spectrum utility - OpenLibra

ect if you've loaded the code in
at 32256 but start anywhere
else, and all the jumps will be
wrong. Hence the crash. The
answer — alter the addresses.
Program 3 (supplied by our illustrious
Editor) will do just that.
In essence what it does is to
POKE new (corrected) values into
those JP commands, the
values being calculated from
where you start. SAVE it with
SAVE "print" CODE (start
address), 300
and remember to CLEAR (start
address — 1 ) before you LOAD
it back in as part of a program.
One word of warning. Don't
answer the "start address" prompt
with a value greater than
65067 or you'll lose the end of
the routine!
UDCS
That's relocation solved, now
for problem two. Run Program
2, but first replace the "H" with
a user-defined graphic (UDG for
short). Any letter will do. What
appears on screen? Nothing.
The reason for this is again
found in Table A. Look at the line
of op code at address 32272.
The number in brackets is the
address of a system variable
called CHARS. This holds a two
byte (number) value — normally
0 and 60 — which tells the computer
where to look (in the ROM)
for the character set — the 8x8
patterns of pixels which make
up each of the characters. The
Print machine code looks
through this set to find a match
for the character you want
printed. As you design the UDGs
no match is found — hence the
blank screen.
The solution to the problem is
quite simple (although it took me
some time to fathom.) What you
have to do is convince the
machine that the character set it
wants to look at is not in the
ROM but up in the UDG area.
Look at Program 4. Line 5 is
there simply to make the UDGs
different from normal capitals
(by underlining top and bottom).
In line 10 "ABCDE" must be
entered in Graphics mode,
although they won't appear
underlined until after the program
is RUN. The POKEs following
are the key. They tell the
machine that the set it wants
begins at address 64216