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It was only when I decided to disassemble what was actually in memory that I found the problem. The assembler that I was using was inserting the wrong opcode for the conditional jump instruction… Even as recently as within the last year, I've had Visual Studio 2022 generate incorrect code! In this ...

I guess the question here is what was causing the error in BBC BASIC, given that the line as posted looked perfectly reasonable. Agree that an error shouldn't cause it to stop, but why was that line causing an error at all? Was BeebEm getting input faster than it could be consumed, or was there some...

Do you still have the original file which was failing? There's no need to pin the blame on anyone, but you can help improve things for everyone by reporting the issue to the BeebEm team with a solid way to reproduce it. In my experience, part of being a programmer is to understand that all software ...

Have you tried pressing the Turbo button on the server?

Honestly a lot of bots in the forums at the moment. Right now I see Ahrefs [Bot], Bing [Bot], Bytespider [Bot], Google [Bot], GPTBot [Bot], Majestic-12 [Bot], PetalBot [Bot], Semrush [Bot] and YandexBot [Bot]. Don't know what half of them are, but it'd be good to sort out the bot group permissions a...

I wasn't aware of having 2FA set up on my Github accounts, although I just logged into an infrequently used one and it asked for a verification code sent to my email. Maybe that's 2FA, but it didn't involve needing a phone, if that's your pain point!

To me this looks like interlaced sync and video not working correctly - I don't think the non-S 6845s support this either, so what you're effectively getting is a regular interlaced mode with 19 scanline high characters!

Yep, that's the alpha max plus beta min algorithm I linked to above! An amazing approximation!

I think it's subtle but the convention seems to be: x 2 = 4 x = ±2 Here x clearly has two solutions, so the answer is ±2. x = 4 √x = 2 Here we just consider the principal value of the function. This is equally true of things like inverse trigonometric functions which in some sense have an infinite n...

Born in '75, grew up in Surrey. I don't remember BODMAS being explicitly used to teach this, although I remember the mnemonic. It's overkill for what is a simple concept, and also not particularly clear. Also what does the O stand for?! By the time you're writing expressions with 'proper' notation (...

Another thought, given your use case. Since you have a limited range of inputs, perhaps you can do this with two (and a half) tables: - One table mapping (dx,dy) -> arctan(dy/dx), scaled to 0...128 for 0...90 degrees - Another table mapping x -> sin(x/128 * pi/2), and an overlapping one for cos. So ...

Looks great! The frustrating bit is still that I have to divide the existing components by the magnitude, unless I use a reciprocal lookup for the magnitude. But I'm using fixed point numbers so that would be quite a challenge. I don't think so - the fractional part of your fixed point values will c...

This is another useful magnitude calculation hack, a bit better than just taking the maximum component: Alpha max plus beta min algorithm It can be expanded to handle 3d vectors in the following way: // get vector components in min, med, max value order min = abs(vec.x) med = abs(vec.y) max = abs(ve...

I think the point is more that triangles are a kind of basic primitive - you can build any filled polygon from triangles - however, a dedicated quad renderer will always be faster than rendering the equivalent two adjacent triangles. But consider that such a quad renderer will almost certainly be ex...

I'm not feeling well, but could you swap a and X and then cpx #3 ADC #&ff I don't think so - I was trying to find some way of doing that, but the carry flag needs to be inverted (where's the complement carry flag instruction?) and then ADC #0. I guess you could put EOR #255 either side... or CL...

I have always just used LDA &FE64:LDX &FE65:CMP #3:BCS P%+3:INX:STA &80:STX &81 to read the 16-bit timer value. It doesn't quite run in constant time but I've never found it to matter much (if you really cared I guess you could align the code so that the taken branch crossed a page b...

The falling edge of 1MHzE and 2MHzE always coincide. Yellow = 2MHzE Blue = 1MHzE even case: https://stardot.org.uk/forums/download/file.php?id=49508&t=1 odd case: https://stardot.org.uk/forums/download/file.php?id=49507&t=1 Excuse the bump! I was looking at the circuit diagram and saw that ...

[The 6522 datasheets are clear: writing ORB clears the CB1 interrupt flag. Ergo the CB1 IRQ can go missing in a small race window. With the caveat that, if you write the non-handshake version (&FE4F), that doesn't happen. And I think that's generally what the OS does, though I might be mistaken!

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It's not really very obvious to be honest. ~ isn't really an operator in the sense that you can use it anywhere to turn a numeric value into a hex string - it's just a special case hacked into STR$ and PRINT.

My old build system used to work something like this: File S.Core: 10 LOMEM=&3000:HIMEM=&5000:top%=TOP-2:code%=&1100 20 FOR pass%=4 TO 6 STEP 2 30 P%=code%:O%=HIMEM 40 FOR file%=1 TO 5 50 OSCLI"LOAD S.Part"+STR$file%+" "+STR$~top% 60 PROCassemble 70 NEXT 80 NEXT 90 OS...

Yeah I guess that's what I was thinking! Though you would probably want to pass the pointers in R0 and R1, and then use R13/14 as scratch registers once you've saved them somewhere. Note that in your case you were looking at plotting smaller sprites than 32 pixels wide, so you wouldn't use so many r...

I started a thread here on this subject which was interesting until it got disrupted by bickering. Maybe some useful stuff there to start with?

What are homebrew Beeb builders doing for parts which are no longer available, like the Video or Serial ULAs? Raiding old dead Beebs for parts, or are there modern alternatives à la VideoNuLA?

I think the more surprising thing about JSR (particularly from the perspective of an emulator author) is the order it does its memory accesses. Most absolute mode opcodes fetch the lab and msb of the operand on the second and third cycles, but if you look at Visual 6502, or the 6502 hardware manual,...

I'm sure there's already a Beeb rendition of the theme tune out there somewhere!

(Edit: yes, of course there is...
https://www.youtube.com/watch?v=tvfu25w4hWg )

Never really played with ROMFS or created anything for it, but could never really understand how or if it were possible to have a file spanning more than one ROM. Could one ROM hand off to another mid-file? Or how would two independent ROMs cooperate on providing ROMFS content if they would generall...

Yes, I think I once concluded that the result of INKEY(-256) was undefined, but yielded different and consistent values in OS 0.1 and OS 1.20. And people realised they could use it as a hacky way of determining the OS version (maybe OS 0.1 didn't have OSBYTE 0?), so they did. And after that it was a...

beebjit, while not implementing an instruction level emulator, does emulate the internal register usage and general timing implied by our reverse engineering effort. This is apparently enough to reproduce many, if not all, of the corner cases and hackery which we see on real hardware! It'd certainly...

DEC wouldn't work though if the source block overlaps with the destination block (I don't know how big the block is that's being copied). Honestly the very simplest workaround if possible is to index with Y, and use X as a counter for the memory pages you copy.