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

set. Thus, the Z-flag will be indicative of a problem in effecting the transfer. If, by
chance, PROGRAM-B needs to provide a return code, it must be done by using register pair
DE, IX, or IY, as registers AF, BC, and HL are used to perform the transfer (or some
other technique such as altering the return transfer address to a known error trapping
routine).
PROGRAM-B should contain code that is very similar to that shown earlier. For example,
PROGRAM-B could be:
PROGB PUSH BC ;Save old bank data
PUSH HL ;Save the RET address
.
your PROGRAM-B routines
.
POP HL ;Recover transfer address
POP BC ;Get bank transfer data
LD A,@BANK ;Identify the SVC
RST 40
JR NZ,ERROR ;Whatever error trap
PROGRAM-B saves the bank data (register BC). Don't forget that a transfer was effected
and register C has bit-7 already set when PROGRAM-B is entered.
PROGRAM-B also saves the address it needs to transfer back (which is in HL). It then
performs whatever routines it has been coded for, recovers the transfer data, and issues
the bank transfer request. As explained earlier, an NZ return condition from the @BANK
SVC indicates that the bank transfer was not performed. A recommendation is to verify
that your application has not violated the integrity of the stack where the transfer data
was stored.
Never place disk drivers, device drivers, device filters, or interrupt task service
routines in banks other than bank-0. It is possible to segment one of the above modules
and place segments in banks 1 through 7 provided the segment containing the primary entry
is placed in bank 0. All three types of divisions are incorporated into the system's
spooler with transfer between segments being accomplished by the bank transfer techniques
discussed above.
It sometimes is necessary to transfer a page of memory from one bank to another. This can
only be done in one of two ways. Either a character(s) at a time is passed in a
register(s) or a page buffer below X'8000' is used. The system uses the last page of the
system overlay region (X'2300'-X'23FF') as an overlay buffer (except for SYS5/SYS which
loads into the region). This buffer is generally available for use as a page transfer
buffer. Do not use this location if your memory transfer routine is a background task or
is using the RAM bank as a disk cache buffer.
8.9 INTERFACING TO @KITSK
Consider for a moment that disk I/O can not take place during an interrupt task. How then
can we write "background" routines that perform disk I/O? The system printer spooler does
its despooling function as a background task. If we cannot perform disk I/O during
interrupt tasks, how can we despool? We achieve this by being able to invoke a background
task in a way that does not depend on the interrupt task processor. A function frequently
requested in almost every application is that of obtaining characters from the keyboard.
If we can "hook into" this keyboard request, we can execute a task every time the
keyboard is scanned. For those tasks that require disk I/O, we can make use of this
keyboard task process.
At the beginning of the system keyboard driver code is a call to @KITSK. This means that
any time that @KBD is called, the @KITSK vector is likewise called (actually, the type-
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