Major accuracy improvements have happened over the past few days. They easily warrant a new beta release.
First, it turns out that every emulator to date; not only for the SNES, but for the Apple II GS as well, incorrectly computed ADC (add) and SBC (subtract) flags in BCD (binary-coded decimal) mode. At least fifteen years of emulating the 65816 processor, at least five known investigations into their behavior, and we all still had it wrong.
So I wrote some tests that dumped every possible combination of adc and sbc with every possible input and every possible flag, and recorded both the accumulator result and status flag register. From here, blargg figured out the underlying trick: the CPU was computing overflow before the top-nibble's BCD correction pass. With the routines rewritten, bsnes perfectly matches real hardware now.
Next, some background. The whole reason I got into SNES emulation was because I was tired of writing code that ran perfectly fine on emulators, but failed miserably on real hardware. The number one problem was emulators allowing video RAM to be written while the screen was being rendered. This single bug has broken dozens of fan translations and ROM hacks. Some have been updated to work around this bug, and many others are left in a permanently broken state (such as the translations of Dragon Quest I & II and Sailor Moon: Another Story, to name just two.) After asking emulator authors to fix this since 1997, I finally had enough in 2004 and started on bsnes. For this particular bug, I'm very happy to report that all but one SNES emulator now properly blocks these invalid accesses. Although sadly one still offers a configuration setting for backwards compatibility with these translations. What an ironic shame ... emulating an emulator. And in the process, sapping the motivation to ever go back and fix these
titles to ever run on real hardware. But I digress ...
The second biggest problem that causes software created under emulation to break on real hardware has, without a doubt, been the hardware delays as the S-CPU computes MUL (multiplication) and DIV (division) results. To date, whenever you used this hardware functionality, emulators have immmediately furnished the correct results. But on real hardware, multiplication requires eight CPU cycles, and division requires sixteen. Each step computes one bit of the source operand and updates the results. Reading the output registers early thus provides the partially computed results.
This is obscure. It isn't well known, and many people writing software for the SNES probably aren't even aware of this limitation. Because of the partial computation results, outright delaying the computation would break many commercial software titles. But by not emulating the delay at all, we were causing a great disservice to anyone wishing to use an emulator for development purposes.
Now, once again, thanks to blargg's algorithm help, he has determined the underlying multiplication and division algorithms. Combined with my expertise of SNES analysis and hardware testing, I was able to determine when and how the ALU (arithmetic logic unit) stepped through each work cycle. Our work combined, bsnes now also perfectly emulates the hardware MUL and DIV delays.
Again, this isn't going to fix commercial software titles. They would have realized that they were reading back invalid MUL and DIV values, and fixed their code. This is for all of the software developed using emulators. This is an extension of my commitment to create a hardware emulator, and not a video game emulator.
We also verified that the S-PPU multiplication interface does indeed return instant results with no delay. So emulation of that interface was already correct.
I'm only labelling this release a beta because it hasn't been tested. But I'm fairly confident that it is stable, and I seriously recommend upgrading from v060 or prior releases. This is easily one of the last major pieces missing from emulation.
The last notable elements are: S-CPU auto joypad poll timing, S-CPUr1 HDMA crash detection, S-CPU<>S-SMP port ORing, S-SMP timer glitching, S-DSP mute pulse, and full cycle-level emulation of the S-PPU. With all of the aforementioned items, I will consider a v1.0 release of bsnes ;)
Lastly, I'll post this screenshot just for fun. When d4s translated Breath of Fire II to German, he added some code that relies on the incorrect emulation of the DIV register to detect emulators. With this emulated properly, you now see the following screen:
./sshots/bs_349.png
Sorry to spoil that, but the secret's already out, as the MESS team reported on it publicly already.
I intend to add pseudo-randomness support shortly, which should eliminate one of the last vectors possible to distinguish bsnes from real hardware :)
A million thanks to blargg for making this release possible.
Complete rewrite of adc + sbc opcodes, should fix:
- adc BCD overflow flag
- sbc BCD overflow flag
- sbc BCD invalid input value
Testing is appreciated, I believe Sim Earth is probably the most
likely to observe any difference.
This is an experimental release, as such it is posted only to Google Code.
Changelog:
- 21fx API moved to pre-finalized form as S-MSU1; more about this on the forum
- OpenGL driver now uses GL_CLAMP_TO_BORDER instead of GL_CLAMP_TO_EDGE to support screen curvature shader
- rewrote file open dialog; code is greatly simplified, interface is improved
- all cheat code columns are now enquoted, and empty codes at the bottom of the file are omitted (format is compatible with previous releases still)
- debugger: added missing DMA variables to S-CPU properties viewer
- snesfilter: added OpenMP (multi-threading) support to HQ2x filter
- lots of other miscellaneous code cleanup work
Eight hours of non-stop work, for the sole purpose of trying to
separate the file browser underlying mechanics from the bsnes-specific
stuff.
So far, it's going quite well. 95% of the functionality is there with
only 25% of the code size. Forked the underlying stuff to
nall/qt/file-dialog.moc.hpp, which is now designed to support
open+save+folder selection natively. Save mode adds a text box to
enter your own file name, and folder mode hides the filter drop-down
and all files automatically. The top bar now spans 100% of the width.
I like it more this way. I also killed the tree view in favor of a
list view, for the sole reason that I really can't stand how when you
go up a folder and the deeper tree is still open. Since the
QFileSystemModel is asynchronous, I can't close the tree nodes when
navigating up.
The simplifications were needed because it was getting damned-near
impossible to edit that mess of a file (diskbrowser.cpp.) Compare to
filebrowser.cpp, much cleaner. Now I should be able to add open-folder
concept for BS-X, ST and SGB games much easier. And of course, I
should be able to offer the base QFileDialog as an option, too.
After that, I'll probably export the hex editor to a generic class,
and then export the Qb stuff (window geometry save/restore, stock
check / radio menu buttons.)
Also, I just wiped out Windows XP and put Windows 7 back, just to fix
the video tearing issue relating to DWM and ... it works perfectly
fine. Zero tearing, zero skipping, zero audio popping. All I did was
start bsnes v059.04, set audio sync to the usual 31960, and it was
just fine. What the hell are people complaining about, exactly?
Changelog:
- added folder-up button to the file loading window
- hid new-folder button except on path selection window
- removed "Assign Modifiers as Keys" button; replaced with input.modifierEnable in the configuration file
- fixed a Qt signal issue that was causing ROM loading to take an extra second or two longer than necessary
- scale 5x setting will now maintain an exact multiple in both width and height for both NTSC and PAL modes
- re-added group assignment and unassignment to the input settings window
- re-wrote mouse capture code to be more intuitive, now uses buttons to set assignment
- re-added input.allowInvalidInput check to stop up+down and left+right key combinations by default [Jonas Quinn]
- split "Tools Dialog" menu option into separate items for each tool (Cheat Editor, Cheat Finder, State Manager)
- added S-SMP and S-DSP property information readouts to the debugger
