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Apple ][ is 280x200 I believe (each character block is 7 pixels across, bizarrely). So it may well have come from Applesoft BASIC originally.

The reason is so that 'forward searches' (such as to find the ELSE in an IF clause) can be performed very efficiently, because you know you won't encounter the token for ELSE (in this case) in any other context. If you allow arbitrary binary constants to appear in a program, any such search needs t...

I've occasionally contemplated whether you could improve the performance of 6502 BBC BASIC by tokenising numeric literals. With an extra 16k of ROM there'd almost certainly be room to do that along with other features. I'd imagine replacing the ASCII representation of the number with a token byte, f...

In the third program edit line 60 to change the plus to a minus: - 60 PRINT CHR$(32+3*(C-INT(C)<.4)); + 60 PRINT CHR$(32-3*(C-INT(C)<.4)); I guess Apple BASIC uses +1 for TRUE. Yes, I also spotted that. There's also a boolean expression in line 40 which needs changing: - 40 A=ATN(Y/X)+(X<0)*SGN(Y)*...

From memory, I think B-Em scrimps on ADC emulation and doesn't generate and handle as many IRQs as real hardware. Have you compared with jsbeeb to see if you get more reasonable results? Also try turning off the ADC (*FX16) to see if that gives closer results on B-Em. Can't speak at all for BeebEm a...

It's amazing to think how we did it back then: armed with just BBC BASIC with its built-in assembler, its clunky line editing, and ingenious ways of working around the memory limits. Matt and I did the first version of Blurp and a few other things like that, although we'd written a ROM which gave us...

Another possibility is only allowing time-limited editing, apart from the first post of a topic, via this extension: https://www.phpbb.com/customise/db/extension/gn36_firstpostedit If you've already read a post, it's easy to miss any subsequent edits to it, so this doesn't seem like a bad idea to me...

I have had a look over lunch, it does seem fairly easy to make a forum theme based on the current one that omits the edited message footer unless someone puts something in the reason box. If this sounds like it would mitigate most concerns then I'll put that into play this evening - it will require...

A good while back, I think the forum used to report all edits, and then this disappeared with one of the big upgrades. Since then, if I edit one of my own posts to add additional content or correct issues, I nearly always prefix it with 'Edit:' myself to make it clear that it's been added. I think t...

Cheers for the answer. However it looks a bit complicated for me. I am using a SD collection of games so looks like I need that specific drive with the game and load it manually? I do use special menu otherwise http://joxi.ru/v29Xeq1i3pY95m Yes - mount the disk image which contains Labyrinth and tr...

Any idea why that was done? :?: Maybe they thought it gave the graphics a strange swirly ethereal quality (which it certainly did on my TV) and thought it fitted the game. Or more likely it was just a mistake. It also has the effect of lowering the frame rate to 49.3fps but I doubt they did it to w...

Found it! This works on the Acornsoft release on the STH site - haven't tested any other versions.

Yeah Labyrinth programs the video controller with non-standard values, specifically the horizontal timing is too long for a PAL display. My TV even used to have some small problems with it back in the day. It'll be easy enough to patch I suspect, but it'll need someone to find where CRTC R0 is writt...

timmy wrote: ↑

Thu May 24, 2018 8:52 am

My school favourite was to do a quick:

Code: Select all

?&FE40=0:CLS

on all the machines before the teacher came and then sit down and wait for people to start typing...

What does that do? Enables sound chip writes at the same time as keyboard scan? It doesn't work on jsbeeb or B-Em... I wonder why?

Amazing the technology you can get these days.

I fully expect GitHub to end up with adverts everywhere though The annoying animating type that ended up all over Skype and MSN Messenger before it.

A counter which is preloaded and counts down to zero is very much preferable - but the block diagram doesn't indicate that.) And indeed we know it not to be the case, as it's possible to change registers during the CRTC frame, and their new value will be correctly used as the upper bound. Are there...

Just pondering this again this morning. I think, in terms of simplicity, everything makes sense apart from one thing. Are we sure that R12/13 take effect when set in the final scanline of the previous frame? Kieran, do you have concrete evidence of this working OK in your demo, or is there a chance ...

This does make sense. Looking at the b-em source code it's doing approximately the following: IF HorizontalCharacterCount==R0 AND ScanlineCount==R9 AND VerticalCharacterCount==R4 THEN VerticalCharacterCount=0; FrameCount++ Presumably jsbeeb similar? What do we think that logic should look like? Bot...

The easiest (and hence cheapest) counters have unsynchronised outputs. That is, propagation delay of the logic circuits causes the count to ripple through each stage, with the output of each stage changing one after another. This results in the output having invalid data for brief time periods. The...

1024MAK wrote: ↑

Wed Jun 06, 2018 9:35 am

Here is a diagram of a 74LS688 comparator from a TI datasheet:

Happy I guessed that right, at least Even if I didn't realise it would invert the inputs (or indeed the output!).

There are some curious things I haven't yet figured. For example, the Scanline Counter has as its MC input: SLC==R9 OR (something from the Linear Address Generator). What can that be doing? Also interesting to note that the Linear Address Generator gets Hend as an input - this is so that it can cac...

Looks like my interpretation of MC is wrong - thanks Mark. So MC will perform an immediate reset then. (Edit: although here 's an example of the sort of circuit I was imagining: a synchronous counter with a synchronous reset.) By the way, I think CC on the diagram is just another CO that's scanned b...

Edit: Mark just posted a load of great stuff, but going to post what I had here anyway, in reply to Ed! That sounds more complicated Ed! I assume a comparator is no more than a bunch of ANDed XNORs which surely can't have a prohibitively high transistor count. My mental model for this is as follows:...

That all sounds very plausible, Kieran! It makes sense once you realise that the timer itself will trigger a scanline early; it also helps to explain why the first scanline was apparently displaying OK when, by the same account, you'd expect setting R4=0 during the first visible scanline to be ignor...

With the diversion on restoring 'normal' CRTC conditions, I think I've lost the discussion here! Kieran, can you explain again what problems you're seeing now on real hardware, compared to emulation? During the demo (and also during the transition)? Just to make sure I understand your code well enou...

Yeah, that was my take on it as well (the "race conditions" I speculated about above). I wouldn't be surprised if every register which holds "value minus 1" is matched to the internal counter early, and the results latched until the following time round. But it still doesn't quite explain some other...

Turning off the ADC converter got about 77px/s. Interestingly, now jsbeeb and b-em are reporting the same times. I think jsbeeb might be skimping on the ADC emulation... Memory fails me now, but I think it's the other way round. We had a timing test where both B-Em and jsbeeb were out compared to r...

I think at this stage, the OS interrupt routine is going to be adding noise to the timing results. I'd be inclined to go to 100% assembler, and shut out the OS, so you could use all the zp and have complete control over IRQ handling, but short of that, you could try reducing the OS IRQ footprint. Tr...

@rtw: d'oh! Obvious in hindsight. Sadly, tsb only has zp and abs addressing modes, so I still need an ora (screenptr); sta (screenptr), but that's a nice 15 px/s. Hence my other suggestion to find a way to move the routine (or some of it) into the zp so that the operand of the TSB abs can actually ...