In my childhood days when I was messing about with the Speccy and learning BASIC, etc. I was always fascinated by the OUT keyword. I don't think I knew what it meant, at that time: I probably thought it was a sort of more dangerous super-POKE. Today I obviously know what it means: it is outputting a byte to a port. But still, what does that mean to the system?
Since I had no friends, I would sometimes try a lot of OUT x,y combinations to see if I would get anything cool or scary. Sometimes cool and scary. Quite often, an OUT command would hang the machine and slowly fill the screen with a pattern. And I remember one particular pattern that was quite common: it was blue, black and magenta squares (not properly aligned, so they made a kind of chessboard), with white "ladders" on top.
Presumably this was the result of writing a certain sequence of values repeatedly into memory, like an LDIR, and crossing over the screen memory at 16384 upwards. But why so common, and what was the sequence? Does anyone recognise the thing I am describing? I could almost draw it. (I have also seen this happen when programs crash. Maybe only in 128K mode.)
Update: artist's impression:
