If you have the assembly listing you can type it in, modify it and reassemble.
Or you can put a 2 byte JR to the end of the program in place of the LDI, and
put the new code at the end with conditional and unconditional jumps back to
the required places. If you adopt the latter course, note that the code proper
ends at 0D48H with a C9, but the next 2 bytes are used as a store.
BASIC IN RAM. This one led to some embarrassment for me because I
overlooked the cause and made a long distance telephone call to someone who
kindly pointed out my ‘basic’ error. When BASIC is executed it asks how much
RAM to use. If you do not wish to reserve any and press enter, BASIC will
‘measure’ the RAM. If there is no ROM or ‘empty’ RAM at the top, just below
BASIC, the NASCOM ROM BASIC will corrupt itself as it is not protected against
this possibility (since it expects to be in ROM). The solution is to enter a
limit of say 55000 in response to the query SIZE?.
NAS-SYS HANDSHAKE. One of my very few ‘hates’ about NASCOM Software
has been the need to have a number of ‘USER’ routines, each one marginally
different, to enable printer handshaking to ZEAP, NASPEN, NAS-SYS etc. Since
it is now easy to overwrite parts of the firmware on a temporary basis, I have
included a routine, accessed from my MENU, that provides H/S by altering the
SRLX routine in NAS-SYS. The SRLX routine is used to send bytes to the serial
port, so it is used by all devices talking to a serial printer. I use an
printer at present, so my routine is written to suit this printer but the
software can easily be changed if necessary. The space allocated to SRLX is
not sufficient to allow the modified routine to be put in the same locations,
and the next routines may be needed by the Programs that are using the
Printer. One routine that is most unlikely to be required during the execution
of a program is the ‘M’ routine. My overwrite routine therefore looks like
this (NAS-SYS 3):
At the end of the program, a RESET restores NAS-SYS to its unmodified state.
The same overwrite routine could be used with NS1 but addresses would have to
This system has been in use for about three months now and the ability
to switch rapidly between systems and formats has been very useful. Particular
advantages over the old MULTIMAPPING system have been the ability to use 64K
of RAM, to load NS firmware and utilities instantly and to overwrite NAS-SYS
and to restore it at will. It is also very useful to be able to load software
into memory under one operating system, and to switch to the other system
although this was also possible with the old MM setup.