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by scarybeasts
Sun Jul 28, 2019 10:11 pm
Forum: 8-bit acorn emulators
Topic: Sampled sound on the sn76489 -- tests
Replies: 35
Views: 2810

Re: Sampled sound on the sn76489 -- tests

Regarding sound chip write enable: b2 does run the speech demo , as it has a fudge for this: allow writes to the sound chip at any time, if write is enabled . There's an internal counter that corresponds to the 16 µsec delay you're supposed to have from the 6502 end, but judging by the commit comme...
by scarybeasts
Sun Jul 28, 2019 9:46 pm
Forum: 8-bit acorn emulators
Topic: Sampled sound on the sn76489 -- tests
Replies: 35
Views: 2810

Re: Sampled sound on the sn76489 -- tests

Regarding noise output bit 0: b2 uses the inverse of bit 0 when reading the next value for the noise output , because John K's SN76489 page mentioned this. And the forum post linked to above also mentions this, too. But this can't be right, surely, because the initial output would then be 1. And th...
by scarybeasts
Sun Jul 28, 2019 9:37 am
Forum: 8-bit acorn emulators
Topic: Sampled sound on the sn76489 -- tests
Replies: 35
Views: 2810

Re: Sampled sound on the sn76489 -- tests

This is a very interesting topic. I'm unsure how it might work though, because as I understand it, the noise LFSR is reset such that the top bit (bit 15) is set and the low bit (bit 0) is clear (0x8000), and it is the low bit 0 that is output to the mixer, so that means the output is low on reset -...
by scarybeasts
Sun Jul 28, 2019 9:32 am
Forum: 8-bit acorn emulators
Topic: Sampled sound on the sn76489 -- tests
Replies: 35
Views: 2810

Re: Sampled sound on the sn76489 -- tests

Just a note that I have read conflicting opinion on whether or not setting the tone register to 1 generates a square wave or a constant level. If the chip logic is: decrement counter if 0, flip output bit, reload counter Then we'd expect a 125Khz square wave if the tone register is set to 1. Howeve...
by scarybeasts
Sun Jul 28, 2019 7:28 am
Forum: 8-bit acorn emulators
Topic: Sampled sound on the sn76489 -- tests
Replies: 35
Views: 2810

Re: Sampled sound on the sn76489 -- tests

Here is a recording of them on a ModelB, recorded via the TV on a mobile phone with a CTRL-G at the start for comparison. https://www.dropbox.com/s/pvrpnroueedxgn1/noise.3gpp?dl=0 no login or password should be required. The first one seemed to make the odd occasional very quiet click. I would say ...
by scarybeasts
Sat Jul 27, 2019 9:46 am
Forum: 8-bit acorn emulators
Topic: Sampled sound on the sn76489 -- tests
Replies: 35
Views: 2810

Re: Sampled sound on the sn76489 -- tests

Ping on this thread -- anyone able to run the test cases on a real beeb to see what they sound like?

I can paste the file content here as text if that would help.
by scarybeasts
Sat Jul 27, 2019 9:40 am
Forum: 8-bit acorn emulators
Topic: Uridium and other games not working on JSBeeb
Replies: 30
Views: 1395

Re: Detecting BeebEm

If a disc-image works in BeebEm and on a real Beeb but not in JSBeeb then I think it’s fair to say that it’s JSBeeb that’s at fault. :idea: Yes, I think that's fair. However, in the specific case of Uridium, I'm unconvinced there's anything to do: 1) Plain BBC B. While the Uridium disk image refere...
by scarybeasts
Sat Jul 27, 2019 6:29 am
Forum: 8-bit acorn emulators
Topic: Uridium and other games not working on JSBeeb
Replies: 30
Views: 1395

Re: Detecting BeebEm

This thread has inspired me to look at fixing any prominent jsbeeb accuracy issues. It's a welcome break from hacking away at my own proto-emulator and ultimately will probably be of more use to people. I have a PR up to fix Snapper, for example, which is broken (all versions) but we understand why ...
by scarybeasts
Wed Jul 24, 2019 11:11 am
Forum: 8-bit acorn emulators
Topic: Sampled sound on the sn76489 -- tests
Replies: 35
Views: 2810

Sampled sound on the sn76489 -- tests

Hi, As is well documented, the sn76489 in the BBC is capable of sampled sound playback. See e.g. Spy Hunter loading screen or game start of sideways RAM version Exile. This is achieved by rapidly modulating the 4-bit volume of one or more of the three tone channels, where said channel is programmed ...
by scarybeasts
Sat Jul 20, 2019 9:22 pm
Forum: 8-bit acorn emulators
Topic: Reads of write-only CRTC / ULA registers -- tests
Replies: 31
Views: 1691

Re: Reads of write-only CRTC / ULA registers -- tests

This is most like the Beeb's most common CRTC (although, as we've discovered, there are Masters if not Beebs with different versions from different manufacturers!): http://www.cpcwiki.eu/imgs/c/c0/Hd6845.hitachi.pdf This datasheet covers the skew control (R8), variable VSync pulse width, and specif...
by scarybeasts
Fri Jul 19, 2019 5:04 am
Forum: 8-bit acorn emulators
Topic: Reads of write-only CRTC / ULA registers -- tests
Replies: 31
Views: 1691

