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

CKPAWS LD A,@FLAGS$ ;Get Flags pointer
RST 40 ; into reg IY
LD A,(IY+'K'-'A') ;P/u the KFLAG$
RRCA ;Bit 0 to carry
JP C,GOTBRK ;Go on BREAK
RRCA ;Bit 1 to carry
RET NC ;Return if no pause
reads the KFLAG$ contents. The @FLAGS$ SuperVisor Call is used to obtain the flags
pointer from the DOS. Be aware that if your application is using the IY index register,
then you better save and restore it within the CKPAWS routine (alternatively, you could
use memory loads in lieu of IY indexing, use @FLAGS at the beginning of your application
to calculate the location of KFLAG$, and stuff the address into the CKPAWS memory LD
instructions.) The first rotate instruction places the BREAK bit into the carry flag.
Thus, if a condition was in effect, the sub-routine would branch to "GOTBRK" -
which is your break handling routine. If there is no pending BREAK, the second rotate
places what was originally in the PAUSE bit into the carry flag. If a condition
is not in effect, the routine returns to the caller. This sequence of code gives a higher
priority to (i.e. if both BREAK and PAUSE conditions are pending, the
condition has precedence). It is important to note that the GOTBRK routine needs to clear
the KFLAG$ bits after it services the condition. This is simply done via a call
to RESKFL.
The next part of the routine is executed on a condition.
CALL RESKFL ;Reset the flag
PUSH DE ;Don't alter reg DE
FLUSH LD A,@KBD ;Flush type-ahead
RST 40 ; buffer while
JR Z,FLUSH ; ignoring errors
POP DE
First the KFLAG$ bits are reset via the call to RESKFL. Next, we take care of removing
any characters that are stored in the type-ahead buffer (the system will automatically
clear the type-ahead buffer when a BREAK condition is latched). This can be done by
repeatedly invoking the @KBD request until it returns a "no character available"
condition code.
Now that the routine is in a PAUSEd state and the type-ahead buffer is cleared, it must
wait for a key input. The following routine does this:
PROMPT PUSH DE
LD A,@KEY ;Wait on key entry
RST 40
POP DE
CP 80H ;Go on
JP Z,GOTBRK
CP 60H ;Ignore
JR Z,PROMPT ; else ...
The PROMPT routine is coded to accept a and branch to your BREAK handling routine
so that the user can "abort" from a PAUSE. It will ignore repeated entries (the
60H is the standard byte value that is interpreted as a PAUSE entry). Any other character
will cause it to fall through to the following routine which clears the KFLAG$ latch.
RESKFL PUSH HL ;Reset KFLAG$ without
PUSH AF ; altering AF or HL
LD A,@FLAGS$ ;P/u flags pointer
RST 40 ; into reg IY
RESKFL1 LD A,(IY+'K'-'A') ;P/u the flag
AND 0F8H ;Strip ENTER,
8-4