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That screen would compress down to 30% I'm sure!

Sorry, this won't work in BeebAsm. String variables aren't supported at all, be they regular symbols or macro parameters. String literals are really no more than a special case hack for whichever keywords take them (EQUS, INCLUDE, etc)

My guess is to defer the sound/music update until after the blanking period, so the game has all the CPU time available to update as much of the screen as possible before the next frame starts being rasterised.

That indeed seems to be the approach taken by the code in that Acorn User article!

Oh, I quite like the ninja TRACE and unTRACE. And good to hear that extending Basic by trapping syntax errors (or other errors) is practically workable. I found a couple of articles in Micro User March-April 1988 which shows how this is done, but they weren't the ones I was thinking of. Perhaps it ...

That's clever! I guess there are various ways you could do this - another way would be to search forward for the next BASIC line on the interrupt, and replace the first character with something which would never occur (even a control code), and then catch that using the same mechanism with the BRKV,...

In fact, this is the sort of thing that would go in a sideways ROM, not in the MOS. The MOS provides the core, sideways ROMs provide extensions to the core. I think there were certainly some missed opportunities in OS 1.20, and the input handling was one of them, as good as the split cursor editing...

I thought that was a Master-only thing, but trying it now on jsbeeb, I see it works in BBC B mode too! Live and learn...

I remember a Micro User article which added new keywords to BASIC (a hack involving intercepting the BRK vector, and responding to BASIC errors when it encountered a "Mistake" by parsing the current BASIC line, doing the new action, and then jumping directly back into the BASIC ROM to continue execu...

I was thinking about that myself. It could've been implemented in the BBC OS with minimal memory overhead by remembering the last buffer address passed to OSWORD 0, and storing the length and a CRC of the last typed line. Then, on a subsequent call, if the buffer address matches, and the CRC of memo...

Shift Lock was always totally useless, but I got to thinking what could have gone in its place and the only thing I could think of was "Scroll Lock". I guess that could've had its uses.

It's probably a few hundred lines of BASIC!

It looks like it directly calls various routines in the OS which have no public entry point, e.g. the routine which plots a fast horizontal line between two points, filling byte at a time. So the specific version of the GXR ROM has to match the installed OS. I can only guess that the GXR ROM was alr...

I think Wizadore would've fitted the Ultimate catalogue perfectly, but I guess the platform was a bit too niche compared to Spectrum/C64. It's actually well programmed too, with quite a few big sprites at once and some nice scrolling (including a stationary part at the bottom), but it's a shame it's...

Reasonably sure this is to do with the value the RAM starts off at on different motherboard revisions. The Beeb doesn't initialise any of the zero page, or the first byte of every page on reset, and on early Beebs they will be 255, and on later ones, 0 (I'm not really clear why this is). So if a gam...

Sometime I'll have to dig out my old laptop and try and tidy up the source code and get it on Github. These days, it pales into insignificance compared to some of the amazing stuff Tricky has produced. But it pioneered a few cool techniques back in its day :) I found a disk image of it, so you can s...

Watching a playthrough on YouTube, I noticed that the sword is emblazoned with the familiar initials ACG (Ashby Computers and Graphics) as per various other Ultimate Play the Game titles. So it seems as if Wizadore was to be published by them at some point. It's very much their "type" of game. Pagin...

Yes, I was going to say the same thing. Perhaps it would've been better to have the language ROMs paged over &C000...&FC00 (with the final 1k never paged out), but I think it would've been hard to lose 1k of code from the BASIC ROM. Skip that, it would never have worked. I think the memory map was p...

In the particular example you gave (ROR abs,x), here's how the cycles break down. Let's say we're executing ROR &32F0,X with X=&28. - Fetch opcode ROR abs,X, increment PC * - Fetch low byte of address (&F0), increment PC - Fetch high byte of address (&32), add X to low byte without carry, increment ...

Hmm, true. I don't think there's any synchronisation with vsync at all, so it could indeed happen at any moment and result in the occasional frame without vertical display disable. Perhaps some monitors expect no signal during vsync, and won't sync if there is.

That's weird. From Diminished's disassembly, I only see it setting CRTC R6 (vertical displayed) to 2 while it redraws the screen - nothing there which would affect VSync, or have that kind of strange effect. It also plays with R5 (vertical adjust) during gameplay, to shake the screen when the player...

BeebEm, in spite of its continued popularity, isn't exactly renowned for its accuracy. I suspect B-Em and jsbeeb are doing the right thing, but the question is why? You definitely need 16 cycles between the two writes (I normally put two dummy LDA (&00,X) instructions, or try to fill the time with u...

+1

Always happy when this thread resurfaces! Citadel has always been a standout game on the Beeb, and it's fascinating to learn some of its inner secrets.

lurkio wrote: ↑

Thu Oct 08, 2020 3:28 pm

EDIT: Looks like you need to change 254 to 253 (on line 40).

Thanks, yes, you're completely right! I couldn't do 2 EOR 255 in my head properly. Have edited my post accordingly!

I use VDU 5 for that literal corner case as suggested before There is also another way, if you are targeting 8 bit machines without a second processor. 10 MODE 2 20 ?&D0=?&D0 OR 2 30 PRINT TAB(19, 31);"*"; 40 ?&D0=?&D0 AND 253 Line 20 will disable scrolling entirely, and line 40 re-enables it. It's...

This disassembly more or less confirms the original names for the COLOUR, GCOL and SOUND keywords that I speculated about here!

TBAS01 wrote:

Code: Select all

 = "COLOUR"
 = TTEXT,2

Code: Select all

 = "SOUND"
 = TBEEP,2

Code: Select all

 = "GCOL"
 = TGRAPH,2
 

No hints on what VDU used to be called though (the label is also called TVDU).

There's the "castle and cloud" demo on the Master Welcome Disc if that counts? Uses shadow RAM to double buffer the cloud animation.

Isn't Master Elite less flickery, due to the graphics being undrawn/redrawn on a line-by-line basis, instead of undrawing an entire model and then redrawing it? I think Tube Elite (like the original) used the latter method.

I remember it well! When Castle Quest was circulating in the school computer room as the new game du jour, I couldn't wait to get home and try it on my Master, with exactly that result! Wasn't the first time my Master had let me down (Elite, Knight Lore, Frak, Zalaga, Contraption, Citadel...).

That's awesome.

And you may also possibly have written the most niche program ever written!