Acorn's series of BBC Micro were pretty popular in the UK, and had some presence elsewhere too. They have a lot of I/O and expansion capability and a relatively sophisticated OS with an official API. There's an active retrocomputing scene, with several emulators and with new hardware and new software being produced. For an overview, see https://en.wikipedia.org/wiki/BBC_Micro

This is a thread for helping the curious and interested to get to grips with the machine. Of course it boots into Basic by default, so you don't have to know much to get started.

The basics:
- 2MHz 6502 CPU, builtin keyboard, video to TV or monitor, speaker, cassette interface.
- Graphics in teletext mode or bitmapped, from 1k to 20k memory footprint, up to 8 colours and flashing colours.
- 16k Basic with commands for graphics and sound, inbuilt assembler, 5 byte floats, fast integer variables, file i/o support, some support for structured programming.
- 32k RAM at bottom of memory shared between OS and application. Screen RAM is a variable amount at the high end. There's a 1k screen mode which the machine boots into.
- The next 16k is for application and language ROMs (and sometimes RAMs.) They interact with an API to register their capabilities, respond to OS requests, claim and free resources. Basic is a language ROM, factory-fitted, leaving 3 on-board sockets free. There are 16 logical slots, and internal expansion boards were popular to extend beyond the 4 supplied.
- OS is 16k at top of memory, offers a fixed ABI at top of memory. The OS has the idea of a current filing system and a current language. Also a current input stream and current output stream. It's interrupt-driven to handle buffered I/O, sound output, keyboard scanning.
- there are 3 pages of I/O space just below the top page. All on-board peripherals are in the highest of these pages. Notably two VIAs, control of ROM switching, of video config, of serial config, sound chip.
- The OS provides a filing system interface, but only implements cassette files. A floppy disk filing system would be served by an application ROM, via the OS. Two popular disk filing systems are DFS and ADFS - ADFS being advanced, and the later and more capable one.
- The OS supports a second processor, whereby the Beeb becomes an I/O server and the attached device becomes the application machine. Generally the attached device is faster and has a large flat memory map: just 2k of ROM and I/O at the top of the 6502 second processor for example, and the rest is RAM. See the User Guide. Note that most Beebs did not have a second processor fitted, and most games don't work in that configuration. Serious software and a few games do. In the case of a non-6502 second processor, the OS that the user sees will be appropriate for the CPU: CP/M, DOS, etc.

As for peripherals, these are built in:
- keyboard, screen (composite, RGB, UHF)
- cassette i/o with motor control
- sound (internal speaker only)
- parallel printer port
- serial port
- user port (VIA)
- Analogue I/O
- 1 MHz bus expansion
- Tube interface for second processor (faster 6502, Z80, x86, NS32016, ARM, etc)
Optional, but fitted internally:
- Floppy disk controller
- Econet local area network
- speech chip with access to serial ROMs
- cartridge port
These days it's popular to fit an internal SDcard board for solid state storage.

With all the I/O and with application ROMs, it was popular for instrumentation and control uses. The model B was succeeded by the Master, which has 128k RAM and a banking scheme which makes rather more of the low 32k available for applications and populates some of the 16 application slots with RAM. The Master also allows for an internally-fitted second processor.

A couple of very useful reference sites:
http://mdfs.net/
http://beebwiki.mdfs.net/
http://stardot.org.uk/mirrors/www.bbcdocs.com/
http://www.sprow.co.uk/bbc/reference.htm
(and there's also the StarDot forums where there's a lot of friendly expertise.)

