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This BBC Micro is a Time Sink

It's weirdly compelling. This is proper low-level stuff right at the metal, something like a PIC microcontroller with a screen, and a keyboard, but I'm starting to realise that it's the architecture of the hardware and software in almost perfect usison that make this computer a bit of a masterpiece. I shouldn't be suprised, I suppose, since these are the people who were already conceiving ARM CPUs.

Okay, so it's custom chips are banal by Jay Miner standards, a serial processor and a video chip (the ULA). It has no hardware sprites, it has eight colours, two of which are black and white, and the other six are garish, and what passes for hardware scrolling is an inadvertent side-effect of the ULA design. On the other hand, here's what stands out to me on this 8-bit micro:

Two 8-bit bi-direcional IO ports (one of which is dedicated to the printer port).
A four channel 12-bit Analogue to Digital Converter.
Hardware timers with interrupts.
Multiple ROM sockets that can easily accept RAM chips - paged memory.
An IO bus - a 34 pin external port connected directly to the CPU.
An unusually sophisticated operating system (MOS) that feels like a proper kernel.
A PCB that's entirely through hole at 2.54mm (0.1") pitch. Huge!
Its BASIC interpreter includes a 6502 assembler.
BASIC programs can have 6502 code interspersed.
Detailed documentation of every bit in its address space.
Option to add LAN support (called Econet)
Option to add a speech synthesiser.

When I say it has what seems like a proper OS kernel, I mean that there's a wealth of services exposed by what are termed OSBYTE calls, whereby you set the A register with the service, using X & Y registers for parameters and return values, and make a call to address 0xFFF4 to invoke some service. Here's a selction of some of the stuff the OS provides:

00 Print operating system version
03 Select output stream
04 Enable/disable cursor editing
05 Select printer destination
07 Set RS423 baud rate for receiving data
0B Set keyboard auto-repeat delay interval
10 Select ADC channels to be sampled
13 Wait for vertical sync
15 Flush specific buffer
75 Read VDU status
76 Reflect keyboard status in LEDs
79 Perform keyboard scan
7F Check for EOF on an open file
81 Read key with time limit
84 Read bottom of display RAM address (HIMEM)
86 Read text cursor position (POS and VPOS)
87 Read character at cursor position
SF Issue paged ROM service request
92 Read from FRED, 1 MHz bus
96 Read from SHEILA, mapped I/O
99 Insert character into input buffer
9B Write to video ULA palette register and copy
9C Read/write 6850 control register and copy
9F Write to speech processor
A6 Read start address of OS variables
B0 Read/write CFS timeout counter
B7 Read/write cassette/ROM filing system switch
BA Read/write ROM number active at last BRK (error)
C6 Read/write *EXEC file handle

There are many more, and all of these calls can be invoked from BASIC, or from machine code, or from the command line.

There are also VDU commands that provide services to draw graphics primitives to the framebuffer, render text, control cursors, and drive printers.

The BASIC ROM is organised in a similar way to the OS, where pretty much every feature that BASIC offers can be invoked from your machine code, so if you want to evaluate an expression, or convert a number to a string, or access a BASIC variable from machine code, then there's a consistent way to do all this.

Even with my superficial understanding of this system, I can already see that I could write a BASIC program, enhance its performance with machine code, and then migrate the whole thing to machine code, making use of the ROM routines that are exposed. Really powerful stuff.

And I haven't even started on the use of vectors as a mechanism of plugin system extensions, and the way the ROMs can identify themselves and provide OS handlers for their commands. You can tell this was all designed by people who really knew what they were doing.

One thing that I will add here, on a slightly philosophical note. There's something compelling about a computer that's immediately useable, with absolutely no distractions. As soon as I flick the power switch, the machine beeps and I'm ready to go. It's profoundly joyful, and whatever the underlying reason, I think it's the same thing that makes Gemini so appealing.

I currently have it on our livingroom floor, hooked up to our household telly. I'm waiting for an SRAM chip to wire up as two banks of paged RAM (or simulated ROM), and I'm going to repurpose the printer port as a general purpose IO port. I feel a little sad hacking something so old and relatively pristine, but it does seem made for exactly that purpose.

Some stuff

A review of the BBC Micro from 1982. Note how detiled this is, compared to typical 'reviews' that we get today. "The keyboard feels good and has an auto repeat facility" - charming!