GDSX Manual

DEVICE^HANDLER Example, Design
Extended General Device Support (GDSX) Manual–134303
5-15
Device Control Blocks
To make the source code visible under Inspect, in high-level Inspect at the Inspect
terminal, enter:
- SOURCE ON
- SCOPE #DEVICE^HANDLER
- SOURCE
Enter a few carriage returns until you see the CASE RQST^TYPE statement. Set a
breakpoint at this line:
- BREAK #line-num
Then press “R” to resume execution of your program.
At the application terminal, enter a short text string followed by a carriage return. This
should force the GDSX process to a breakpoint, which is displayed at the Inspect
terminal. (The requester has issued a WRITE to send the modified text string back to its
OUT file.)
At the Inspect terminal, use the DISPLAY command to find the current value of
rqst^type:
- DISPLAY RQST^TYPE
Check the LITERALs, defined near the beginning of the DEVICE^HANDLER code, to
determine what kind of operation has been initiated by the requester. (It should be a
WRITE.)
Device Control Blocks
Now you can examine some of the pertinent control structures for the
DEVICE^HANDLER task. The DCB is the starting point. A DCB is the internal
representation of an SU and is used to manage a DEVICE^HANDLER task’s
communication with a file. When the requester application in this example opened
terminal $TC1, TSCODE opened the terminal, assigned a DCB to manage the terminal,
and created a DEVICE^HANDLER task that maps to the DCB.
The DCB contains:
•
The variable name that contains the name of the SU object corresponding to the
DCB (this example uses a dynamic SU, so the variable name is set to $TC1)
•
The access name or file name actually opened for the corresponding SU ($TC1 in
this example)
•
The variable tcbaddr, a pointer to the associated task control block (discussed below)
ULHDECSE contains USKELEX’s dcb^template and is included in Appendix C.
Storage for DCBs is declared at run time by TSCODE and is contained in extended
memory. The size of a DCB for a GDSX process depends on the EXTSUSIZE
configuration parameter, which can be supplied at startup time. Therefore, storage for
DCBs cannot be determined at compile time.
At startup time, depending on the EXTSUSIZE and MAXTERMS configuration
parameters, the maximum number of SUs that can be active simultaneously within a