
Mon Oct  8 16:18:27 2007 UTC
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- <html>
+ <html><head><title>GXemul documentation: Technical details</title>
- <head><title>GXemul documentation: Technical details</title>
+ <meta name="robots" content="noarchive,nofollow,noindex"></head>
- </head>
  <body bgcolor="#f8f8f8" text="#000000" link="#4040f0" vlink="#404040" alink="#ff0000">
  <table border=0 width=100% bgcolor="#d0d0d0"><tr>
  <td width=100% align=center valign=center><table border=0 width=100%><tr>
-Line 8
+Line 7
  <b>GXemul documentation:</b></font>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  <font color="#000000" size="6"><b>Technical details</b>
  </font></td></tr></table></td></tr></table><p>
- <!-- The first 10 lines are cut away by the homepage updating script.  -->
  <!--
- $Id: technical.html,v 1.49 2005/04/16 00:29:45 debug Exp $
+ $Id: technical.html,v 1.53 2005/06/27 17:31:50 debug Exp $
  Copyright (C) 2004-2005  Anders Gavare.  All rights reserved.
-Line 48 
 SUCH DAMAGE.
+Line 45 
 SUCH DAMAGE.
  <p><br>
  <h2>Technical details</h2>
- <p>
+ <p>This page describes some of the internals of GXemul.
- This page describes some of the internals of GXemul.
  <p>
  <ul>
-   <li><a href="#overview">Overview</a>
+   <li><a href="#speed">Speed and emulation modes</a>
-   <li><a href="#speed">Speed</a>
    <li><a href="#net">Networking</a>
    <li><a href="#devices">Emulation of hardware devices</a>
    <li><a href="#regtest">Regression tests</a>
-Line 63 
 This page describes some of the internal
+Line 58 
 This page describes some of the internal
- <p><br>
- <a name="overview"></a>
- <h3>Overview</h3>
- In simple terms, GXemul is just a simple fetch-and-execute
- loop; an instruction is fetched from memory, and executed.
- <p>
- In reality, a lot of things need to be handled. Before each instruction is
- executed, the emulator checks to see if any interrupts are asserted which
- are not masked away. If so, then an INT exception is generated. Exceptions
- cause the program counter to be set to a specific value, and some of the
- system coprocessor's registers to be set to values signifying what kind of
- exception it was (an interrupt exception in this case).
- <p>
- Reading instructions from memory is done through a TLB, a translation
- lookaside buffer. The TLB on MIPS is software controlled, which means that
- the program running inside the emulator (for example an operating system
- kernel) has to take care of manually updating the TLB. Some memory
- addresses are translated into physical addresses directly, some are
- translated into valid physical addresses via the TLB, and some memory
- references are not valid. Invalid memory references cause exceptions.
- <p>
- After an instruction has been read from memory, the emulator checks which
- opcode it contains and executes the instruction. Executing an instruction
- usually involves reading some register and writing some register, or perhaps a
- load from memory (or a store to memory). The program counter is increased
- for every instruction.
- <p>
- Some memory references point to physical addresses which are not in the
- normal RAM address space. They may point to hardware devices. If that is
- the case, then loads and stores are converted into calls to a device
- access function. The device access function is then responsible for
- handling these reads and writes.  For example, a graphical framebuffer
- device may put a pixel on the screen when a value is written to it, or a
- serial controller device may output a character to stdout when written to.
  <p><br>
  <a name="speed"></a>
- <h3>Speed</h3>
+ <h3>Speed and emulation modes</h3>
- There are two modes in which the emulator can run, <b>a</b>) a straight forward
- loop which fetches one instruction from emulated RAM and executes it
- (described in the previous section), and <b>b</b>)
- using dynamic binary translation.
- <p>
- Mode <b>a</b> is very slow. On a 2.8 GHz Intel Xeon host the resulting
- emulated machine is rougly equal to a 7 MHz R3000 (or a 3.5 MHz R4000).
- The actual performance varies a lot, maybe between 5 and 10 million
- instructions per second, depending on workload.
