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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!

I was really hoping for something like this. Are they writeable, then? Can you just bypass the byte processor, and program the bit processor directly? Because that would be rad. Externally you could "write special register" and hit the bit processor directly, yes; whether the latency involved in th...

I think the second one from the left table has an error. The bits are 0000xxxx, not 0001xxxx. The '0' select line folds around from underneath the next row of bits. Yes, that would make sense as there's already a 0001xxxx on the left hand side (and there isn't a 0000xxxx). It didn't make any differ...

How do the registers work, Rich? I see a lot of MOV Ix type stuff, but then you have references to Rx in the comments. What about the register banks? What's the dealio with those? So, we know there are 32 internal registers (we can see them in the photo!). But this is a lot to encode inside an opco...

The PLA just takes 16 inputs, 8 matching set opcode bits and 8 matching unset opcode bits. Each column represents some property or behaviour of a given opcode; most opcodes will fire more than one PLA column as they will share broad functionality with others, while specific behaviour will be due to ...

What's on the left-hand side of each cell? Are they likely to be another set of PC values in there? I'm not sure why there would be values encoded in silicon there that aren't used for anything. They are a 10 bit key, broken up as (2, 4, 4). There are three opcodes which build a key in certain dist...

Have you correlated your assumed opcodes back to the physical columns of the two blocks (or have I misunderstood where all that's come from)? Yes - in as much as we can determine the smallest blocks of opcodes which the PLA handles in the same way, and presume them to be small variants of the same ...

What are you calling the 2-bit id? The extra two inputs to the narrow ROM (jump table?), or the high two output bits? I've not kept up with what names you're using for different parts sorry. The inputs to the narrow ROM are what I'm calling the 2-bit ID - the high two output bits are just the top t...

No, the unknown 2-bit ID remains unknown! At least in some cases it seems to be hardcoded according to the event which occurs (index pulse, parameter register write, etc); there also seems to be an opcode ($EE) which forces it to 2, along with a 4-bit index from the current selected register bank (o...

I think the breakthrough sequence was approximately: ...rewind a bit more! It's interesting to document how it's possible to start extracting order from 1k of random data. - The very first breakthrough was Chris reversing the dumped bits and noticing 'interesting' looking constants like E5 (default...

Hey, that's great work! =D> Do you think you could run this process through the best photos we currently have of the 8271 and get a new dump of the ROM (as we've found a few transcription errors here and there, but there may be more). Also, will the tool work on the narrow ROM which contains the ent...

melchett wrote: ↑

Mon Sep 07, 2020 8:19 pm

Did I hear correctly that Peter passed away a few years ago?

Thankfully no! His latest venture seems to be Soluble Apps, and he's active on Facebook too.

By the way, I really don't know that FC is CALL or FE is JMP. I've just been assuming that because it makes most sense in the context of the ROM dump, and we can see if it helps make sense of things as we carry on tracing lines. But let's not be surprised if it turns out to be completely wrong.

Here's one interesting thing, Rich -- there are fourteen columns in the left table which don't have any don't care-looking thingies in them. And the number of columns in the right table which don't have any don't care-looking thingies in them is ... also fourteen. Money's on them being the same opc...

Incidentally, one mystery I'd like to solve is how 2 byte opcodes are described. There doesn't seem to be much of a pattern in that, and I imagine that, of all those signals, some of them will be OR'd together to specify 2 byte opcodes. (I guess the signal is then latched, and causes the subsequent ...

https://stardot.org.uk/forums/download/file.php?id=60806&t=1 Great job! Here's my interpretation of this table in terms of opcode ranges: 111x xxxx E0-FF 0001 xxxx 10-1F 1111 1100 FC 1111 1101 FD 01xx xxxx 40-7F 1111 1110 FE 1000 1xxx 88-8F 1110 11xx EC-EF 1111 1111 FF 0001 xxxx 10-1F 1111 1000 F8 ...

That was the exact code I was going to post, although I would've left the LDA #19 outside the loop

Y'know... For speed!

The NMI routine is presumably located directly at D00 for maximum speed, that way saving even an indirect JMP through a vector. Acorn presumably already planned to use NMIs for peripherals which required immediate and frequent response. It's a shame there wasn't a way to at least sidestep the IRQ vs...

What would be amazing would be if we could start tracing the bits of the instruction register into the PLA -> registers and ALU. Yes! Assuming that the decode PLAs are the two things left and right of centre, it'd be great to start trying to get a broad overview of how the opcodes are structured. O...

Interesting... it's very common for routines to contain sequences like this: ; ** entry point 19 ** 1AC F5 E5 ; sector fill byte data 1AE F4 FF ; sector fill byte clock 1B0 FC D1 ; CALL 1D1? 1B2 98 E5 ; LD A,#E5 1B4 FC EF ; CALL 1EF? Add byte to CRC? 1B6 0D <------- (segment?) 1B7 EE <------- (yield...

Who designed the BBC Micro character set in the OS? It's very distinctive, and different from any other home computer of the era. Why the enormous unrolled routine to clear the screen in OS 1.20? Most of the OS code is extraordinarily concise, so it's surprising to see so much memory spent on this. ...

Thanks both! These addresses make considerably more sense when entered into the ROM "disassembly"! In particular, it marks out some specific addresses which I already had earmarked as entry points. https://docs.google.com/document/d/1bQTvncIcgRfO0zOvBB16x37WfTh71c4U5WWCyQ4lFVM I've revised some idea...

Wow, just had a chance to look at this! More detail there than I could possibly have imagined - great work! And nice to confirm that it really uses the Minsky hack for rotating things in place by small angles. Will look forward to browsing through this properly when I can!

That's great! And now we have some good guesses as to where separate routines might be in the ROM, which will help with corroborating observations from the die. I'm now a bit lost on whether we still think the narrow ROM is responsible for holding these addresses, and what its inputs and outputs are...

If we could somehow get a list of routine start addresses, that would be very helpful. I'm working with Rich to try and work out the opcodes from the ROM. We do have some good leads but deciphering an unknown ISA isn't easy. I figured the best thing at this point is just to share what we've got so ...

Has anyone yet managed to transcribe the contents of the narrow ROM to the right of the program ROM (bottom left), which may be something to do with the multitasking madness? I'm intrigued by what it might contain.

Ed, I think you're right about offsets into the ROM for entry points. So where's the decode PLA? Surely there must be one?

Ahem, yes, good point. And indeed we see the PLA controller for the bit level processor is full - so yes, indeed, it's the other application. What's still slightly surprising about this is that if the photo from the paper is an 8273, I'd still expect the decode PLA for the byte processor to be the ...

I moved the topic into a more appropriate forum, but left a shadow topic in place so the OP would be able to find it! I guess... quirk of the phpBB code?