The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

If the module is a device driver or filter, then it was assigned a Device Control Block
when the driver or filter was invoked with the SET command. The SET command passes a
pointer to this DCB in register pair DE when the initializing program first executes. It
is the responsibility of the initializing program to load the DCB pointer into the 2-byte
MODDCB storage field. The system requires this pointer for proper operation of its
character I/O device chains.
The last 2-byte field is loaded with a binary zero. It's use is reserved by the operating
system. You may conveniently use the memory region after this address for the storage of
any data. Thus, the pointer returned from a successful @GTMOD search for the module will
be easily used to index the data area.
8.6 INTERRUPT TASK PROCESSOR INTERFACING
The operating system is designed to function on hardware that can provide a maskable
interrupt (mode 1). This interrupt can be generated either by a standard Clock Timer Chip
(CTC) or it can be derived by other clocking methods (synchronized to the AC line
frequency or decoded from some other frequency generator). An operating system Task
Processor (TP) manages this interrupt to perform background tasks neccessary to perform
specific functions of the DOS (such as the time clock where a hardware clock is not
provided, blinking cursor where a CRTC blinking cursor is not provided, etc.).
The TP provides twelve individual TASK SLOTS that are executed on a "time-sharing" basis.
The interrupt rate is software divided into three different timing groups spread across
the task slots. One of these task slots is considered "high priority" and functions
approximately 60 times a second (the exact time period depends on the interrupt rate
provided). Three are considered "medium priority" and execute 30 times a second. The
remaining eight are considered "low priority" and execute at a rate of 30/8 times a
second (or 15 times every four seconds). The task task slots are numbered 0-11 with 0-7
being "low priority" tasks, 8-10 being "medium priority" tasks, and 11 being a "high
priority" task.
The DOS maintains a Task Control Block Vector Table (TCBVT) which contains 12 vectors -
one for each of the 12 possible task slots numbered from zero through eleven. Five system
SuperVisor Calls that manage the task vectors are provided. These and their functions
are:
@CKTSK = Check if a task slot is unused or active
@ADTSK = Add a task to the TCBVT
@RMTSK = Remove a task from the TCBVT
@KLTSK = Remove the currently executing task
@RPTSK = Replace the TCB address for the current task
The next point must be completely understood since it has caused confusion to many
attempting to learn how to interface to the TP. The Task Control Block Vector Table
(TCBVT) contains vector pointers. The TCBVT vectors POINT TO A 16-BIT LOCATION IN MEMORY
WHICH CONTAINS THE ADDRESS OF THE SERVICING ROUTINE. Thus, the tasks themselves are twice
indirectly addressed (those programmers familiar with C will observe that the TCBVT is an
array of pointers to pointers). Make sure you keep this in mind! When you program an
interrupt service routine, the entry point of the routine needs to be stored in memory.
If we call this storage location the beginning of a Task Control Block (TCB), the reason
for the indirect method of addressing interrupt tasks will become more clear. Let's
illustrate an example TCB.
MYTCB DW MYTASK
COUNTER DB 15
TEMPY DS 1
MYTASK RET
8-15