This project is dedicated to the memory of William Morris (aka Frags), who was the main contributor to the bounty but was unable to see the final result.

Thursday, February 21, 2013

After the second year

Well, here we are again. Another year had passed - namely the second - since I started the project.

Obviously, it is not done yet, otherwise you would see Dancing Bananas around the blog. But we are slowly getting there. There are outstanding bugs and yet a fair share of work is to be done.
Since we passed the first year I gave up giving any estimate on how long will this take. As it seems I am pretty bad in estimates. Yet I hope that third is the charm! :)

Until then: Keep calm and Amiga On!

Sunday, February 17, 2013

Watch for the LED

Since there were too many complaints (too many > 2) about that it is hard to tell whether the JIT compiled code is active or not, I decided to implement a small on-screen indicator for it in this tiny update.

How does it work?

I extended the already available on-screen status line with one more "LED" which says: JIT.
If you turn on the OSD status line by adding the following lines into the configuration:


Then you will find one more block at the end of the status line. This "LED" will lit up in bright greenish color as soon as the emulator executes JIT compiled code (instead of interpreted code).

Now, since the emulation is (and always will be) a mixture of JIT compiled and interpreted execution, it is not a simple task to find out how much of the executed code was JIT compiled and how much is interpreted.
To overcome of this complication the block will show you the ratio between the compiled and the interpreted code by changing the background color:
  • If more compiled code was executed then it is more vivid light green.
  • If the interpreted code dominated the execution then the color dims toward black
  • When the JIT is inactive (turned off by the configuration) or for any reason the compiled code is not used (cache turned off, blocks are too short, etc.) then the background color will be completely black.

Here you can see some example screenshots:

The indicator lits up light green: JIT compiled code is running mostly.

The indicator is still green, but more dark green:
a mixture of JIT compiled code and interpreted code is executed.
The darker means less JIT code.

The indicator is black:
JIT is turned off or the compiled code is not executed for some reason.
So, now you can tell by simply looking at the screen if the JIT is active and how much of the executed code makes use of the JIT compiling.

Tiny-tiny catch

I didn't want to add extensive statistical data collection into the compiled code, it would make it run a lot slower.
This implementation slows down the emulation a tiny bit, but not that much. Probably later on I will either remove it completely or add some configuration around it, so it could be turned off.

How does it really work?

To tell you the truth: the indicator doesn't show you exactly how much of the actual instructions are executed by the JIT or the interpretive, but it collects the data from executed code block types. This is cheating beacuse:
  1. The length of the blocks vary between 1 instruction and 20-30 (or sometimes even more) instructions. Still one block counts exactly once in the calculation, regardless of the size.
    This could be improved, but I didn't want to put too much effort into this implementation.
  2. Even inside the compiled blocks not all of the instructions are JIT compiled (as I described this in earlier posts). Thus it is possible that none of the instructions in the "compiled" block consist of actual JIT code, but simply calls to the interpretive implementation. It would be more fair to calculate the ration based on the executed instruction types instead of the block types.
    Now, I would rather not change this for sake of performance. As long as most of the instructions are implemented, this won't affect the results too much. (And that is not true just yet, but will be improved overtime.)
So, bottom line: take this indicator results with a grain of salt.

Still cool, eh? :)

Saturday, February 16, 2013

When things go bogo

I came around fixing the reported issue with the enabled bogomem (fake Fast Ram) setting, you can find the simple fix in the recent update:
  • Fixed MOVEA.W reg,Ax - source data was not sign-extended
Now the Kickstart starts with both bogomem enabled or disabled configuration.

Some details on the fix

Debugging an emulator needs a completely different approach than debugging any other application type, simply because even if one or two instructions were misbehaving it doesn't mean that the emulated program is not working at all. It just does weird things, but not on the good way.

In this specific case when the error was: if the bogomem configuration was enabled then the Kickstart went into the dreaded reboot-loop, which is basically the result of an internal crash, usually because of a wrong access of memory somewhere or an exception while another exception is executed.

I had a closer look on what is going on and I have found that an instruction is trying to write into a custom register for the disk controller which is read-only. Never a good sign, especially if the Kickstart is trying to do that which is always playing by the (hardware) rules.
It was an even more interesting fact that the wrong-doing instruction was only for reading from memory.
My reaction was a confused face with a hint of suspicious look. I am still a bit puzzled by this, even after the fix - this must never happen ever.

But at least we have a crash!
It is always easier with a crash, it gives a starting point (or so I thought at least).

