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<b>GXemul documentation:</b></font> |
<b>Gavare's eXperimental Emulator: </b></font> |
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<font color="#000000" size="6"><b>Technical details</b> |
<font color="#000000" size="6"><b>Technical details</b> |
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$Id: technical.html,v 1.50 2005/05/14 18:31:16 debug Exp $ |
$Id: technical.html,v 1.67 2005/11/24 12:32:10 debug Exp $ |
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Copyright (C) 2004-2005 Anders Gavare. All rights reserved. |
Copyright (C) 2004-2005 Anders Gavare. All rights reserved. |
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<a href="./">Back to the index</a> |
<a href="./">Back to the index</a> |
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<p><br> |
<p><br> |
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<h2>Technical details</h2> |
<h2>Technical details</h2> |
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<p> |
<p>This page describes some of the internals of GXemul. |
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This page describes some of the internals of GXemul. |
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<p> |
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<font color="#e00000"><b>NOTE: This page is probably not |
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very up-to-date by now.</b></font> |
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<p> |
<p> |
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<ul> |
<ul> |
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<li><a href="#overview">Overview</a> |
<li><a href="#speed">Speed and emulation modes</a> |
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<li><a href="#speed">Speed</a> |
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<li><a href="#net">Networking</a> |
<li><a href="#net">Networking</a> |
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<li><a href="#devices">Emulation of hardware devices</a> |
<li><a href="#devices">Emulation of hardware devices</a> |
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<li><a href="#regtest">Regression tests</a> |
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</ul> |
</ul> |
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<p><br> |
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<a name="overview"></a> |
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<h3>Overview</h3> |
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In simple terms, GXemul is just a simple fetch-and-execute |
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loop; an instruction is fetched from memory, and executed. |
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<p> |
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In reality, a lot of things need to be handled. Before each instruction is |
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executed, the emulator checks to see if any interrupts are asserted which |
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are not masked away. If so, then an INT exception is generated. Exceptions |
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cause the program counter to be set to a specific value, and some of the |
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system coprocessor's registers to be set to values signifying what kind of |
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exception it was (an interrupt exception in this case). |
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<p> |
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Reading instructions from memory is done through a TLB, a translation |
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lookaside buffer. The TLB on MIPS is software controlled, which means that |
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the program running inside the emulator (for example an operating system |
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kernel) has to take care of manually updating the TLB. Some memory |
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addresses are translated into physical addresses directly, some are |
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translated into valid physical addresses via the TLB, and some memory |
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references are not valid. Invalid memory references cause exceptions. |
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<p> |
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After an instruction has been read from memory, the emulator checks which |
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opcode it contains and executes the instruction. Executing an instruction |
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usually involves reading some register and writing some register, or perhaps a |
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load from memory (or a store to memory). The program counter is increased |
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for every instruction. |
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<p> |
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Some memory references point to physical addresses which are not in the |
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normal RAM address space. They may point to hardware devices. If that is |
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the case, then loads and stores are converted into calls to a device |
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access function. The device access function is then responsible for |
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handling these reads and writes. For example, a graphical framebuffer |
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device may put a pixel on the screen when a value is written to it, or a |
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serial controller device may output a character to stdout when written to. |
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<p><br> |
<p><br> |
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<a name="speed"></a> |
<a name="speed"></a> |
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<h3>Speed</h3> |
<h3>Speed and emulation modes</h3> |
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There are two modes in which the emulator can run, <b>a</b>) a straight forward |
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loop which fetches one instruction from emulated RAM and executes it |
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(described in the previous section), and <b>b</b>) |
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using dynamic binary translation. |
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<p> |
So, how fast is GXemul? There is no short answer to this. There is |
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Mode <b>a</b> is very slow. On a 2.8 GHz Intel Xeon host the resulting |
especially no answer to the question <b>What is the slowdown factor?</b>, |
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emulated machine is rougly equal to a 7 MHz R3000 (or a 3.5 MHz R4000). |
because the host architecture and emulated architecture can usually not be |
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The actual performance varies a lot, maybe between 5 and 10 million |
compared just like that. |
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instructions per second, depending on workload. |