Two standard reference books as pdf
- BBC Microcomputer System User Guide (supplied with the machine)
- The Advanced User Guide for the BBC Microcomputer (an aftermarket book, owned by every serious user)

Some specific references:
- Introduction to the BBC Operating System
- System Memory Map
- BBC Basic's memory use
- Clickable mainboard photo
- the Beeb's circuit diagram
- documentation for each OS call
- annotated disassembly of the OS
- Thread here about the second processor interface and architecture

Emulation (generally supporting Beeb or Master, with or without 6502 second processor)
- JSBeeb (in-browser, highly accurate.)
- lib6502 (CLI, small, fast, portable, no sound or graphics! No second processor emulation supplied - build your own!)
- BeebEm is a BBC Micro and Master 128 emulator. (Runs OK in wine on linux and Mac.)
- B-Em for Linux and Windows.
- Big list of emulators

Other popular 6502 machines from Acorn include the Atom, a predecessor machine with different OS and hardware organisation, and the Electron, a cost-reduced machine offering a high degree of compatibility with the Beeb but with some crucial differences, and rather fewer on-board peripherals. Both machines had popularity in their day and still have their adherents. There are current projects to fix up both machines with "missing" capabilities such as a Tube interface, and in the case of the Atom, sound and colour.

I've never seen one in person, but there's a ton of videos on YouTube by owners and collectors. They look like they're built like a tank and will be running well into the next century.

Here's are link to two computerphile videos about them:

https://www.youtube.com/watch?v=do6xydtcVPk

https://www.youtube.com/watch?v=y4WG549i3YY

Huh! -- now that is a thought that gives me pause. Can you imagine how a 22nd-century person would view a Beeb??? Kinda gives a whole new meaning to the term "Retro" ! Electronics itself might be no more than a quaint notion by then -- maybe all the CPU's will be optical.

Thanks for the posts and the links, Ed and Martin.

-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

It's true that Beebs are physically robust, and relatively cheap and plentiful. The one thing which fails - the line-voltage filter capacitor - can be readily replaced, and is often replaced preemptively.

Unfortunately, electrically they are not quite so clever: the original Beeb's databus is overloaded, and it has proved tricky - but not impossible - to make designs which will happily interface to both the overloaded TTL Beeb and the less loaded CMOS Master.

Thanks for the video links Martin - the mention of the case screws is amusing, because these screws are missing from many actively-used Beebs: they just slow down the opening of the case to fit a ROM or other internal expansion.

If they're actually still possible to find in working conditions at reasonable prices I'll try to get one. I always wanted one, from I first saw a BBC B described in Personal Computer World. But that one was about a 100 pounds more expensive than I could afford during that period.
The last time I looked at getting one it didn't seem particularly easy so I left it at that.

-Tor

The regulars over on stardot often have spare machines, so if you ask over there as a hobbyist you could get a good fair deal. Best way to start would probably be an "introduce yourself" post to give your background.

You're quite right, they were never a bargain back in the day - which explains to some degree the popularity of the Electron. (A CPU-replacing speedup board is possible, and an interesting project. But the CPU isn't socketed.)

The "beeb" was very popular in schools when I was a kid. It always seemed like a more "serious" computer compared to the Spectrums most people had at home. But by the time I got to do IT in secondary school (1988 or so) they were just starting to replace them all with Archimedes, so I too didn't really get to use Beebs much. I do remember the lovely monitors, and crystal clear graphics compared to what you'd get from using a computer with a TV, which is what I was used to.

I miss those days so much sometimes.

http://www.ebay.co.uk/itm/221907400747 - 170 pounds for what looks like a good specimen. I would probably get it, if I had the room!

_________________
8 bit fun and games: https://www.aslak.net/

I used to drool over the BBC when at school. It felt like a proper machine, whilst the ZX Spectrum seemed very limited. My own first machine was an Oric-1 (also 6502 based) back early 1984, when that model was already on it's way out - and hence was cheap.

As a weird circle of life thing, I am building my own 6502 computer now on breadboard. I decided I needed a case, and so I got a non-working BBC from ebay and used that...!

