GDSX (Extended General Device Support) Manual

Design and Development
Extended General Device Support (GDSX) Manual—529931-001
2-40
The GDSX Generation Process
A semaphore is first initialized with INIT^SEM. The semaphore may be defined either
in global or extended memory; however, it is recommended that it be declared in global
memory, within a user private block, for example. If a backup GDSX process exists,
the semaphore must be initialized in both primary and backup GDSX processes. The
semaphore may then be acquired by calls to PSEM and released by calls to VSEM.
A task takes control of a semaphore when a call to PSEM inserts the address of the
task’s TCB (@TCB) into the first element of the semaphore structure (described in the
table below). If the semaphore is already owned, a task requesting ownership will
have its TCB address inserted at the end of a linked list of tasks waiting for the same
resource (tasks are linked via tcb.locklink); the task’s tcb.statex.wait^cause is set to 1;
and the task waits on DISP^RESOURCE. The linked list is also part of the semaphore
structure, which has the three elements shown in the following table:
For any semaphore other than the checkpoint semaphore, the VSEM procedure is
used to release the semaphore so that other tasks can use the associated resource.
When a task calls VSEM, TSCODE verifies that the caller owns the semaphore, then
checks the semaphore’s wait list. If this list is not empty, the first task on the list is
given the semaphore and a DISP^RESOURCE event is caused for the task.
A description of how tasks owning semaphores are restarted following a takeover is
given in Takeover Processing
on page 2-34.
A user may wish to define a semaphore to ensure serial I/O to a disk file. This may be
accomplished in the USER^INITIALIZE exit by calls on ^OPEN and INIT^SEM to open
the disk file and initialize a semaphore for the file. See the example in the
considerations for the ^OPEN procedure in Section 8, Service Routines
.
The GDSX Generation Process
Every runnable GDSX process is a combination of code supplied by HP and user-
supplied code. The user-supplied code is typically written in TAL. (Only USCODE
written in TAL has been tested; although USCODE may be written in other languages,
GDSX has not been tested with other languages.) When your source code file is
compiled, declarations contained in files supplied by HP (DCCBDECS and PEXTGDS)
are sourced in (see Figure 2-3 on page 2-42); depending on the application, user-
Variable Name Description Contents
sem.semowner Current owner of
semaphore
0 if semaphore is available; owner’s TCB address
if semaphore is owned
sem.semhead Head of the wait list Address of SEMHEAD if no tasks are waiting;
otherwise, first waiter’s TCB address
sem.semtail Tail of the wait list Address of SEMHEAD if no tasks are waiting;
otherwise, last waiter’s TCB address
Caution. If a task owning one or more semaphores stops, the primary GDSX process stops.
If a task owning one or more semaphores faults with noretn equal to 1, the primary GDSX
process stops.