In the PC bootup sequence, the first sector of the boot disk is loaded into memory and executed. If this is the DOS boot sector, it loads the first three sectors of IO.SYS into memory and transfers control to it. IO.SYS then:
However, v3.3 allows sector 4 and higher to be fragmented. v5.0 allows the first 3 sectors of IO.SYS to be allocated anywhere (as long as they are contiguous). COMMAND.COM can be treated like any ordinary file.