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Old 03-15-2010, 08:09 AM   #1
Azul Fria
Ice Queen Zero
Join Date: Jul 2008
Posts: 521
Default bsnes v0.62 beta

Another byuutiful update to the bsnes has been released. Here's a word from the author.

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

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.
Download it here or at http://byuu.org/

Last edited by Reaper man; 03-15-2010 at 03:38 PM. Reason: slightly modified as we have no screenshot available in post
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