FDS $4023 Testing

[TakuikaNinja]

Or more bluntly, "nobody checked what 'disabling FDS registers' via $4023 actually meant until now".

(I swear I posted here last night... whatever)

So it turns out that the read-only registers are mapped and functional even they're disabled via $4023. I haven't checked the sound registers yet but I would expect them to behave in the same way. This means that only the write-only registers are actually disabled in this state. I've seen some emulators map the read-only registers to open bus when "disabled", so those are incorrect.

Exhibit 1 - $4030-$4033 are still readable when $4023 = 0:
registers_disabled_but_still_readable.png


Exhibit 2 - $4032 can still recognise the disk being inserted/ejected while in this state (first/second reads = while inserted, third read = after ejection):
registers_disabled_but_eject_is_recognised.png

I finally got around to making a program to better visualise/test this behaviour. I will be making separate threads for individual FDS test programs from now on as to not clutter the FDS emulation thread with different topics.

Summary:

• $4023 state at top of screen. %10000011 on startup/reset, bit 7 cannot be changed.
• Hex dump of $4020-$409F below, updated in realtime so it can recognise actions such as ejecting/inserting a disk.
• B toggles $4023.D1 (sound registers), A toggles $4023.D0 (timer IRQ & disk I/O registers).
• Measures have been put in place to reduce the chance of PRG-RAM corruption caused by leaving $4023.D0 = 0 (DRAM refresh watchdog is disabled). However, it may be necessary to do a full power-cycle/reload should the program ever halt or otherwise fail to reset properly.

Source code: https://github.com/TakuikaNinja/FDS-4023-Test

I'll just have the program attached here. I might get a GitHub release going once I get results from other consoles/emulators. I think I need to update the mirroring test as a result of this new one, and then get back to the DRAM refresh watchdog at some point... Not to mention the tests I also want to make for the byte transfer flag and its IRQs.

Hardware recordings from my Twin Famicom will be posted here soon.

[TakuikaNinja]

I have a video recording now and will get that up on YouTube in a short moment. Here's a minor update with an adjusted palette for the hex dump - I've found that my EasyCAP doesn't handle grey-on-black very well.

[TakuikaNinja]

Right, here's the recording from my Twin Famicom: https://youtu.be/rbvNSA2-kNI

Here's the list of actions taken:

1. Initial load, $4023 = %10000011
2. Disk eject + insert
3. Set $4023.D0 = 0 (disk I/O registers)
4. Disk eject + insert
5. Toggle $4023.D1 (sound registers) a few times
6. Press reset a few times

[TakuikaNinja]

Main findings so far:

• Readable disk I/O registers ($4030-$4033) are still readable when $4023.D0 = 0.
• $4030.D3 (nametable arrangement status) is seemingly cleared when setting $4023.D0 = 0. It stays cleared, despite setting $4023.D0 = 1 again, until reset (when $4025.D3 is newly written to?). I'll have to test this further in my mirroring test.
• $4032 can still recognise the disk being inserted/ejected while $4023.D0 = 0.
• Readable sound registers are still readable when $4023.D1 = 0, though a few of the mod table registers ($4093, $4095) seem to change values?
• Wavetable RAM ($4040-$407F) seems to return an initial state (note: top 2 bits are open bus), albeit likely influenced by the active wavetable position/accumulators (judging by the reset flakiness)?

Things to further test/research:

• Does $4030 have any other responses to setting $4023.D0 = 0?
• Does $4033 reflect the FDS expansion port state while $4023.D0 = 0?
• Does Wavetable RAM ($4040-$407F) have an actual initial state, or is it undefined?
• Sound registers in general. I personally don't have a good understanding of them.

[TakuikaNinja]

Update on the wavetable RAM: Loading the program from power-on as early as possible (by having the FDSKey remember the disk image between power cycles) resulted in the below hexdump. On my unit, it seems to return open bus bits | $02 and doesn't change across resets.