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<p>Performance depends on several factors, including (but not limited to) |
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<p> |
host architecture, host clock speed, which compiler and compiler flags |
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Mode <b>b</b> ("bintrans") is still to be considered experimental, but |
were used to build the emulator, what the workload is, and so on. For |
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gives higher performance than mode <b>a</b>. It translates MIPS machine |
example, if an emulated operating system tries to read a block from disk, |
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code into machine code that can be executed on the host machine |
from its point of view the read was instantaneous (no waiting). So 1 MIPS |
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on-the-fly. The translation itself obviously takes some time, but this is |
in an emulated OS might have taken more than one million instructions on a |
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usually made up for by the fact that the translated code chunks are |
real machine. |
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executed multiple times. |
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To run the emulator with binary translation enabled, just add |
<p>Also, if the emulator says it has executed 1 million instructions, and |
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<tt><b>-b</b></tt> to the command line. |
the CPU family in question was capable of scalar execution (i.e. one cycle |
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per instruction), it might still have taken more than 1 million cycles on |
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a real machine because of cache misses and similar micro-architectural |
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penalties that are not simulated by GXemul. |
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<p>Because of these issues, it is in my opinion best to measure |
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performance as the actual (real-world) time it takes to perform a task |
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with the emulator. Typical examples would be "How long does it take to |
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install NetBSD?", or "How long does it take to compile XYZ inside NetBSD |
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in the emulator?". |
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<p>So, how fast is it? :-) Answer: it varies. |
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<p>The emulation technique used varies depending on which processor type |
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is being emulated. (One of my main goals with GXemul is to experiment with |
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different kinds of emulation, so these might change in the future.) |
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<p> |
<ul> |
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Only small pieces of MIPS machine code are translated, usually the size of |
<li><b>MIPS:</b><br> |
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a function, or less. There is no "intermediate representation" code, so |
There are two emulation modes. The most important one is an |
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all translations are done directly from MIPS to host machine code. |
implementation of a <i>dynamic binary translator</i>. |
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(Compared to real binary translators, though, GXemul's bintrans |
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<p> |
subsystem is very simple and does not perform very well.) |
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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 |
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<tt>-DDEFAULT_BINTRANS_SIZE_IN_MB=<i>xx</i></tt> to your CFLAGS environment |
mode is simple interpretation, where an instruction is read from |
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variable before running the configure script, or by using the |
emulated memory, and interpreted one-at-a-time. (Slow, but it |
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<tt>bintrans_size()</tt> configuration file option when running the emulator. |
works. It can be forcefully used by using the <tt>-B</tt> command |
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line option.) |
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<p> |
<p> |
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By default, an emulated OS running under DECstation emulation which listens to |
<li><b>All other modes:</b><br> |
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interrupts from the mc146818 clock will get interrupts that are close to the |
These use a kind of dynamic translation system. (This system does |
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host's clock. That is, if the emulated OS says it wants 100 interrupts per |
not use host-specific backends, so it is not "recompilation" or |
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second, it will get approximately 100 interrupts per real second. |
anything like that.) Speed is slower than real binary translation, |
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but faster than traditional interpretation, and with some tricks |
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<p> |
it will hopefully still give reasonable speed. The ARM and PowerPC |
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There is however a <tt><b>-I</b></tt> option, which sets the number of |
emulation modes uses this kind of translation. |
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emulated cycles per seconds to a fixed value. Let's say you wish to make the |
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emulated OS think it is running on a 40 MHz DECstation, and not a 7 MHz one, |
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then you can add <tt><b>-I 40000000</b></tt> to the command line. This will not |
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make the emulation faster, of course. It might even make it seem slower; for |
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example, if NetBSD/pmax waits 2 seconds for SCSI devices to settle during |
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bootup, those 2 seconds will take 2*40000000 cycles (which will take more |
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time than 2*7000000). |
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<p> |
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The <b><tt>-I</tt></b> option is also necessary if you want to run |
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deterministic experiments, if a mc146818 (or similar) device is present. |
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<p> |
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Some emulators make claims such as "x times slowdown," but in the case of |
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GXemul, the host is often not a MIPS-based machine, and hence comparing |
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one MIPS instruction to a host instruction doesn't work. Performance depends on |
