Every procedure in OS that accepts a parameter, say buffer, of an open array type also accepts a pair of parameters, say start and length, designating a subarray. The start and length parameters always have defaults of zero and LAST(CARDINAL), respectively. We use the notation buffer[start FOR length] for the subarray of buffer determined by start and length, namely, the elements with indexes:
start, start+1, ... MIN(start+length-1, HIGH(buffer))
The subarray is empty if length is zero or start is greater than HIGH(buffer).