- <p>
+ So, how fast is GXemul? There is no good answer to this. There is
- Mode <b>b</b> ("bintrans") is still to be considered experimental, but
+ especially no answer to the question <b>What is the slowdown factor?</b>,
- gives higher performance than mode <b>a</b>. It translates MIPS machine
+ because the host architecture and emulated architecture can usually not be
- code into machine code that can be executed on the host machine
+ compared just like that.
- on-the-fly. The translation itself obviously takes some time, but this is
- usually made up for by the fact that the translated code chunks are
+ <p>Performance depends on several factors, including (but not limited to)
- executed multiple times.
+ host architecture, host clock speed, which compiler and compiler flags
- To run the emulator with binary translation enabled, just add <b>-b</b>
+ were used to build the emulator, what the workload is, and so on. For
- to the command line.
+ 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. Typical examples would be "How long does it take to
+ install NetBSD?", or "How long does it take to compile XYZ inside NetBSD
+ in the emulator?".
+ <p>The emulation technique used varies depending on which processor type
+ is being emulated. (One of my main goals with GXemul is to experiment with
+ different kinds of emulation, so these might change in the future.)
- <p>
+ <ul>
- Only small pieces of MIPS machine code are translated, usually the size of
+   <li><b>MIPS</b><br>
- a function, or less. There is no "intermediate representation" code, so
+         There are two emulation modes. The most important one is an
- all translations are done directly from MIPS to host machine code.
+         implementation of a <i>dynamic binary translator</i>.
+         (Compared to real binary translators, though, GXemul's bintrans
- <p>
+         subsystem is very simple and does not perform very well.)
- The default bintrans cache size is 16 MB, but you can change this by adding
+         This mode can be used on Alpha and i386 host. The other emulation
- -DDEFAULT_BINTRANS_SIZE_IN_MB=<i>xx</i> to your CFLAGS environment variable
+         mode is simple interpretation, where an instruction is read from
- before running the configure script, or by using the bintrans_size()
+         emulated memory, and interpreted one-at-a-time. (Slow, but it
- configuration file option when running the emulator.
+         works. It can be forcefully used by using the <tt>-B</tt> command
+         line option.)
+   <p>
+   <li><b>ARM</b><br>
+         This mode does not really work yet, but will use
+         dynamic translation, but not binary translation. Stay tuned. :-)
+   <p>
+   <li><b>URISC</b><br>
+         Simple interpretation, one instruction at a time. There is probably
+         no other way to emulate URISC, because it relies too heavily
+         on self-modifying code.
+   <p>
+   <li><b>POWER/PowerPC</b><br>
+         This emulation mode is very much unfinished, but still enabled by
+         default. So far it uses plain interpretation, where an instruction
+         is read from emulated memory, and interpreted one at a time.
+         Slow. Not very interesting.
+   <p>
+   <li><b>x86</b><br>
+         Although too unstable and non-working to be enabled by default,
+         there is some code for emulating x86 machines. It simply reads
+         one instruction at a time from emulated memory, and executes it.
+         This is as slow as it gets. Not very interesting.
+ </ul>
- <p>
- By default, an emulated OS running under DECstation emulation which listens to
- interrupts from the mc146818 clock will get interrupts that are close to the
- host's clock. That is, if the emulated OS says it wants 100 interrupts per
- second, it will get approximately 100 interrupts per real second.
- <p>
- There is however a -I option, which sets the number of emulated cycles per
- seconds to a fixed value. Let's say you wish to make the emulated OS think it
- is running on a 40 MHz DECstation, and not a 7 MHz one, then you can add
- -I 40000000 to the command line. This will not make the emulation faster, of
- course. It might even make it seem slower; for example, if NetBSD/pmax waits
-seconds for SCSI devices to settle during bootup, those 2 seconds will take
-*40000000 cycles (which will take more time than 2*7000000).
- <p>
- The -I option is also necessary if you want to run deterministic experiments,
- if a mc146818 device is present.
- <p>
- Some emulators make claims such as "x times slowdown," but in the case of
- GXemul, the host is often not a MIPS-based machine, and hence comparing
- one MIPS instruction to a host instruction doesn't work. Performance depends on
- a lot of factors, including (but not limited to) host architecture, host speed,
- which compiler and compiler flags were used to build GXemul, what the
- workload is, and so on. 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.  Because of this, imho it is best
- to measure performance as the actual (real-world) time it takes to perform
- a task with the emulator.