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a lot of factors, including (but not limited to) host architecture, host speed, |
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which compiler and compiler flags were used to build GXemul, what the |
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workload is, and so on. For example, if an emulated operating system tries |
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to read a block from disk, from its point of view the read was instantaneous |
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(no waiting). So 1 MIPS in an emulated OS might have taken more than one |
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million instructions on a real machine. Because of this, imho it is best |
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to measure performance as the actual (real-world) time it takes to perform |
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a task with the emulator. |
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<a name="net"></a> |
<a name="net"></a> |
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<h3>Networking</h3> |
<h3>Networking</h3> |
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Running an entire operating system under emulation is very interesting in |
<font color="#ff0000">NOTE/TODO: This section is very old and a bit |
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itself, but for several reasons, running a modern OS without access to |
out of date.</font> |
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TCP/IP networking is a bit akward. Hence, I feel the need to implement TCP/IP |
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(networking) support in the emulator. |
<p>Running an entire operating system under emulation is very interesting |
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in itself, but for several reasons, running a modern OS without access to |
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TCP/IP networking is a bit akward. Hence, I feel the need to implement |
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TCP/IP (networking) support in the emulator. |
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<p> |
<p> |
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As far as I have understood it, there seems to be two different ways to go: |
As far as I have understood it, there seems to be two different ways to go: |
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<p><br> |
<p><br> |
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<a name="devices"></a> |
<a name="devices"></a> |
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<h3>Emulation of hardware devices</h3> |
<h3>Emulation of hardware devices</h3> |
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Each file in the device/ directory is responsible for one hardware device. |
Each file called <tt>dev_*.c</tt> in the <tt>src/device/</tt> directory is |
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These are used from src/machine.c, when initializing which hardware a |
responsible for one hardware device. These are used from |
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particular machine model will be using, or when adding devices to a |
<tt>src/machine.c</tt>, when initializing which hardware a particular |
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machine using the <b>device()</b> command in configuration files. |
machine model will be using, or when adding devices to a machine using the |
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<tt>device()</tt> command in configuration files. |
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<font color="#ff0000">NOTE: 2005-02-26: I'm currently rewriting the |
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device registry subsystem.</font> |
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<p> |
<p>(I'll be using the name "<tt>foo</tt>" as the name of the device in all |
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(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 |
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examples. This is pseudo code, it might need some modification to |
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actually compile and run.) |
actually compile and run.) |
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<p> |
<p>Each device should have the following: |
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Each device should have the following: |
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<p> |
<p> |
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<ul> |
<ul> |
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<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 |
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something like this: |
would typically look something like this: |
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<pre> |
<pre> |
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/* |
/* |
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* devinit_foo(): |
* devinit_foo(): |
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memory_device_register(devinit->machine->memory, devinit->name, |
memory_device_register(devinit->machine->memory, devinit->name, |
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devinit->addr, DEV_FOO_LENGTH, |
devinit->addr, DEV_FOO_LENGTH, |
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dev_foo_access, (void *)d, MEM_DEFAULT, NULL); |
dev_foo_access, (void *)d, DM_DEFAULT, NULL); |
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/* This should only be here if the device |
/* This should only be here if the device |
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has a tick function: */ |
has a tick function: */ |
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} |
} |
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</pre><br> |
</pre><br> |
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<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 |
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should be defined. |
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<pre> |
<pre> |
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struct foo_data { |
struct foo_data { |
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int irq_nr; |
int irq_nr; |
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/* ... */ |
/* ... */ |
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} |
} |
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</pre><br> |
</pre><br> |
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(There is an exception to this rule; ugly hacks which allow |
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<li>If foo has a tick function (that is, something that needs to be |
code in <tt>src/machine.c</tt> to use some structures makes it |
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run at regular intervals) then FOO_TICKSHIFT and a tick function |
necessary to place the <tt>struct foo_data</tt> in |
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need to be defined as well: |
<tt>src/include/devices.h</tt> instead of in <tt>dev_foo.c</tt> |
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itself. This is useful for example for interrupt controllers.) |
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<p> |
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<li>If <tt>foo</tt> has a tick function (that is, something that needs to be |
