Emulating a processor

Hi all,

I'm interested to understand the processor architecture in depth. So i
decided on emulating the processor itself (as my project). The best one
to start would be 386.

So i wud require some documents which explains on how to emulate any
processor or devices. ( Apart from the Intel Architecture documents
available).
How to emulate a 386 processor. I want to kno how usually this is done.
I wonder how bochs has been developed so elegantly...the resource
they've used.

It wud be appreciable if someone can guide me on any documents or
reference books avaliable on Emulating processors and devices.

..
Wht are the resources I should have in hand to start up up this
project.

Thanks,
Gromer

why not take a look at the Bochs source code? Of course, it emulates
an entire PC system, not just the processor...

basically you have two choices:

full emulation: you read a byte from (emulated) ram and
interpret/execute it (use a huge switch-statement). then advance to the
next byte and interpret/execute it, ...


the virtualpc/qemu-with-accelerator/vmware/...-way: run the code as
ring3-process. catch every exception and emulate the thing that caused
the exception (i/o ports, mmu-stuff, ...)


but i suggest starting with something similar to dosbox
(http://dosbox.sf.net/). create a real-mode-only-emulator first. messing
around with page-tables and exceptions makes emulation quite difficult
to get right...

regards,
simon


Gromer wrote:
> Hi all,
>
> I'm interested to understand the processor architecture in depth. So i
> decided on emulating the processor itself (as my project). The best one
> to start would be 386.
>
> So i wud require some documents which explains on how to emulate any
> processor or devices. ( Apart from the Intel Architecture documents
> available).
> How to emulate a 386 processor. I want to kno how usually this is done.
> I wonder how bochs has been developed so elegantly...the resource
> they've used.
>
> It wud be appreciable if someone can guide me on any documents or
> reference books avaliable on Emulating processors and devices.
>
> .
> Wht are the resources I should have in hand to start up up this
> project.
>
> Thanks,
> Gromer
>

[F'up2 cut down --- OP neglected doing it.]

In comp.arch.embedded Gromer <> wrote:

> I'm interested to understand the processor architecture in depth. So
> i decided on emulating the processor itself (as my project). The
> best one to start would be 386.

I really wonder by what strange criteria the 386 could possibly come
out as "the best". Actually, the x86 is almost certainly the single
worst possible CPU line to do any theoretical work on. It's just too
plagued with silly exceptions due to its long history of compatibility
kept at almost any cost.

If you really want to learn something about CPU emulation, I'd
recommend Professor Knuth's work on the MIX and MMIX platforms ---
those are CPUs designed purely for educational purpose, and they only
ever existed as emulators, the original ones written by his Knuth-ness
in person.

--
Hans-Bernhard Broeker ()
Even if all the snow were burnt, ashes would remain.

Hi Gromer

Why not to take an approach to vhdl.
Instead of emulating you can simulate and even synthesize.
If your aim is to better understand how a processor work this may give you a
deeper understanding.
Also there are numerous processor cores available as open source in vhdl,
mainly microcontrollers.


"Gromer" <> ha scritto nel messaggio
news: ups.com...
> Hi all,
>
> I'm interested to understand the processor architecture in depth. So i
> decided on emulating the processor itself (as my project). The best one
> to start would be 386.
>
> So i wud require some documents which explains on how to emulate any
> processor or devices. ( Apart from the Intel Architecture documents
> available).
> How to emulate a 386 processor. I want to kno how usually this is done.
> I wonder how bochs has been developed so elegantly...the resource
> they've used.
>
> It wud be appreciable if someone can guide me on any documents or
> reference books avaliable on Emulating processors and devices.
>
> .
> Wht are the resources I should have in hand to start up up this
> project.
>
> Thanks,
> Gromer
>

["Followup-To:" nach comp.arch.embedded gesetzt.]

Simon Felix <> schrieb:

> the virtualpc/qemu-with-accelerator/vmware/...-way: run the code as
> ring3-process. catch every exception and emulate the thing that caused
> the exception (i/o ports, mmu-stuff, ...)

VMWare does not work this way.

Markus

On 2005-11-22, Gromer <> wrote:

> I'm interested to understand the processor architecture in depth. So i
> decided on emulating the processor itself (as my project). The best one
> to start would be 386.

Yikes. Since you've never written an emulator, I would have
recommended starting with a decent architecture. Something like
a PDP-11 or 6809 or Z80. The '386 is a really nasty bit of
work. It's 3 or 4 badly designed processors rolled into one.

> So i wud require some documents which explains on how to
> emulate any processor or devices. ( Apart from the Intel
> Architecture documents available). How to emulate a 386
> processor. I want to kno how usually this is done. I wonder
> how bochs has been developed so elegantly...the resource
> they've used.

Well, if you want to know how bochs as done, then look at the
bochs source code and documentation.

> It wud be appreciable if someone can guide me on any documents or
> reference books avaliable on Emulating processors and devices.
>
> Wht are the resources I should have in hand to start up up this
> project.

