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

8.12 SYSTEM OVERLAY ACCESS
Practically all of the functions contained in the system overlays are accessed via
library commands or standard SuperVisor Calls. Only in a few unique cases is access to
overlay functions through the SYSTEM SVC required. The two cases, calculating the file
specification hash code and the password string hash code, have been discussed. The
system functions provided in the overlays will usually have a standard SuperVisor Call
assigned. These SVCs have been discussed in chapter 7. The system translates standard SVC
numbers within SYSRES to the overlay entry number in order to process the user
request. Although it is possible to directly access a function via its overlay entry
number or ISAM entry number, this should not be done. The standard SVC linkage protocol
should be used to address the overlay functions since there is no guarantee that the
routines servicing the overlay functions will remain in the overlay presently assigned.
A user SVC request is via a RST 40 instruction which places the return address at the top
of the stack. Since the process to translate the user request to a system overlay request
also uses a RST instruction (to minimize the length of the translation code), an extra
return address is placed on the stack. The SVC processor adjusts for this by popping the
extraneous return address when it is processing a system overlay request. The system's
request is easily identifiable since all system request codes have bit-7 set. Because of
this, if a user requests a system overlay function directly, it is necessary to CALL the
RST instruction so that the return address that is kept on the stack is a pointer to the
address following the CALL instruction. System overlays one through five and nine through
thirteen, can be loaded into the overlay region by means of the following code:
LD A,8