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run at regular intervals) then <tt>FOO_TICKSHIFT</tt> and a tick |
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function need to be defined as well: |
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<pre> |
<pre> |
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#define FOO_TICKSHIFT 10 |
#define FOO_TICKSHIFT 14 |
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void dev_foo_tick(struct cpu *cpu, void *extra) |
void dev_foo_tick(struct cpu *cpu, void *extra) |
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{ |
{ |
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} |
} |
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</pre><br> |
</pre><br> |
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<li>Does this device belong to a standard bus? |
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<ul> |
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<li>If this device should be detectable as a PCI device, then |
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glue code should be added to |
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<tt>src/devices/bus_pci.c</tt>. |
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<li>If this is a legacy ISA device which should be usable by |
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any machine which has an ISA bus, then the device should |
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be added to <tt>src/devices/bus_isa.c</tt>. |
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</ul> |
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<p> |
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<li>And last but not least, the device should have an access function. |
<li>And last but not least, the device should have an access function. |
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The access function is called whenever there is a load or store |
The access function is called whenever there is a load or store |
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to an address which is in the device' memory mapped region. |
to an address which is in the device' memory mapped region. |
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<p><br> |
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<a name="regtest"></a> |
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<h3>Regression tests</h3> |
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In order to make sure that the emulator actually works like it is supposed |
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to, it must be tested. For this purpose, there is a simple regression |
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testing framework in the <tt>tests/</tt> directory. |
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<p> |
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<i>NOTE: The regression testing framework is basically just a skeleton so far. |
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Regression tests are very good to have. However, the fact that complete |
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operating systems can run in the emulator indicate that the emulation is |
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probably not too incorrect. This makes it less of a priority to write |
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regression tests.</i> |
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<p> |
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To run all the regression tests, type <tt>make regtest</tt>. Each assembly |
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language file matching the pattern <tt>test_*.S</tt> will be compiled and |
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linked into a 64-bit MIPS ELF (using a gcc cross compiler), and run in the |
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emulator. If everything goes well, you should see something like this: |
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<pre> |
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$ make regtest |
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cd tests; make run_tests; cd .. |
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gcc33 -Wall -fomit-frame-pointer -fmove-all-movables -fpeephole -O2 |
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-mcpu=ev5 -I/usr/X11R6/include -lm -L/usr/X11R6/lib -lX11 do_tests.c |
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-o do_tests |
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do_tests.c: In function `main': |
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do_tests.c:173: warning: unused variable `s' |
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/var/tmp//ccFOupvD.o: In function `do_tests': |
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/var/tmp//ccFOupvD.o(.text+0x3a8): warning: tmpnam() possibly used |
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unsafely; consider using mkstemp() |
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mips64-unknown-elf-gcc -g -O3 -fno-builtin -fschedule-insns -mips64 |
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-mabi=64 test_common.c -c -o test_common.o |
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./do_tests "mips64-unknown-elf-gcc -g -O3 -fno-builtin -fschedule-insns |
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-mips64 -mabi=64" "mips64-unknown-elf-as -mabi=64 -mips64" |
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"mips64-unknown-elf-ld -Ttext 0xa800000000030000 -e main |
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--oformat=elf64-bigmips" "../gxemul" |
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Starting tests: |
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test_addu.S (-a) |
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test_addu.S (-a -b) |
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test_clo_clz.S (-a) |
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test_clo_clz.S (-a -b) |
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.. |
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test_unaligned.S (-a) |
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test_unaligned.S (-a -b) |
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Done. (12 tests done) |
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PASS: 12 |
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FAIL: 0 |
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---------------- |
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All tests OK |
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---------------- |
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</pre> |
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<p> |
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Each test writes output to stdout, and there is a <tt>test_*.good</tt> for |
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each <tt>.S</tt> file which contains the wanted output. If the actual |
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output matches the <tt>.good</tt> file, then the test passes, otherwise it |
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fails. |
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<p> |
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Read <tt>tests/README</tt> for more information. |
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</body> |
</body> |
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</html> |
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