--
Grant Edwards grante Yow! Yow! Are we in the
at perfect mood?
visi.com

Grant Edwards wrote:

>Yikes. Since you've never written an emulator, I would have
>recommended starting with a decent architecture. Something like
>a PDP-11 or 6809 or Z80. The '386 is a really nasty bit of
>work. It's 3 or 4 badly designed processors rolled into one.

Agreed.

I wrote a Z80 simulator (I believe that the correct term for this thread's
topic is "simulator" rather than "emulator".) 20 years ao in 8088 assembler
on an 8 MHz AT&T 6300 that, at the time, had two 5.25" floppies and no hard
disk!

I used a rather straight forward (brute force?) method of allocating data
structures for the internal registers and look up tables for the op-codes.
The tables where used to jump to functions that "executed" the op-codes.
An Intel hex file decoder read program files into the simulated Z80 memory
space.

As usual, the hardest part was the user interface, especially the displays
of registers, and memory and I/O spaces. Interrupt generation was the
program's weak point and the thing I most planned to improve but never
got around to.

The greatest benefit of writing this program was the education concerning
the Z80's internals. I was writing embedded Z80 systems so this was a
good thing!

I had long thought that I had lost the source to this program but, in a
office renovation, I recently came across a disk that appears to contain
it. The trouble is that I no longer have a machine with a 5.25" floppy
drive. I might have to buy such a drive on eBay soon and see if the disk
is still readable!

--
================================================== ======================
Michael Kesti | "And like, one and one don't make
| two, one and one make one."
mrkesti at comcast dot net | - The Who, Bargain

In article <>, says...
> Grant Edwards wrote:
>
> >Yikes. Since you've never written an emulator, I would have
> >recommended starting with a decent architecture. Something like
> >a PDP-11 or 6809 or Z80. The '386 is a really nasty bit of
> >work. It's 3 or 4 badly designed processors rolled into one.
>
> Agreed.
>
> I wrote a Z80 simulator (I believe that the correct term for this thread's
> topic is "simulator" rather than "emulator".) 20 years ao in 8088 assembler
> on an 8 MHz AT&T 6300 that, at the time, had two 5.25" floppies and no hard
> disk!
>
> I used a rather straight forward (brute force?) method of allocating data
> structures for the internal registers and look up tables for the op-codes.
> The tables where used to jump to functions that "executed" the op-codes.
> An Intel hex file decoder read program files into the simulated Z80 memory
> space.
>
> As usual, the hardest part was the user interface, especially the displays
> of registers, and memory and I/O spaces. Interrupt generation was the
> program's weak point and the thing I most planned to improve but never
> got around to.
>
> The greatest benefit of writing this program was the education concerning
> the Z80's internals. I was writing embedded Z80 systems so this was a
> good thing!
>
> I had long thought that I had lost the source to this program but, in a
> office renovation, I recently came across a disk that appears to contain
> it. The trouble is that I no longer have a machine with a 5.25" floppy
> drive. I might have to buy such a drive on eBay soon and see if the disk
> is still readable!
>
>
I did the same thing for the 68K processor. I also simulated a simple
serial port and a few interrupts. As you say, the most difficult part
was the user interface. My simulator was done on a Macintosh and worked
well enough that I used it when I taught an introductory course on
computer architecture. It turned out to be most useful in illustrating
what happens in memory when you use stack-based parameters for
subroutines. It beat the heck out of keeping track of the stack with
paper and pencil!

My simulator would interpret M68K assembly language in a text file--
stepping through the source code and showing effects on registers
and memory. It was cool at the time (mid 80's), but now you can
get a better simulator with just about every IDE designed to
do cross-compilation and debugging with an embedded processor.

Mark Borgerson

Mark Borgerson wrote:

>I did the same thing for the 68K processor.

When I showed my simulator to the division engineering manager he asked
me if I could do a 68K version because we were about to embark on a 68K
project. I told him that I probably wouldn't finish it before the project's
final testing phase and that it probably wouldn't be all that useful even
if it was available sooner than that.

> I also simulated a simple
>serial port and a few interrupts. As you say, the most difficult part
>was the user interface. My simulator was done on a Macintosh and worked
>well enough that I used it when I taught an introductory course on
>computer architecture. It turned out to be most useful in illustrating
>what happens in memory when you use stack-based parameters for
>subroutines. It beat the heck out of keeping track of the stack with
>paper and pencil!

Yes, I can see that a classroom environment may be the best place for
serious use of such programs.

>My simulator would interpret M68K assembly language in a text file--
>stepping through the source code and showing effects on registers
>and memory.

It didn't occur to me to have my simulator also be an assembler, but I
can see that this would be cool. How did you deal with forward references?

> It was cool at the time (mid 80's), but now you can
>get a better simulator with just about every IDE designed to
>do cross-compilation and debugging with an embedded processor.

I've used plenty of IDE-based cross compilers and debuggers but don't
recall any of them being equipped with simlators. Can you provide some
examples of such?

--
================================================== ======================
Michael Kesti | "And like, one and one don't make
| two, one and one make one."
mrkesti at comcast dot net | - The Who, Bargain