The Lucas Advanced Video Controller provides an easily used,
moderately high-definition colour graphics facility on the
Nascom range of computers.
However, I have seen no mention of the fact that it can also
provide a very useful addition to the memory capacity of the
machine in non-graphical applications, by virtue of the fact
that the board contains 48 kilo-bytes of dynamic RAM, arranged
in 3 pages of 16K each.
These three pages are all located at the same address
(normally 8000H to BFFFH, although there is provision for
changing this by means of hard-wired links). Page selection is
via port B2, but only operates after execution of the
control software (G32 or G48). As the execution of G32 or G48
seems to require the prior initialisation of Basic, even if all
subsequent work is to be in machine code, it is also a wise
precaution to set the Basic memory ceiling below 8000H. The
response “32767” or less to the question “Memory size?” will
Using Sys, the keyboard command
O B2 1
will select page 1 (normally the red page),
O B2 2
will select page 2 (green) and
O B2 3
will select page 3 (blue), all of which are 16K long.
If the Nascom also has its normal RAM extending over the
sane address area, O B2 0 will select the normal RAM, giving, in
effect, a choice of 4 separate pages of 16K each, in the
location 8000H to BFFFH.
Obviously, the page selection can also be carried out within
a program, and it is here that the full benefit can be realised.
Listing 1 shows one way of achieving this in machine code
whilst, in Basic, only a single-line instruction is required as
in listing 2. (If using Basic, the memory ceiling limitation
referred to earlier should not be applied.)
Don’t forget, of course, that if your control program is in
the area of memory that you have just paged out, even a Nascom