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Does BeebEm emulate video at a cycle-accurate level alongside the CPU yet? Last time I looked (many years ago now) it was batching video update scanline at a time, which could well explain why you're getting a scanline out, even if the VSync pulse width emulation is basically correct.

People are naming games I've never even heard of here...

The other obvious approach (and the one that it looks as if 6502 BASIC does) is to scale the entire range of the RND function into 1...x for RND(x). Basically a multiplication by x/RAND_MAX. No idea what Z80 or ARM might be doing though! Edit: actually 6502 BASIC is scaling up the results of RND(1),...

An alternative strategy, if you were trying to avoid the cost of a modulus, might be to mask out excess bits (AND 127 in this case), and then try again with the next value in the sequence if it were out of range.

I had a quick browse through some of the source code. Amusing to see so much profanity liberally scattered throughout. This stuff probably wouldn't pass code review these days! There are no doubt assembler labels with daft names too (from when you need a label when hurriedly writing code, but don't ...

As far as I know, there wasn't an FDI, only an FSD. And the FSD format has shortcomings here, in that it doesn't model the data properly, but just marks the sector as returning a Data CRC error (&0E) when read, without specifying why. In this case, the fact that there are weak/fuzzy bits requires ex...

scarybeasts wrote: ↑

Thu Feb 13, 2020 10:59 pm

[ITRP] 0492: LDA #$7F
[ITRP] 0494: LDX $0490
[ITRP] 0497: LDY $0491
[ITRP] 049A: JSR $FFF1
[ITRP] 049D: LDA #$0E
[ITRP] 049F: NOP
[ITRP] 04A0: CMP #$0E
[ITRP] 04A2: BNE $049D

^ Looks like you've hacked out the test there so that it always loads. LDA #$0E:NOP should presumably be LDA $04A0!

Warning -- loading Boulderdash is a real nail-biter, it takes a long time and the disc will pause for seconds at a time while it does some CPU intensive unpacking. Is that just because of the decryption code, or is it because the disk controller is having to retry multiple times to find sectors (ma...

I think it's fair to say that every language has its idioms, and BBC BASIC is no exception. I think you've sold the construct pretty well, and I can see how it's useful in those contexts. I haven't seen it used very much, and the last time I did was as a hack to allow a one line program to call itse...

Makes sense, yeah! And, in general, the required lengths of things are double for MFM than for FM, e.g. MFM requires 12 zero bytes in gaps, compared to 6 for FM. So it seems like they've done some pretty thorough research for that 1770 emulator!

Yes, I'm also a big proponent of that, but I'm not sure if BASIC, in general, promotes that concept because it's not strictly scoped: a variable, once created, exists and is visible globally for evermore (unless hidden by an explicit more local scope, in BBC BASIC). With global variables, if they mu...

In that case what alternative approach would you propose for creating a variable if it doesn't already exist, but not changing its value if it does? Again as I mention in the other thread, such functionality is useful in a cleanup routine, which may be run as the result of an asynchronous event suc...

Yes, fair enough. I remember from MS BASIC on my old Oric-1 that variables were created on first use (along with the other MS quirks of only the first two characters being significant, and arrays being dimensioned by default to 10 elements!). Still not really fond of the paradigm personally, but if ...

I guess I have the C++ orthodoxy of "declare all variables before using them" too ingrained!

Wow, that Boulderdash format is weird. I just looked to see if there were any way it could fit, and even reducing gaps 1 and 3 to zero, it still needs 3146 bytes. So is data slightly more squeezed together (written at less than 300rpm?) or is the sector actually incomplete? Gap1: 0+6 For 9 sectors: ...

I'd not spotted that example before, but it's not a safe substitution. 'Klingon%', as a normal dynamic variable, will be deleted by a CLEAR (or CHAIN) but 'K%', as a static integer variable, won't. This could cause a program to misbehave, for example: 10 Klingon% = 123 20 CLEAR 30 Klingon% = Klingo...

The only time this could be a problem, as far as I can tell, is in the case highlighted by Richard's example in the other thread, where you write: A=A+1 without having previously defined A. While this is apparently valid BBC BASIC, I suspect the language wasn't designed with this in mind, and only w...

Out of interest, how do you arrive at the 14.74 MHz figure, Richard? I've never seen any rigorous method for determining the pixel aspect ratio on the Beeb.

Here's a nice Bresenham filled circle routine in BBC BASIC which I wrote with the intention of converting to 6502 a while back: 10 MODE4 20 GCOL 3,1 30 FOR n%=100 TO 10 STEP -10 40 PROCcircle(160,128,n%) 50 NEXT 60 END 70 : 80 DEFPROCplot(x1%,x2%,y%) 90 MOVE x1%*4,1023-y%*4 100 DRAW x2%*4,1023-y%*4 ...

Yes I would prefer to de-interleave clock and data bits, for human readability and for greater compressibility too. Whether this were done at the byte level or greater, I don't know. However in a real disk surface, it'll be possible and even likely that the amount of data per track is not a whole nu...

I would actually argue in favour of IPF, simply because it's easier for emulators to consume, leads to smaller files (I still care about that, even if few do), can represent all known forms of 'exotic' disk surfaces, and is quickly establishing itself as a de facto standard. It's a curious omission ...

Ah, this is the code that was said to be a disk formatting routine! Not quite as evil then, but, in a way, more irritating if it just stops disk writes from working reliably.

Aw, thank-you! Glad you enjoyed the videos, it’s great to hear people appreciated them. I’m currently in the process of splicing all the videos together with the annotations back (thanks to scarybeasts and garfield) into one single (or possibly 3-4?) long videos. Thank-you SO much to the people on ...

Re: B-EM, BCPL, SBC and the Overflow Flag [this post about testing generally] Nightshade protection [technical] BBC Master timing-sensitive protection systems jsbeeb - a Javascript emulator Cycle stretching when accessing 1MHz devices More curious 6502 behaviour (mentions Lunar Jetman) Plea for hel...

Pretty certain the Rydeen track in Striker's Run is from Super Locomotive, because they also nicked the 'Bonus Stage' music from Super Locomotive to be the Striker's Run high score entry music. https://www.youtube.com/watch?v=o89fWnolC4U But this also is a slightly different rendition! But, yeah, S...

Sounds like a different rendition to me - a lot closer to the original YMO version in fact!

I think it's far less useful if it can't present a flat memory model. But I'm not sure how the timing could work to handle the bit 15-16 carry (which as you say would only need to check a single bit discrepancy). I think potentially the CPU would need to be stopped twice - first time after loading z...

The SWI system will always have needed some kind of macro wrapper around it in C, generating the correct instruction, because the SWI number was part of the opcode, rather than any kind of value set up in a register. I think, when Acorn introduced the ARM6 or 7, which had a separate data and instruc...

Simply an alternative to a comma, I think; not sure why semicolon is accepted here at all. I think semicolon was the "standard" BASIC syntax for an INPUT command. Certainly MS BASIC expects a semicolon (comma is a syntax error). That's a new one on me. I assume it's a RISC OS feature and if so it m...