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PiTubeDirect: A Raspberry Pi as a BBC Micro Second Processor (github.com)
70 points by fanf2 on Jan 10, 2018 | hide | past | web | favorite | 24 comments

For more background on the whole Second Processor system, see the "Acorn's Second Processors and the Tube - what, and why" thread at Stardot: http://www.stardot.org.uk/forums/viewtopic.php?f=3&t=14211

On that note, Stardot is a small but thriving community around the BBC Micro, and Acorn Computers' other machines -- the Acorn Atom, Acorn Electron, BBC Master, BBC Master Compact, and the Archimedes/RISC PC machines. The author of PiTubeDirect is an active poster there, and the tone is generally very friendly and on-topic. I've been reading/posting for the last year or so and it's turned Acorn-focussed retro hardware into my favourite hobby. I'd highly recommend taking a look if you used/loved one of these machines back in the 80s/90s.

Wait...Acorn as in 'Acorn RISC Machine'/ARM? I didn't know they made actual consumer hardware, that's pretty cool. Sort of funny that the modern BBC Micro:bit went on to run on a Cortex-M0 core.

I also kind of love that the BBC's learning hardware calls its method of routing instructions to coprocessors 'The Tube'.

The early ARM machines didn’t really make it out of the U.K. I had some early Acorn/ARM stuff in 1988. It was by far the most advanced bit of kit at the time. Literally smoked everything and did so for a number of years afterwards.

Example: 1987 graphics demo: https://youtu.be/653Ger80ros

They even did a line of unix workstations!

Nice video on the matter here from 1987: https://youtu.be/hrj-EEnsacQ

Acorn Archimedes - A Technical Introduction - 1987 BBC VHS Video. [1] Quoting from YouTube description: "Fred Harris talks to Roger Wilson about the technical aspects of the Acorn Archimedes A305. Also features footage from Zarch by David Braben. Produced by the BBC. Digitised from VHS Video Tape by The Centre for Computing History."

According to Wikipedia [2]: "The Acorn Archimedes was the first RISC-based home computer." and "The Archimedes was one of the most powerful home computers available during the late 1980s and early 1990s". Article also includes basic performance comparisons with m68k (Amiga & Atari).

[1] https://www.youtube.com/watch?v=KKTa54UikgE

[2] https://en.wikipedia.org/wiki/Acorn_Archimedes

The ARM was protoyped, designed an simulated on a Beeb, first silicon worked unmodified.

The BBC was the ultimate tinkerers platform, the underside of the case is a long row of headers for various expansions.

Well it was primarily designed as an educational tool, and thus a direct inspiration to the RPi.

After all, David Braben, creator of Elite (that originated on a BBC Micro), is also co-founder of the RPi foundation.

I can highly recommend the BBC drama "Micro Men", about the early years of the British home computer boom and the rivalry between Sinclair and Acorn. It gives a real flavour of the mad, maverick years before the IBM clones took over.


Not technically routing instructions since each processor executed only from its own memory. To execute code on the other processor you'd have to do a block memory transfer over the Tube, then send a request to execute code at a stated address.

Driving the databus GPIO pins using the VideoCore as a second processor is a really clever hack, especially on the single-core Pi Zero. I guess it's logical that the GPIO can be driven by the GPU in this architecture but I never really thought to do it before.

Came to say this. I wonder if this hack is widely known, because it seems like this is a pretty clever way to use the Pi Zero in some applications that would be better served with a multi-core processor and don't require the GPU for video tasks. Hacking the Pi Zero to be a dual-core system essentially.

Could four Pi Zeroes become a Pi 3? :)

Clustering the Pi has always been fairly easy [0].

[0] http://www.southampton.ac.uk/~sjc/raspberrypi/

Hah, neat to see the 32016 mentioned, I still have a pre-production series sample of that chip somewhere in storage.

Super cool this, I'm almost tempted to score a Master or a Model-B on ebay or mp to replicate this but there just isn't time enough in the day to get sucked into retro computing. Extra points if they ever manage to port the basic to the native Pi including inline assembly, that would be a pretty neat environment to develop stuff for the Pi in.

There is a native port to the Pi of RISC OS, the ARM-based operating system that ran on Acorn's successor to the BBC (the Acorn Archimedes). The BASIC on that is also BBC Basic, including inline ARM assembler.


BBC Basic for x86 systems also includes the inline assembler.


Now that’s what I come here for. I had a Master with 65C102 coprocessor (internal one) and I still claim to this day that it was the finest machine I have ever used. Rather cool what you can achieve with the tube even to this day.

You’re not wrong. Mine is setup on my desk at home and still gets regularly used.

Glad to hear it. I’m scouting for one on eBay now :)

For some reason i thought this involved the Micro:Bit.


To really close the circle, someone should implement this hack with a BBC Micro:bit, which is the spiritual successor to the original Micro.

Are BBC Micros or other Acorn machines generally available outside of the UK? I don't recall having seen one on eBay in the US (at least, not when I went looking..) I would imagine 80s systems were designed to use PAL rather than digital output?

I've seen a few from US-based sellers on eBay, but most are in the UK. Pretty much everyone I've met in the USA with Acorn hardware grew up in the UK and moved here.

I use an RGB-to-HDMI converter (this one -- https://retrocomputing.stackexchange.com/questions/2553/acor... ) and connect my BBC and Electron up to a normal HDMI monitor. The native video format is 1-bit RGB with TTL levels, with a 50Hz refresh rate (possibly interlaced -- I forget).

Not just designed.

IIRC, C64 games often ran slower on the European variant. This because the CPU clock had to be synced with the video output.

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