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

What could be more simple? We identify what task slot to remove by the value placed into
register C, then issue the supervisor call. Another method can be used if we want to
remove the task WHILE WE ARE EXECUTING IT. Consider the routine modified as follows:
MYTCB DW MYTASK
COUNTER DB 10
TEMPY DB 0
MYTASK DEC (IX+2)
RET NZ
LD A,@KLTSK ;Identify the SVC
RST 40 ;Invoke the service call
The @KLTSK service routine will remove the currently executing task. Since this task is
currently executing, it is the one that gets removed from the TCBVT table. The system
will not return to your routine but will continue as if you had executed an "RET"
instruction. Therefore, the "@KLTSK" SuperVisor Call should be the last instruction you
want executed. In this example, MYTASK will decrement the counter by one on each entry to
the task. When the counter reaches zero, the task will be removed from slot 2 (remember
it was placed in slot 2).
One additional TP SuperVisor Call is @RPTSK. The function is easy to say in words;
however, its function is best illustrated. The @RPTSK function will update the TCB
storage vector (the vector address in your task control block) to be the address
immediately following the @RPTSK SVC instruction. This is also another case where the
system will NOT return to your task routine after the SVC is made but rather continues on
with the TP. To illustrate how this TP function is used in a program, the final example
should be examined:
First, let's point out that this task routine contains no method of relocating it to
protected RAM. The statements starting at label, BEGIN, add the task to TCBVT slot zero
(without checking for its availability) and return to DOS Ready. The task contains a four
second down counter and a routine to put a character in video RAM (80th character of row
0). At four second intervals, the character toggles between '|' and '-'. The toggling is
achieved by toggling the execution of two separate routines which perform the character
display. Use is made of the @RPTSK TP call to implement the routine toggling. Examine
this task closely to ascertain the functioning of @RPTSK.
BEGIN LD DE,TCB ;Point to TCB & add the
LD C,0 ; task to slot 0
LD A,@ADTSK
RST 40
LD A,@EXIT ;Exit to DOS
RST 40
TCB DW TASK
COUNTER DB 15
TASKA LD A,@RPTSK ;Replace current
RST 40 ; task with TASKA
TASK LD BC,2