ZX Computings - OpenLibra

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Lines 700-706:
Line 710:
Lines 735-780:
Lines 790-840:
Line 999:
PRINT the fourth "question" down the right
hand side of the screen,
waits until a letter(s) are entered. Line 715
causes the program to jump to line 730 if a
"Y" is entered. If an "N" is entered then the
program STOPs, otherwise the program
waits at the INPUT statement at line 710.
display the fifth "question" and wait for a
proper INPUT at line 775.
execute the various subroutines,
causes the program to halt. Equivalent to an
END statement.
SUBROUTINE AT LINE 1000:
"btacks-in" the left % of the screen.
Line 1010 contains 23 inverse spaces.
Line 1025 puts the sun (an inverse asterisk)
in the centre of the black square.
SUBROUTINE AT LINE 1 050:
displays the planets in their starting
positions.
SUBROUTINE AT LINE 1 100:
displays the period.
SUBROUTINE AT LINE 1145:
removes any print statements on the righthand
side by PRINTing blank tines. Nine
spaces are contained in the string in tine
1155.
IMER 1982 ZX COMPUTING SUMMER 1982
ZX81 Programs
SUBROUTINES AT LINES 1 495, 1 595, 1 695 and 1 795:
contain the different pieces of data on each
of the four planets.
SUBROUTINE AT LINE 1995:
PRINTs the general headings for the data.
SUBROUTINE AT LINE 2100:
converts plot co-ordinates to PRINT AT coordinates
and then displays each planet's
initial letter by the pixel square representing it
on the display.
Line 2145 contains an inverse "H".
Line 21 50 contains an inverse "V".
Line 2155 contains an inverse "E".
Line 21 60 contains an inverse "M".
The Display
(a) PRINT AT co-ordinates:
The ZX81 PRINT AT statement
is of the form — 9999
PRINT AT (line), (column);
"{whatever is to be
•PRINTed)" - where:
line numbers are from 0 to
21 inclusive.
column numbers are from 0
to 31 inclusive.
(b) PLOT/UNPLOT co-ordinates:
The ZX81 PLOT/UNPLOT
statements are of the form
- 9999 PLOT (X pixel
coord), (Y pixel coord) . . .
"blacks-in" pixel
9999 UNPLOT (X pixel
coord), (Y pixel coord) . . .
"whites-out" pixel —
where:
X coords are from 0 to 63 inclusive
Y coords are from 0 to 43 inclusive
Normally, the ZX81 prints
BLACK ON WHITE. During the
simulation, the first 23 columns
(down all 22 lines) are
"blacked-in" (Line 1010).
So that the planets and sun
can be seen on the bfack back-
Space Saving
tf the program IS to be used as
part of some space-borne
adventure game then its length
will undoubtedly want to be
shortened. I suggest the following
course of action:
1. Remove the REM statements.
2. Remove the introduction
and the first THREE "questions".
3. Replace the INPUT statements
for the three "questions"
by LETs.
ground an UNPLOT statement
DISPLAYS the planet, whilst a
PLOT statement will "btack-in"
the square again. If your computer
normally writes WHITE
ON BLACK then it may be found
better to make the right hand
side of your screen WHITE with
your inverse spaces. If you do
so remember to change all
writing in PRINT statements
after tine 620 to your inverse
graphics, delete lines 1000,
1005, 1010 and 1015, and
change the characters in lines
1025,2145,2150,2155and
2160 to your "normal" and
NOT inverse characters.
It may have been noticed
that in lines 560-575 and 590-
605 the Y coords are being
deducted from 44 (The
greatest Y pixel coord). This
causes the planets to revolve
ANTICLOCKWISE about the
sun. If the "44-" is omitted
th en the planets revolve
CLOCKWISE. All PRINT AT,
PLOT/UNPLOT statements are
referenced to the top LEFT-
HAND corner of the screen.
4. Put variables AH, AV, AE,
AM, SXH & SYH, SXV &
SYV, SXE & SYE, SXM &
SYM, RH, RV, RE and RM into
their respective routines.
(5. Reduce the number of
planets.)
Again there is no reason why
the planets should be called
Mercury, Venus or whatever.
The program was written in a
"modular" form to aid conversion
but this does make it rather
lengthy.