-Line 181 
 a task with the emulator.
+Line 137 
 a task with the emulator.
  <a name="net"></a>
  <h3>Networking</h3>
- Running an entire operating system under emulation is very interesting in
+ <font color="#ff0000">NOTE/TODO: This section is very old and a bit
- itself, but for several reasons, running a modern OS without access to
+ out of date.</font>
- TCP/IP networking is a bit akward. Hence, I feel the need to implement TCP/IP
- (networking) support in the emulator.
+ <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:
-Line 205 
 As far as I have understood it, there se
+Line 164 
 As far as I have understood it, there se
                  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
-Line 233 
 As far as I have understood it, there se
+Line 194 
 As far as I have understood it, there se
          </ul>
  </ol>
- Other emulators that I have heard of seem to use the first one, if they
+ <p>
- support networking.
+ 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
-Line 249 
 emulation (-E dec -e 3max):
+Line 212 
 emulation (-E dec -e 3max):
          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>ping</b> program) are interpreted and converted to ICMP echo
+         <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
-Line 263 
 emulation (-E dec -e 3max):
+Line 226 
 emulation (-E dec -e 3max):
          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
- has a fixed IPv4 address of 10.0.0.254. An OS running in the emulator
+ usually has a fixed IPv4 address of <tt>10.0.0.254</tt>. An OS running in
- can thus have any 10.x.x.x address; a typical choice would be 10.0.0.1.
+ 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 or OpenBSD, running the following commands should
+ Inside emulated NetBSD/pmax or OpenBSD/pmax, running the following
- configure the emulated NIC:
+ 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 129.16.1.3 with the actual real-world IP address of your
+ (But replace <tt>129.16.1.3</tt> with the actual real-world IP address of
- nearest nameserver.)
+ your nearest nameserver.)
  <p>
  Now, host lookups should work:
  <pre>
-Line 309 
 Now, host lookups should work:
+Line 278 
 Now, host lookups should work:
          uucp-gw-2.pa.dec.com    172799 IN       A       204.123.2.19
  </pre>
- To transfer files via UDP, you can use the tftp program.
+ <p>
+ At this point, UDP and TCP should (mostly) work.
- <pre>
-         # <b>tftp 12.34.56.78</b>
-         tftp> <b>get filename</b>
-         Received XXXXXX bytes in X.X seconds
-         tftp> <b>quit</b>
-         #
- </pre>
- or, to do it non-interactively (with ugly output):
- <pre>
-         # <b>echo get filename | tftp 12.34.56.78</b>
-         tftp> Received XXXXXX bytes in X.X seconds
-         tftp> #
- </pre>
- This, of course, requires that you have put the file <i>filename</i> in
+ <p>
- the root directory of the tftp server (12.34.56.78).
+ Here is an example of how to configure a server machine and an emulated
+ client machine for sharing files via NFS:
  <p>
- It is also possible to run NFS via UDP. This is very useful if you want to
+ (This is very useful if you want to share entire directory trees
- share entire directory trees between the emulated environment and another
+ between the emulated environment and another machine. These instruction
- machine. These instruction will work for FreeBSD, if you are running
+ will work for FreeBSD, if you are running something else, use your
- something else, use your imagination to modify them:
+ 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>
-Line 374 
 a CDROM ISO image. You can use a read-wr
+Line 330 
 a CDROM ISO image. You can use a read-wr
  files in both directions, but then you should be aware of the
  fragmentation issue mentioned above.
- <p>
+ <p>TODO: Write a section on how to connect multiple emulator instances.
- TCP is implemented to some extent, but should not be considered to be
+ (Using the <tt>local_port</tt> and <tt>add_remote</tt> configuration file
- stable yet. It is enough to let NetBSD/pmax and OpenBSD/pmax install via
+ commands.)
- ftp, though.
-Line 386 
 ftp, though.
+Line 343 
 ftp, though.
  <a name="devices"></a>
  <h3>Emulation of hardware devices</h3>
- Each file in the device/ directory is responsible for one hardware device.
