ZX Computings - OpenLibra

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The display file is a block of
addresses that contain the
CHR$ codes that form the
display on the TV screen. At
the beginning of the file we
find a Newline CHR$, and
there are also Newline CHR$
at the end of each line. As we
can print up to 32 CHR$ per
line, every 33rd CHR$ = a
Newline CHRS. These are
used by the ZX81 ROM as
references and should not be
altered.
To find the position of the
display file we can use: 5 LET
Z= 1 + PEEK 16396 + 256 *
PEEK 16397. "Z" will now be
the address of the first print
position on the screen. If we
POKE Z with any CHR$ code
then this CHR$ will be printed
n the first position on the
position, and so on, as shown
on the chart.
To prevent printing, or
should I say POKEing, to the
33rd CHR$ there are generally
three different approaches.
1. To use a dedicated POKE
routine that can never work
out to hit a 33rd CHR$.
2. To peek at the position on the
screen before POKEing, ie if
PEEK (position) = 118
(Newline CHR$) then don't
POKE here.
3. To use a formula that will
automatically skip every 33rd
CHR$, ie let us say X is the
position we wish to poke then
we can use POKE
(Z + X + INT(X/32),(CHR$
code) this will automatically
add one after every 32nd
CHR$ making 33 = 34
effectively, and so on for
every line. A simple program
using this formula :-
5 LET Z= 1 + PEEK 16396
+ 256*PEEK 16397
10 FOR X='TO 763 STEP 7
20 POKE (Z + X + INT(X/32)),8
30 NEXT X
40 STOP
(Note:- That this formula
automatically adds one to
every 33rd position. The
memory map chart must be
read as (position) — (line
number on left) to give the
correct number per position).
ANOTHER SHORT PROGRAM
5 LET Z = 1 + PEEK 16396
+ 256*PEEK 16397
10 LET A = 0
20 LET B = 33
25 IF A< 0 OR A> 767
THEN GOTO 35
30 POKE Z + A + INT/A/32),
128
35 LET A = A + B
40 IF A>0 THEN LET B - 33
50 fF A>767
THEN LET B = -31
60 IF A = 27 THEN CIS
-70 GOTO 25
POKEing
ZX8t
One thing to remember is that
the display file is using the
memory immediately after the
BASIC program. This means
that if the BASIC program is
altered then "run" is required
to find the new value for "Z",
or at least the new value for
"Z" must be calculated.
An idea of a simple program
may be as follows. We have a
gun at the bottom left-hand
side of the screen. Pressing
any key from 0 to 9 will fire
the gun at a target at the to
line on the screen, "0" to fire
straight up, "9" to fire
farthest right, we will also
need to PEEK at the top line in
the relevant place to see if we
- D H O l V f
- PROGRAMMING
have hit the target.
Looking at this idea we can
see that system 1 can be used
as we don't intend to POKE
over the edges of the screen,
and the plotting involved has a
limited path.
After the normal line 5 to
find "2" we then POKE
Z + 726,1 28 to put a square
block for the gun. Then to fire
straight up to the top line is
easy, we simply POKE
Z + 693,23 then successively
- 33 from this, poking all the
way up to 0. Taking the top
line to equal 32 CHR$ wide
and the number of lines down
to the gun being 22 we must
shift to the right 32 + 22 =
1.5 positions per line to hit the
top right-hand corner.
Considering we have 9 shifted
to the right options (0 to 9) if
we use 1.5 + 9 = 0.166*
(the INKEY$ number) this will
give us the right amount of
shift per line to plot our gun
fire. I've rounded down to
0.16 so there is no chance of
over-stepping the corner if 9 is
selected. So the basic program
is shown as, an INKEYS input,
followed by the fire routine.
(Note: - The line numbering
should be followed to leave
room for the rest of the
program).
UMMER 1983 z x COMPUTING SUMMER 1982 53