This program simulates a five-horse race at “Sportsmans Park” using the ZX81’s low-resolution pixel graphics. Each horse is represented by a single lit pixel moving horizontally across one of five fixed pixel rows, with the winning condition triggered when any horse’s x-coordinate reaches 60. A random number in the range 0–4 selects which horse advances one pixel per iteration, producing a simple probabilistic race. After a winner is determined, a flashing “WINNER” message is displayed by alternately printing and erasing the text in a loop before restarting the race.
Program Analysis
Program Structure
The program is divided into three logical phases:
- Initialisation (lines 1–20): Variables are zeroed, the track header and lane markers are printed, and a finish-line is drawn using a
FOR/PLOTloop at x=61. - Race loop (lines 22–93): Horse positions are plotted, a random horse is chosen, its pixel is unplotted, its coordinate incremented, and replotted, repeating until a horse reaches x=60.
- Winner display and restart (lines 200–260): A flashing “WINNER” banner loops 20 times, then
RUNrestarts the whole program.
Variable Usage
| Variable | Role |
|---|---|
A | Horse 1 x-position (pixel row y=6) |
B | Horse 2 x-position (pixel row y=12) |
E | Horse 3 x-position (pixel row y=18) |
F | Horse 4 x-position (pixel row y=24) |
G | Horse 5 x-position (pixel row y=30) |
C | Random selector (0–4) choosing which horse advances |
D | Loop counter for drawing the finish line |
X | Loop counter for flashing the winner message |
Graphics Technique
Each horse is a single pixel, manipulated with UNPLOT to erase and PLOT to redraw at the incremented position. The pixel rows are spaced 6 units apart (y = 6, 12, 18, 24, 30), neatly separating the five lanes within the ZX81’s 44×64 pixel display. The finish line is drawn as a vertical column of pixels at x=61 using a FOR D=0 TO 38 / PLOT 61,D / NEXT D loop, covering the full usable vertical extent.
FAST/SLOW Mode Usage
The program switches to FAST mode during initialisation (line 2) to speed up the screen setup, then switches to SLOW mode at line 20 before the race begins. This ensures the pixel animation is visible rather than flickering by in a single unobserved burst.
Race Logic and Dispatch
At line 40, C=INT(RND*5) generates a value from 0 to 4. A chain of five IF C=n THEN GOTO statements (lines 44–52) dispatches to the appropriate horse-advance subroutine at lines 70, 75, 80, 85, or 90. Each of those three-line blocks unplots, increments, replots, and loops back to line 40. This pattern is equivalent to a computed GO TO, which the ZX81 lacks natively.
Win Detection
Line 42 checks whether any horse variable equals 60 using a compound OR condition. Importantly, this check occurs before the dispatch, so as soon as any horse reaches x=60 on the previous turn, the race ends. The winner is not explicitly identified; the program simply displays “WINNER” without stating which horse won, which is a notable functional omission.
Winner Flash Effect
Lines 200–220 implement a simple flash by alternately printing "WINNER" and six spaces at the same screen position inside a FOR X=1 TO 20 loop. This produces a toggling on/off effect at the natural speed of the interpreter.
Restart Behaviour
Line 230 uses RUN to restart the program, which on the ZX81 clears all variables and begins from line 1, effectively resetting the race. Lines 240–260 (CLEAR, SAVE, RUN) are unreachable dead code, as RUN at line 230 unconditionally restarts execution.
Anomalies and Notes
- The lane markers printed at line 12 (characters “5” through “1” at column 31) label the horses in reverse order (5 at top, 1 at bottom), which may be counterintuitive.
- No betting or user interaction is implemented despite the “FAST TRACK $2.00 WINDOW” header suggesting a wagering theme.
- The finish line is drawn at x=61, but winning is checked at x=60, meaning a horse stops one pixel short of the drawn line rather than crossing it.
- Lines 240–260 are dead code and will never execute during normal program flow.
Content
Image Gallery
Source Code
1 REM HORSE RACE
2 FAST
3 PRINT AT 0,5;"SPORTSMANS PARK"
4 PRINT AT 1,0;"FAST TRACK $2.00 WINDOW"
5 LET A=0
6 LET B=0
7 LET E=0
8 LET F=0
9 LET G=0
12 PRINT AT 6,31;"5";AT 9,31;"4";AT 12,31;"3";AT 15,31;"2";AT 18,31;"1"
14 FOR D=0 TO 38
16 PLOT 61,D
18 NEXT D
20 SLOW
22 PLOT A,6
24 PLOT B,12
26 PLOT E,18
28 PLOT F,24
30 PLOT G,30
40 LET C=INT (RND*5)
42 IF A=60 OR B=60 OR E=60 OR F=60 OR G=60 THEN GOTO 200
44 IF C=0 THEN GOTO 70
46 IF C=1 THEN GOTO 75
48 IF C=2 THEN GOTO 80
50 IF C=3 THEN GOTO 85
52 IF C=4 THEN GOTO 90
70 UNPLOT A,6
71 LET A=A+1
72 PLOT A,6
73 GOTO 40
75 UNPLOT B,12
76 LET B=B+1
77 PLOT B,12
78 GOTO 40
80 UNPLOT E,18
81 LET E=E+1
82 PLOT E,18
83 GOTO 40
85 UNPLOT F,24
86 LET F=F+1
87 PLOT F,24
88 GOTO 40
90 UNPLOT G,30
91 LET G=G+1
92 PLOT G,30
93 GOTO 40
200 FOR X=1 TO 20
201 PRINT AT 21,25;"WINNER"
210 PRINT AT 21,25;" "
220 NEXT X
230 RUN
240 CLEAR
250 SAVE "1030%7"
260 RUN Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.