When more than one task is referring to the same data set, exclusive control of the block being updated is required to prevent referring to the same record at the same time. Rather than issuing an ENQ macro each time you update a block, you can request exclusive control through the MACRF field of the DCB and the type parameter of the READ macro. The coding example in Figure 1 shows the use of exclusive control. After the READ macro is run, your task has exclusive control of the block being updated. No other task in the system requesting access to the block is given access until the operation started by your WRITE macro is complete. If, however, the block is not to be written, you can release exclusive control using the RELEX macro.