How to toast your write protected Beeb floppy disk

Some of the floppy drives for the BBC Micro were using the Intel 8271 controller, and this little chip is the actual star of this story.

When I was taught a few basics in logic design back in the day, I was told to strictly stick with the documented signal combinations, since everything else is just undefined behavior. Meaning, nobody ever even cared to find out, what may happen. (Everything may happen, due to unknown quantum effects Margaret Thatcher may rise from the grave, the end of days, etc., you get the idea. Just don’t.) Chris Evans, the hero of this story, on the other hand, is apparently a character of courage.

Specifically, it’s about the reset command of the i8271, which is issued by “outputting a one followed by a zero to the Reset register” – and this, by the way, for exactly 10 clock cycles. Which proved to be just a bit too much of a provocation for our daring hero, when playing with the chip for a retrocomputing exercise:

It is no doubt my security background that caused me to read this description and then promptly write the value 100 (not 1 or 0) to this register.

Learn what followed here:

Via HN:


These kinds of ‘undocumented behaviour’ makes you wonder about possible stories about ‘ghosts in the machine’. What can you say if one day, a stray cosmic ray happens to change the value of that register to 100 that one time long ago? :slight_smile:

I guess, this may explain this strange love/fear of zombies that was quite common amongst programmers when there were actual floppy and hard disk controllers in the computers. :slight_smile:
(Mind the rather conspicuous correlation of the decline of zombie films and the incline in SSD distribution.)


The adventure continues, with a write-up of the reverse-engineering of this surprising dual-core chip:

Not only is the 8271 larger than the Beeb’s 6502 CPU by a significant amount, it also cost much more

As we engaged in efforts to try and disassemble the ROM, without any prior knowledge of the Instruction Set Architecture (ISA), I was fortunate enough to have a conversation with Ken Shirriff ( about this interesting chip. Against all odds, he found a detailed conference presentation abstract from 1977 [link to copy of full abstract].