24 lines, for a total of 960 bytes occupying $400 to $7FF. The missing 64 bytes were called "screen holes", peppered throughout that memory range, and were used as scratchpad locations for peripherals. One of those peripherals is the disk drive controller card, and the position of the current track is stored in one of those holes. So if you try to clearing the screen the easy way, by zeroing $400 to $7FF, the disk drive forgets what track it's on and it needs to recalibrate, resulting in an awful noise as the read-write head bangs on a stopper.

I remember modifying that routine into banging the head at different frequencies. You could get into musical note range and turn it into a little organ.

Good point. I forgot about these scratchpad locations. One of my first assembly programs was a reverse scroll functions. Depending on the paddle controller it shifted also one char left/right. Two char spaced apart on each line served boundaries for a street and the car was two stationary char in the middle (depending on the paddle controller pointing left, ahead or right). Due to lack of assembler the whole was hand coded. Due to limited access to the computer it was all written out on paper first.