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

ahead interrupt task bypasses this entry to inhibit calling @KITSK from the interrupt
routine). Therefore, if you want to interface a background routine that does disk I/O,
you must chain into @KITSK
The interfacing procedure to @KITSK is virtually identical to that shown for Boot
Initialization ICNFG Interfacing (except that FLAGS+31 through FLAGS+33 is used to
reference the @KITSK vector) and will not be repeated here. For the sake of clarity, you
may want to write your background routine to start with:
START CALL ROUTINE ;Invoke task
LINK DB 'Roy' ;For @KITSK hook
ROUTINE EQU $ ;Start of the task
.
RET
Now that the procedure has been demonstrated, be aware of one major pitfall. The @KBD
routine is invoked from @CMNDI and @CMNDR which is in SYS1/SYS. This invocation is from
the @KEYIN call which fetches the next command line after issuing the "DOS Ready"
message. If your background task executes and opens or closes a file (or does anything to
cause the execution of a system overlay other than SYS1), then SYS1 will be overwritten
by that system module handling your request). When your routine finishes, the @KEYIN
handler returns to what called it - which was SYS1. Unfortunately, SYS1 is no longer
resident. You have just crashed the system!
ANY TASK CHAINED TO @KITSK WHICH CAUSES
A RESIDENT SYS1 TO BE OVERWRITTEN MUST
RELOAD SYS1 PRIOR TO RETURNING.
Okay, how do you accomplish this without knowing system code (point of information: if
you are writing background tasks, you are writing system support code!)? You will be able
to use the following code to reload SYS1 if SYS1 was resident prior to your task's
execution.
ROUTINE LD A,@FLAGS ;Get flags pointer
RST 40 ; into register IY
LD A,(IY-1) ;P/u resident over-
AND 8FH ; lay and remove
LD (OLDSYS+1),A ; the entry code
.
Rest of your task
.
EXIT EQU $
OLDSYS LD A,0 ;P/u old overlay #
CP 83H ;Was it SYS1?
RET NZ ;Return if not else
RST 40 ;Get SYS1 per reg A
; (no RET needed)
Another method is to determine if the keyboard request originated from the command
interpreter. While the command interpreter is fetching its command line via @KEYIN , it
sets bit-2 in the CFLAG$ (see @FLAGS SuperVisor Call). Thus, if your KITSK routine
discovers that bit set, then the command interpreter originated the line input. If you
cause the system to load some other overlay into the system overlay region, it is your
responsibility to restore SYS1!
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