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.