I remember peeking and poking a PET (or was it a CBM?) we had in the copier room at school until it would make a horrible whining sound requiring turning off th

I remember peeking and poking a PET (or was it a CBM?) we had in the copier room at school until it would make a horrible whining sound requiring turning off the machine. Learned much later that some of the video registers could be poked with values that might damage the hardware :-)