Might be worth adding some comparisons of Acorn's 6502 computer offerings, using the Beeb as a baseline, as a kind of buyer's guide overview:

• Atom - earlier machine, simpler, all-TTL, very expandable, all-in-keyboard style
• BBC Model B - the most popular original config
• Model A - the half-RAM cheaper variation, not popular even at the time
• Electron - the cost-reduced almost-single-chip version, missed its Christmas market but very popular now. All-in-keyboard style with external PSU. Has a cartridge port and expansion bus, but lacks many of the Beeb's I/O devices including the Tube and the Printer, Serial and User Ports. Uses nibble-wide RAM which limits performance, lacks the teletext-style display mode which means less RAM available.
• B+ - a mid-life kicker on the model B, not very popular, more RAM added and, crucially, more RAM available in the main 32k space. Floppy controller factory fitted.
• B+ 128 - has additional RAM in the ROM slots to allow soft ROMs. Introduces the option of running Bas128, which is the usual 16k Basic but with 64k of space available to the programmer, at some performance penalty.
• Master 128 - bigger, heavier and more expensive, but popular now. Upgraded to a 65C12 CPU. Has improved RAM mapping, also cartridge port, a numeric keypad, realtime clock, battery-backed soft configuration, and internal capacity for a Tube coprocessor. Improved and faster Basic. Applications in ROM: View, ViewSheet, Terminal.
• Master Turbo - as the 128, but fitted with a 4MHz 65C102 coprocessor with 64k RAM
• Master 512 - as the 128 but fitted with a 10MHz 80186 with 512k RAM.
• Master Compact - cost reduced version of the 128, no second processor capability, all-in-keyboard design with the PSU and floppy in a monitor plinth box.

Not seen here, the very rare Master Scientific, which is a 32-bit workstation built around a Master motherboard.

More info, reviews, docs and so on, from Chris' Acorns:

BBC Micros
Electron Computers
Master Computers

BTW, the history of how Acorn, a small computer company, got the contract from the BBC, a large public institution with excellent name recognition and a reputation for high quality, to make and sell microcomputers with the BBC name on them, is a very interesting one. If you like that kind of thing, see this thread (over on StarDot):

"Outline specification for the BBC MICROCOMPUTER system"

I've bought a few via ebay. It helps to watch for a while and know how to interpret the listings so you don't pay too much.

The main issue as Ed said is the line filter capacitors - there are 1 or 2 ebay sellers selling kits to replace 3 capacitors. I've used them in the past for lazyness, or buy your own if you know what you're doing. (The X2 capacitor is applicable to most late 70/early 80's computers with switched mode PSUs though - they just don't last).

The "gold standard" monitor of the day was the Microvitec Cub. Big old tubes - most probably still working, but heavy, so you'll often see relatively low-prices sales, but "buyer collect". Disk drives are often 40/80 track switchable and GoTeks are very popular. One issue - caused more by the good OS interface than anything else may be disk filing system incompatibilities - several sellers write their own and shipped their own (sideways) ROMs with their disk interface hardware (e.g. double density) although I think most could read/write standard Acorn disks.

It was a great machine then - cost was always an issue though, but with the "BBC" label, had a huge impact in the educational field - which didn't stop games being written for it. the Apple II really didn't go further than Schools and business in he UK - mostly due to cost - you could get 5 BBC Micros for the price of one Apple II setup.

Technologically brilliant, but we (the UK) were just crap at marketing it. Still.. Without the BBC Micro we would not have the ARM processor, so that's one good thing that did come out of it.

-Gordon

_________________
--
Gordon Henderson.
See my Ruby 6502 and 65816 SBC projects here: https://projects.drogon.net/ruby/

Oh yes, any doubts about the historical significance of the BBC Micro are squashed by the realisation that its (local) success effectively funded the development of the ARM CPU. I think it's still true that there are more ARMs in circulation than any other CPU ISA (despite relatively few desktop computers ever using it as a main CPU, every PC probably has at least half a dozen ARM cores embedded in it, to say nothing of mobile devices), and it's showing no signs of slowing down.

There's an excellent slim brochure from the BBC, dated September 1981, which lays out the architecture and feature set of the Beeb:
BBC Microcomputer System (pdf, "Information sheet G2")
(Links to mirror of Chris's Acorns site, a trove of Acorn, BBC, and many other documents.)