High Quality Content by WIKIPEDIA articles! Cylinder 1024 is the first cylinder of a hard disk that was inaccessible in the original IBM PC compatible hardware specification, interrupt 13h, which uses cylinder-head-sector addressing. At boot time very old BIOS of many PCs could only access the first 1024 cylinders, numbered 0 to 1023 as CHS addressing only defines 10 bits for the cylinder count (2^10=1024).