First I tried to log the full execution and analyze it for a while, but the only thing I had found was that some hardware handling loop runs too long, probably this is why the Kickstart hits the custom register by accident. This is not helping at all, usually it means that some initialization or leaving condition for the loop went wrong, so I had to look further before the loop itself.

Luckily, the Kickstart with this configuration set was working when none of the instructions were compiled. There is a simple method for finding out which instruction(s) causing trouble: turn off compiling of all instructions and add them back one by one while start the emulation with the same settings over and over again.
This sounds tedious but actually it is much easier than scrolling through megabytes of debug logs and looking for something, because it is procedural. Unfortunately, this method does not work for every possible issue, especially when the combinations of the wrong instructions are causing the problem.

At the end I had found that the MOVEA.W AX/DX,AY instruction was the one to blame and a quick look on the compiled code confirmed that the emulation was wrong: for every operation where the target is an address register the involved data must be longword sized.
In this case this simply meant that the word sized source data must be sign-extended while it gets copied into the target address register. I had done this for every other similar instruction, but I missed one case.

Now, you can probably see why there is no way I could find this bug by looking on the execution logs.

Thanks to kas1e, Thunder and MickJT for reporting bugs!

Wednesday, February 6, 2013

Can I haz time machine?

Another busy month passed, here comes the new update with lots of bug fixes and some freshly implemented instructions:
  • Implementation of
    • AND.x reg,reg;
    • EOR.x reg,reg;
    • EOR.x reg,mem;
    • SUBA.x reg,reg;
    • SUBA.x #imm,reg;
    • BSET.L #imm,reg;
    • BCLR.L #imm,reg;
    • EXG.L reg,reg and
    • NOP :) instruction.
  • Reorganized MOVE.x mem,mem instruction: memory reading and writing is separated out into independently callable functions to support the implementation of other similar instructions.
  • Implementation of OR.x reg,reg instruction was adapted to a more generic form.
  • Fixed flag checking for AND(I).(W|B) #imm,reg instructions.
  • Removed confusing OR immediate macroblock and replaced by the already available OR low immediate.
  • Falling back from addressing mode d16(ax) to (ax) if the offset is zero.
  • Fixed depenency flag handling in result check helper function.
  • Cleaned up opcode description table. Removed instructions from the table and from source code which won't be supported by the JIT compiler. Added missing RTM instruction.
  • Temporary register storage slots were moved from the stack frame to the Regs structure (context).
  • Removed useless debug log that flooded the output.
  • Temporarily disabled MOVE.x mem,mem and EOR.x reg,mem instructions to let the Kickstart run.

I have good news and not-so-good news

Which one should we start with? Okay, let me choose for you: not-so-good news first.
At the moment (beside the yet unimplemented instructions) there are two bugs that prevent the OS from running properly:
  1. For some unknown reason when an instruction reads and then writes the memory then the Kickstart goes back to the well-known reboot loop (actually it is a crash, sometimes you can even see the Guru message).
    At the moment I don't have the slightest idea why this happens. I traced it back to the normal MOVE instruction, when it copies data from one address to another. There is nothing wrong with the instruction implementation itself - as far as I can tell. So, this is some weird sh*t again. Lovely.
  2. When the optimize flag is turned on (comp_optimize = true in the config) then the Kickstart crashes very early.To tell you the truth, I am not surprised. When I tried to figure out the register dependency for each macroblock for each instruction then sometimes I mixed up stuff what I found later on. Due to the wrong register dependency settings for some macroblocks, these were accidentally removed, so some code is "optimized away". Most likely there are more mix-ups and missing dependencies in the code, it can be found just matter of time.
Good news:
As you can see in the last item for the update: I temporarily disabled the two already implemented instruction which tries to read and write the memory. The missing instructions are substituted by using the interpretive instructions.
Now, if you disable the optimize option in the configuration (comp_optimize=false) then the Kickstart seems working with some actual JIT compiled instructions! YAY! \o/ (I guess.)

Some more interesting problems

I have got some feedback about issues with running Kickstart 1.3 and some old Amiga500 games when the JIT is enabled. This is interesting indeed because if even if the JIT was turned on it did not emulate these old codes because the cache is never turned on (there was no cache in the Motorola 68000 processors when these programs were written).
As you can read it in the FAQ: the JIT compiling is depending on the cache emulation heavily. So, this is one more question mark. I haven't had much time to investigate it yet.

But most importantly: the Summer Sun is shining, let's go surfing! (Oh, wait. I don't do surfing. Last time when I jumped onto a bodyboard on the beach I bruised my ribs. Embarrassing and totally geeky. Let's just surf the net, shall we?)