+ Each file in the <tt>device/</tt> directory is responsible for one
- These are used from src/machine.c, when initializing which hardware a
+ hardware device. These are used from <tt>src/machine.c</tt>, when
- particular machine model will be using, or when adding devices to a
+ initializing which hardware a particular machine model will be using, or
- machine using the <b>device()</b> command in configuration files.
+ when adding devices to a machine using the <tt>device()</tt> command in
+ configuration files.
- <p>
+ <p><font color="#ff0000">NOTE: The device registry subsystem is currently
- <font color="#ff0000">NOTE: 2005-02-26: I'm currently rewriting the
+ in a state of flux, as it is being redesigned.</font>
- device registry subsystem.</font>
- <p>
+ <p>(I'll be using the name "<tt>foo</tt>" as the name of the device in all
- (I'll be using the name 'foo' as the name of the device in all these
+ these examples.  This is pseudo code, it might need some modification to
- examples.  This is pseudo code, it might need some modification to
  actually compile and run.)
- <p>
+ <p>Each device should have the following:
- Each device should have the following:
  <p>
  <ul>
-   <li>A devinit function in dev_foo.c. It would typically look
+   <li>A <tt>devinit</tt> function in <tt>src/devices/dev_foo.c</tt>. It
-         something like this:
+         would typically look something like this:
  <pre>
          /*
           *  devinit_foo():
-Line 443 
 Each device should have the following:
+Line 398 
 Each device should have the following:
          }
  </pre><br>
-   <li>At the top of dev_foo.c, the foo_data struct should be defined.
+   <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;
-Line 451 
 Each device should have the following:
+Line 407 
 Each device should have the following:
          }
  </pre><br>
-   <li>If foo has a tick function (that is, something that needs to be
+   <li>If <tt>foo</tt> has a tick function (that is, something that needs to be
-         run at regular intervals) then FOO_TICKSHIFT and a tick function
+         run at regular intervals) then <tt>FOO_TICKSHIFT</tt> and a tick
-         need to be defined as well:
+         function need to be defined as well:
  <pre>
          #define FOO_TICKSHIFT           10
-Line 498 
 Each device should have the following:
+Line 454 
 Each device should have the following:
  </ul>
  <p>
- The return value of the access function has until 20040702 been a
+ 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.
-Line 512 
 means that the access failed, and took 5
+Line 468 
 means that the access failed, and took 5
  <p>
  To be compatible with pre-20040702 devices, a return value of 0 is treated
- by the caller (in src/memory.c) as a value of -1.
+ by the caller (in <tt>src/memory_rw.c</tt>) as a value of -1.
-Line 524 
 by the caller (in src/memory.c) as a val
+Line 480 
 by the caller (in src/memory.c) as a val
  In order to make sure that the emulator actually works like it is supposed
  to, it must be tested. For this purpose, there is a simple regression
- testing framework in the <b>tests/</b> directory.
+ testing framework in the <tt>tests/</tt> directory.
  <p>
  <i>NOTE:  The regression testing framework is basically just a skeleton so far.
-Line 534 
 probably not too incorrect. This makes i
+Line 490 
 probably not too incorrect. This makes i
  regression tests.</i>
  <p>
- To run all the regression tests, type <b>make regtest</b>. Each assembly
+ To run all the regression tests, type <tt>make regtest</tt>. Each assembly
- language file matching the pattern <b>test_*.S</b> will be compiled and
+ language file matching the pattern <tt>test_*.S</tt> will be compiled and
  linked into a 64-bit MIPS ELF (using a gcc cross compiler), and run in the
  emulator. If everything goes well, you should see something like this:
-Line 578 
 emulator. If everything goes well, you s
+Line 534 
 emulator. If everything goes well, you s
  </pre>
  <p>
- Each test writes output to stdout, and there is a <b>test_*.good</b> for
+ Each test writes output to stdout, and there is a <tt>test_*.good</tt> for
- each <b>.S</b> file which contains the wanted output. If the actual output
+ each <tt>.S</tt> file which contains the wanted output. If the actual
- matches the <b>.good</b> file, then the test passes, otherwise it fails.
+ output matches the <tt>.good</tt> file, then the test passes, otherwise it
+ fails.
  <p>
- Read <b>tests/README</b> for more information.
+ Read <tt>tests/README</tt> for more information.

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