ZX Computings - OpenLibra

9
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POKEing at
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e a display-
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A/ill, it is very
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es a line full
>rt loop can
ethe
if screen
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ill as spaces
Mile. Up to
inted in this
J that we
have a display-file we can now
take a PEEK at its starting
address. The following
subroutine achieves this and it
is used in all subsequent
listings:-
Of course the display-file
has to exist before we can
start PEEKing and POKEing at
it. If we wish toi POKE onto a
blank screen then it is first
necessary to create a displayfile
full of spaces.
Unfortunately a succession of
PRINT statements will not
achieve this and although a
FOR . . NEXT loop PRINTing
individual spaces will, it is very
cumbersome. Luckily
PRINT . . . creates a line full of
spaces so a short loop can be
used to produce the required
number of screen lines.
Obviously characters can be
used as well as spaces to
create a display-file. Up to 23
lines can be printed in this
way.
Having ensured that we
have a display-file we can now
take a PEEK at its starting
address. The following
subroutine achieves this and it
is used in all subsequent
listings:-
500 LET P = PEEK{ 16397)
510 IFP> 127
THEN LET P = P — 256
520 LET W = PEEK! 16396)
+ P* 256
530 RETURN
It should now be obvious how
we can use this address to
POKE a character onto the
screen. The following program
establishes a blank displayfile,
inputs a row and column
number, POKEs character
code 148 (inverse asterisk) at
the relevant address and then
inputs another "grid
reference". When the program
is run, inverse asterisks appear
at your bidding anywhere on
the screen
10 LET P » 0
20 LET W = 0
30 FOR A = 1 TO 22
40 PRINT
50 NEXT A
60 INPUT A
70 INPUTB
80 IF A>22 OR B> 32
THEN GOTO 60
90 LET Y = (A — 1 > * 33 + 8
100 GOSUB 500
110 POKE W + Y,148
120 GOTO 60
500 LET P = PEEK(16397)
510 IF P> 127 THEN LET
P = P - 256
520 LET W = PEEKI16396)
+ P* 256
530 RETURN
The following two alterations
UMMER 1983 zx COMPUTING SUMMER 1982
to the listing extend this
simple program:-
Specify character to be
POKEd:-
84 INPUT C
110 POKE W + Y,C
(C is relevant character code)
POKE character taken from
the keyboard:-
84 INPUT C$
86 LET X = CODE (C$)
88 IF X> 191
THEN GOTO 84
110 POKE W + Y,X
it will be noticed that the
programs above assign
variables P and W before the
first PEEK. This is because, as
mentioned before, any
variable assignment or initial
input will alter the location of
the display file. If you write
any screen-POKE programs
and find that the characters
are displaced it will almost
certainly be because a variable
in either PEEK or POKE has not
been previously assigned. A
similar case is where an initial
input or an assignment is
made after a previous PEEK or
POKE, when it will be
necessary to take another
PEEK at W before POKEing
again.
careful POKES
Another thing worth
remembering is that POKEing
can be a hazardous occupation
if you happen to POKE in the
wrong place or even if you
POKE an inappropriate
character code in the right
place. Care should therefore
be taken when writing
programs to ensure that
characters are not POKEd
outside the boundaries of the
display-file. Usually such
characters seem to disappear
without trace but sometimes
they can find their way into
your program, invariably with
unpleasant consequences
Some bad POKEs can cause
havoc with the video control.
The codes for all statements,
tokens and operators should
definitely be avoided (ie codes
> 191).
A more subtle problem is
that any extensive use of
screen space is very expensive
in terms of memory. A 23 line
"blank" screen will occupy
760 bytes of RAM, which
does not leave much for the
program if you are using the
basic model ZX80 with 1 K of
memory. You therefore need
to think hard about the
balance of memory
POKElng
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requirements when writing
screen-POKE programs if you
have no memory expansion.
Having grasped the
principles involved in defining
and locating the display-file it
is relatively simple to
manipulate it. Existing
characters on the screen can
be replaced by POKEing an
alternative code at the same
address. If this is the code for
a space (0) then the character
already on the screen
disappears. By PEEKing at the
address you plan to POKE to
you can see what character
already occupies that location,
thus opening up the possibility
of a conditional response. All
the relevant character codes
are identified in the ZX80
handbook.
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Graphic Example
Finally, here is a simple games
program that demonstrates
the features discussed and
which just fits onto the 1K
ZX80. The computer prints up
a display consisting of black
and grey squares in a pattern
determined by a number input
at the beginning of each series
of games. The object of the
game is to get the
'woodworm" Ian asterisk),
which first appears at tine 8
column 1, to eat its way
across the screen to column
32 in the least number of
moves. The snag is that the
black squares represent a
particularly tough kind of
wood and each time one is
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