Re: Reads of write-only CRTC / ULA registers -- tests

Hi, One more thing I noticed. b2 considers CRTC R12 and R13 (screen start address hi / low) to be readable. beebem and the 6845 datasheet do not. If anyone was being charitable, this could be resolved by trying this in a fresh MODE7 bootup: ?&FE00=12:P.?&FE01 It should give 0 if the register is read...
by scarybeasts
Thu Jul 18, 2019 10:12 pm
Forum: 8-bit acorn emulators
Topic: Reads of write-only CRTC / ULA registers -- tests
Replies: 31
Views: 1691

Re: Reads of write-only CRTC / ULA registers -- tests

Rich Talbot-Watkins wrote:
Thu Jul 18, 2019 1:06 pm
I assume the &FE is just what was left on the databus from the LDA &FE21 instruction, but it's weird how Ed got a different result from a power-on.
This capacitance issue is fascinating. I wonder if it could be amusingly used as an RNG on a real beeb?
by scarybeasts
Thu Jul 18, 2019 12:43 pm
Forum: 8-bit acorn emulators
Topic: Reads of write-only CRTC / ULA registers -- tests
Replies: 31
Views: 1691

Re: Reads of write-only CRTC / ULA registers -- tests

Thanks. These results are pretty much as expected, although not every emulator will agree :)

Any chance you could check a test I forgot in my original post?

P.?&FE20

Should be the same as P.?&FE21, i.e. 254, but no harm in checking.


Cheers
Chris
by scarybeasts
Thu Jul 18, 2019 11:23 am
Forum: 8-bit acorn emulators
Topic: Reads of write-only CRTC / ULA registers -- tests
Replies: 31
Views: 1691

Re: Reads of write-only CRTC / ULA registers -- tests

Answers inline... Ed, was that a Model B or a Master? I see different results for a couple on my Model B: FE21 and FEA0 both read back as &FE Dave Thanks. - Just to confirm, does FE20 also return &FE? FE20 -> FE23 should all be mapped to the same thing but you can never be too sure. - While we're a...
by scarybeasts
Thu Jul 18, 2019 11:16 am
Forum: 8-bit acorn emulators
Topic: Reads of write-only CRTC / ULA registers -- tests
Replies: 31
Views: 1691

Re: Reads of write-only CRTC / ULA registers -- tests

Hi Dave, it was a Beeb - normally I do use my Master but I was careful. I even checked that it's got an original 6502 and nothing hanging off the Tube port! Notably, the results I got are at power on. When rerunning, I get slight differences. See post edit above. Edit: hmm, even after a power-cycle...
by scarybeasts
Thu Jul 18, 2019 6:47 am
Forum: 8-bit acorn emulators
Topic: Reads of write-only CRTC / ULA registers -- tests
Replies: 31
Views: 1691

Reads of write-only CRTC / ULA registers -- tests

Hi, [Background: I'm still plugging away at an emulator concept which I hope to release soon.] I noticed that reading of write-only registers is handled pretty inconsistently between emulators: - Some of the stronger emulators even allow the read of a write-only register, which is surprising. - Wher...
by scarybeasts
Sat May 25, 2019 8:11 am
Forum: general
Topic: Remarkable videos recently posted on Youtube
Replies: 332
Views: 43090

Re: Remarkable videos recently posted on Youtube

Great Exile video, nice production and appreciation of the technical brilliance. Thanks for sharing!
by scarybeasts
Mon Apr 22, 2019 3:31 am
Forum: 8-bit acorn software: classic games
Topic: Interesting Camelot bug
Replies: 3
Views: 559

Re: Interesting Camelot bug

0xC0DE wrote:
Sun Apr 21, 2019 8:34 am
Does this count? Also about key handling in Camelot.


viewtopic.php?f=4&t=16774&view=unread#p232025
Sounds like that's the Electron version; I didn't specify but I'm referring to the BBC version. Looks like Camelot has all-around interesting keyboard quirks :-)
by scarybeasts
Sun Apr 21, 2019 3:36 am
Forum: 8-bit acorn software: classic games
Topic: Interesting Camelot bug
Replies: 3
Views: 559

Interesting Camelot bug

Hi, While working on a new experimental emulator, I found a bug in Camelot that seemed interesting / unexpected enough to document. It's pretty easy to trigger: on the game start screen (screenshot attached), press space to start but _hold space down_ until the game has started and Sir Camelot is fi...
by scarybeasts
Wed Apr 03, 2019 9:30 pm
Forum: 8-bit acorn software: classic games
Topic: Who's ready for... guess that game! ;D
Replies: 90
Views: 5023

Re: Who's ready for... guess that game! ;D

Sim? And wasn't that 4 the channel 4 logo from the time?
by scarybeasts
Sat Mar 16, 2019 3:42 am
Forum: 8-bit acorn software: classic games
Topic: Revs special edition?
Replies: 1
Views: 441

Revs special edition?

