Monday, September 25, 2006

PIF fixes

It's about time I posted an update with news of some of the things I've been working on! Annoyingly the '.' key on my keyboard has decided to stop working so apologies in advance for any dodgy punctuation :)

I've been having a look at fixing a couple of issues related to the way that Daedalus handles the N64 PIF (peripheral interface) emulation. The PIF is responsible for a number of different functions, including:

  • Reading the controller status

  • Reading/writing to the eeprom

  • Reading/writing to the controller mempak

Access to the PIF is controlled through a 64 byte region of memory. Writing to this area triggers the PIF to interpret the block of data as a sequence of commands (such as those listed above), and the CPU can read back from this region to obtain the results.

Correctly handling the way the block is formatted is key to emuating the PIF accurately. Over the years I've rewritten this code several times as I've fixed bugs, each time fixing certain incorrect assumptions made during the previous version. Usually these assumptions were made on incredibly useful, but incomplete information (such as early docs from LaC (*waves*) ) which described how to use the PIF rather than how to emulate it. Unfortunately there was lots of gaps in this information and so I'd keep finding roms that used the PIF is various ways that I'd not taken into account.

A few years ago I stumbled across something rather amazing - a document which described the PIF hardware in great detail. Not only was it very thorough - including details on all the various formatting commands I'd spent hours reverse engineering - it also included an insane amount of detail on things such as the chip timings and the even method by which the PIF is used as a 'security system' to lock out pirated roms.

What was this document? Did I find it tucked away on some obscure N64 hacking site? No - it was US Patent 6,394,905, filed by Nintendo in September 2000. Genius!

There are actually a whole series of N64 patents (see 5,426,762 and 4,799,635 for a couple of other similar examples.) It's not all good news though because unfortunately they're all written in a horribly convulted form of Legalese:

There are six JoyChannels available in the present exemplary embodiment. Each Channel's transmit data and receive data byte sizes are all independently assignable by setting size parameters. In the exemplary embodiment, all six channels size parameter setups are required, whether they are used or not.


This is the statement I spotted a couple of weeks ago that explained why one of the previous assumptions I'd made wasn't working in all cases:

If the 64 bit CPU 100 wants to change JoyChannel's Tx/RxData assignment, a 32 bit format flag is used, where a certain bit(s) specify the desired format. For example, when Wr64B or Wr4B is issued when this flag is "1", PIF executes each JoyChannel's Tx/RxData assignment based on each channel's Tx/Rx Size. In other words, unless this flag is set to "1" with Wr64B or Wr4B, Tx/RxData area assignment does not change. After Tx/RxData assignment, this flag is reset to "0" automatically.

After painfully wading through the text, it's basically saying that if you write a '1' to the last byte of the PIF block, you need to reformat all the channels. Otherwise you just assume the same formatting as the previous command.

The bug that I've since fixed was actually caused by two problems. Firstly, I was reformatting the channels regardless of how this last byte was set. I'd assumed that it was always set to '1' by the CPU to indicate that a command was ready, and set to '0' by the PIF when it was finished executing. In a few roms I was seeing '0' being written, but I didn't know why. Quite often when I saw them do this they'd hang shortly aftwerwards.

The other problem that contributed to the bug was that I was accidentally overwritting the 'Tx/RxData assignment' area with 0 as I was processing the PIF block (actually, I was clearing the top 4 bits of each byte, rather than just the top 2). This meant that when I reformatted the Tx/RxData assigments the areas were smaller than expected, so the PIF handling code would abort and return with failure.

These two problems combined to cause certain roms to get confused and hang when trying to access the mempak. The total fix was a few dozen lines of code to separate out the PIF formatting from the command execution, and a single-line fix for the second issue. All the roms I found exhibiting this issue now make further progress (hooray!) There may well be other issues that have been resolved with this fix too :)

It's bedtime for me now (well, maybe a little Dead Rising on the 360 first :) I'll try and get back into the habit of updating a bit more often from this week on. Sorry if I've not replied to your email - it's not personal, it's just me being rubbish.


Sunday, September 17, 2006

I'm still alive

I just wanted to apologise for the lack of updates recently. In between commitments at work and at home I've not had much time to work on Daedalus over the past couple of weeks. Fortunately things are quietening down again so I should have some free time to work on the next release of the emulator this week. Rest assured I'll keep you up to date with all the new developments.


Wednesday, September 06, 2006

Viewport scaling

It's been a fairly slow week for progress on Daedalus. I've had a few busy days with work, and I was away over the weekend so I've not had much time to work on Daedalus since my last update.

One thing I have managed to implement is viewport scaling however. After the previous release I received an email from Chris Meyer-Rassow which had a couple of interesting points. He suggested that I should have a look at providing a setting to allow the emulator to run with an aspect ratio of 4:3, rather than strecthing everything to fit the PSP's 16:9 (or 15.88236:9 to be accurate :)

I had been meaning to add this option for some time, and as it was only a small change I decided to implement this feature for the next release. I've added 3 different modes, as shown the in screenshots below. The first mode simply scales the graphics up to fit the PSP's screen (this is no different from the current release of the emulator):

This renders at 480x272. Notice how the vertial lines for the 't' and 'f' characters look slightly too heavy. This is as a result of scaling the graphics up to fit the PSP's screen. (As an aside, this screenshot clearly shows another issue that I need to address - the zfighting which is clearly visible in the carpet. I have a couple of ideas for fixing this.)

The next shot shows the same scene with the new unscaled 4:3 viewport setting:

This renders at 320x240, and so avoids scaling up any of the graphics. You can see that the problems with the 'f' and 't characters has gone. On the downside, there's a lot of 'wasted' space around the screen.

The final settings shows the new scaled 4:3 viewport setting:

This maintains a 4:3 ratio (so we maintain the correct aspect ratio), but scales things up to 362x272 to make full use of the screen's height. We waste a little less space with this mode, but some of the original problems with the text are still visible.

In the end all three options are a trade off between making full use of the screen and quality. In the next release it will be possible to adjust this setting as the emulator is running so you can pick whichever one looks best for the rom you're currently running.

As a result of doing this work with the viewport, I also fixed a couple of bugs related to how I was emulating the N64's viewport. When I originally wrote the graphics code for the PSP version of Daedalus, I forgot to handle the N64's viewport scaling/translation, which causes problems rendering certain scenes, as demonstrated in Aerogauge below:

Notice how the car is being rendered off-centre (it should be in the top right hand corner). After my viewport fixes, the same scene now looks like this:

The same change also fixes various minor glitches in a number of other roms (including Mariokart and Waverace), so it's a nice bugfix to get in for the next release.