WINOOWS
<l3JECI' REFERENCE
An open window's object reference begins as an object selector and is converted by the IP into an access descriptor for the windowed 432 object. Each open IP window must map a different object in 432 memory, am each obj ect must be represented as a singIe segment of base type data segment (functions may be used to manipulate
When a window is opened on an object, the IP makes the object inaccessible to other IPs by setting the I/O lock bit in the base object's object descriptor; the I/O lock bit in the base object is set when a window is opened on a refinement. The obj ect may, however, remain accessible to GOP processes holding object references for it. If the Peripheral Subsystem requires exclusive access to an object, it must do so by means of a convention. For example, if the object has been defined with a lock field, the IP controller can use the LOCK <l3JECI' function to prevent GOP processes (which observe the convention) from accessing the object. An alternate convention, might be used for objects which do not contain lock fields. For example, a GOP process sending an obj ect to the I/O controller could agree not to access the object, or pass a reference for it to another process, until the I/O controller sends the object as a message back to the GOP process.
The IP supports the 432 philosophy that software should have access to the minimum set of objects needed to perform its function. Therefore, the I/O controller can only open a