Hey,

Fairly random question but does anyone else remember a "Revs special edition" where the boxed game came with some little formula 3 toy racing cars?


Cheers
Chris
by scarybeasts
Fri Feb 01, 2019 3:38 am
Forum: 8-bit acorn emulators
Topic: VIA test case: T2 freeze vs. IRQ
Replies: 2
Views: 416

Re: VIA test case: T2 freeze vs. IRQ

Hi,

Thanks, yeah I thought the IRQ might still fire. Another crazy corner case but probably not one that's super important to emulate.


Cheers
Chris
by scarybeasts
Thu Jan 31, 2019 9:17 pm
Forum: 8-bit acorn emulators
Topic: VIA test case: T2 freeze vs. IRQ
Replies: 2
Views: 416

VIA test case: T2 freeze vs. IRQ

Hi, Unburying this minor unresolved test case from another thread. Cheers Chris 10 REM RESULTS FROM JAN 2018 20 DIM MC% 100 30 DIM R% 16 40 P% = MC% 50 [ 60 OPT 3 70 SEI 80 LDA #&40 90 STA &FE4B 100 LDA #&03 110 STA &FE48 120 LDA #&00 130 STA &FE49 140 LDA #&60 150 STA &FE4B 160 LDA &FE4D 170 STA R%...
by scarybeasts
Sun Jan 27, 2019 8:57 am
Forum: 8-bit acorn emulators
Topic: Two more VIA test cases
Replies: 3
Views: 549

Re: Two more VIA test cases

Thanks! VIA.T12 is about as expected but VIA.T23 is interesting, it appears timer freezing or continuing takes effect one VIA tick later than you'd expect. This does suggest the follow-up test appended below: if the "one VIA tick later" is actually T2 reload, does the IRQ fire or not? Cheers Chris 1...
by scarybeasts
Sat Jan 26, 2019 12:19 pm
Forum: 8-bit acorn emulators
Topic: Two more VIA test cases
Replies: 3
Views: 549

Two more VIA test cases

Hi, Still plugging away trying to get to all the 6522 corner cases for core functionality:-) Finally running low on corner case ideas but here's what has come to mind. The second one could be particularly interesting -- it's writing the T1 latch values at the same time the timer is relatching. As al...
by scarybeasts
Thu Jan 17, 2019 3:28 pm
Forum: 8-bit acorn emulators
Topic: System VIA T2 value on a real MOS1.2 beeb?
Replies: 2
Views: 438

Re: System VIA T2 value on a real MOS1.2 beeb?

Thanks. Power-on counter value is effectively random then.
BigEd wrote:
Thu Jan 17, 2019 9:29 am
First try:

Code: Select all

>PRINT ~?&FE49
        55
>PRINT ~?&FE48
        AE
Second try: 69 and 0.
Third try: 58 and F6
by scarybeasts
Thu Jan 17, 2019 9:23 am
Forum: 8-bit acorn emulators
Topic: System VIA T2 value on a real MOS1.2 beeb?
Replies: 2
Views: 438

System VIA T2 value on a real MOS1.2 beeb?

Hi, TL;DR -- what do the following peeks give on a real BBC B (MOS1.2, vanilla?) on a fresh power-on? Are the values stable across a few tries? PRINT ~?&FE49 PRINT ~?&FE48 It's interesting because MOS1.2 freezes the system VIA timer 2 some short and constant time into initialization. Reading these v...
by scarybeasts
Sat Jan 05, 2019 8:59 pm
Forum: 8-bit acorn emulators
Topic: 6522 VIA emulation: ACR writes vs. timer expiry
Replies: 12
Views: 1127

Re: 6522 VIA emulation: ACR writes vs. timer expiry

I don't know anything about this, but could there be a pipeline delay? Does writing to ACR once cycle before expiry affect the IRQ in a more understandable way? Or maybe that's already been well covered? One of the tests in the most recent dump did the same ACR writes one cycle before IRQ (the coun...
by scarybeasts
Sat Jan 05, 2019 8:57 pm
Forum: 8-bit acorn emulators
Topic: 6522 VIA emulation: ACR writes vs. timer expiry
Replies: 12
Views: 1127

Re: 6522 VIA emulation: ACR writes vs. timer expiry

it is strange to think what could be going on in the silicon to make ACR 00 -> 40 at IRQ time asymmetric with ACR 40 -> 00 at IRQ time. As has been remarked before, it's only "strange" if you assume that everything is synchronous (clocked). For somebody like me who, back in the 1970s and 80s, desig...
by scarybeasts
Sat Jan 05, 2019 11:47 am
Forum: 8-bit acorn emulators
Topic: 6522 VIA emulation: ACR writes vs. timer expiry
Replies: 12
Views: 1127

Re: 6522 VIA emulation: ACR writes vs. timer expiry

Here you go: Thanks BigEd. This confirms the quirky result with no additional quirks uncovered. This is easy enough to emulate but it is strange to think what could be going on in the silicon to make ACR 00 -> 40 at IRQ time asymmetric with ACR 40 -> 00 at IRQ time. In both cases, ACR==00 wins; the...

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