iAPX 286 Operating System Writers Guide 1983

DATA SHARING, ALIASING, AND SYNCHRONIZATIONSystems that implement "pipes" as a form of intertask communication can call mailbox proceduresfrom the device drivers for the pipes.It is possible to use mailboxes as the sole means of interface between applications and operating system.For example, the operating system has one mailbox through which it receives service requests from allapplications; each task has a mailbox through which it receives responses from the operating system.To get more memory, a task would send a memory request message to the operating system; the operatingsystem would return a message containing an alias 'for the allocated memory segment and installthe alias in the task's LDT. The advantage is simplicity. Only two global gates are needed: one forSEND_MESSAGE and one for RECEIVE_MESSAGE. A task can wait for only one purpose: for amessage from a mailbox. The disadvantage is inefficiency. Any implementation of mailboxes is boundto be less efficient than the interlevel CALL instruction normally used to communicate with an operatingsystem.All of these forms of message-passing can use the primitive synchronization and descriptor-manipulationtechniques illustrated in this section.B5-17 121960-001