What is an IO status block?
System routines involving I/O operations often refer to a structure called an “I/O status block”. It is mainly used in asynchronous operations, or to pass additional status information. The IOSB consists of a 16bit word holding the number of bytes transfered, a 16bit word holding the condition value describing the I/O operation’s completion status, and additional information (dependant on the operation and the device). The “I/O Reference Manual” in the documentation set has the gory details. It should always be used with asynchronous operations, because the condition value returned from the system routine only refers to the queuing of the operation, while the actual result of the operation is passed in the IOSB.
