Press A and Return. TURN 0 becomes TURN 1. Do it again for TURN 2. Walk away for a cup of tea and the world waits for you. That is a useful game loop for an adventure, a board game or any other untimed decision.
X leaves the count alone. Q returns to BASIC. The counter stands in for the world update: spend a move, change a room or let the opponents respond only when the command earns it.
Put it in a program
Enter this caller and the routine below as one numbered program on a PAL C64 with BASIC V2, then type RUN. Use uppercase commands as shown. INPUT waits for Return; this is a tiny command decision, not an adventure parser. Empty input, commas and quoted strings follow BASIC’s own input rules.
Initialise once; draw, read, decide and repeat
10 N=0:S=1
20 PRINT CHR$(147);"TURN";N
30 INPUT A$
40 IF A$="Q" THEN END
50 GOSUB 1000
60 GOTO 20N=0:S=1 happens once. The jump returns to line 20, so the next move uses the state left by the last one. Q belongs to the caller: it ends the program before the rule routine runs.
Take the reusable part
Copy lines 1000–1050 into free line numbers and call GOSUB 1000. Put the command in A$, the current count in N and the amount to add in S.
A rule update that owns no keyboard or screen
1000 C=0
1010 IF A$<>"A" THEN RETURN
1020 N=N+S
1030 IF N>255 THEN N=N-256
1040 C=1
1050 RETURN| Variable | Contract |
|---|---|
N |
Integer 0–255; becomes (N+S) modulo 256 for A |
S |
Integer 0–255; preserved |
A$ |
Submitted command; preserved |
C |
Replaced on every call: 1 for accepted, 0 for rejected |
These are ordinary BASIC numbers, stored in the interpreter’s floating-point format. The integer bounds are our agreement with the routine. Their sum is at most 510, so subtracting 256 once is sufficient. Fractional, negative or larger inputs need a different contract.
Use C immediately if it decides whether enemies move. Another routine may reuse that global. Names share BASIC’s namespace; only the first two characters distinguish a numeric name. Rename consistently, reserve the line numbers, and change every call target if you renumber. GOSUB needs its matching RETURN; jumping in with GOTO does not supply one.
The routine reserves no fixed RAM and does no input or drawing. The caller clears and redraws the whole text screen. Move that presentation into your own UI if a status panel must survive each turn.
Make it your own
Replace the caller with this one and keep the same routine. S=3 gives 0 → 3 → 6 for two As.
Three progress units for one accepted command
10 N=0:S=3
20 PRINT CHR$(147);"TURN";N
30 INPUT A$
40 IF A$="Q" THEN END
50 GOSUB 1000
60 GOTO 20
That might spend three energy units while opponents respond once. It does not mean “run the whole game three times”. The change adds no scratch variable or artwork. BASIC still scans the command and redraws after every submission; no speed benchmark follows from the listing’s size.
Common mistake: forgetting the previous turn
This caller is deliberately broken. Line 50 resets N before every rule call.
Broken: persistent state reset inside the loop
10 N=0:S=1
20 PRINT CHR$(147);"TURN";N
30 INPUT A$
40 IF A$="Q" THEN END
50 N=0:GOSUB 1000
60 GOTO 20
A, X, A now gives 1 → 0 → 1, rather than 1 → 1 → 2. Even the rejected command appears to erase progress. The reset belongs before the loop: restore the original caller. The repaired caller was run independently and gives the expected sequence.
When the world cannot wait
INPUT blocks until the player submits a line. Neither approach supplies a turn deadline. The machine’s clocks continue, but the rule state waits. For a countdown, animated cursor or sound service that needs regular calls, retain the rule and replace the waiting policy. Decide what happens if a valid command and an expired deadline arrive together.
The loop comparison separates progression, time, input and presentation. These choices can be combined; they are not a compulsory ladder from BASIC to assembly.
The C64 raster-paced assembly caller makes different ownership choices. Use it when that timing model suits the game; it is not a drop-in replacement for the firmware input reader.
Take the source and evidence
Code status: native-emulator executed, using Emu198x 0.44.1 and the named configuration above. The complete numbered source is typed from a fresh firmware boot. No host write supplies the counter state.
The source folder keeps the caller, routine, customisation, deliberately broken caller and native record together. complete.bas combines the recommended caller and routine for typing.
Six separately started variants cover normal use, step three, the reset fault and repair, byte wraparound and a nonzero start with step 255. Each waits 120 native fields before and after an action, submits X and two As, checks the visible counter and returns to BASIC with Q. The retained checker decodes screen RAM and compares it with independent arithmetic; altered counts, blank screens, absent observations and changed image hashes must fail. The figures are actual framebuffer captures.
These checks establish this small workload. They do not qualify original hardware, every firmware/display variant, a full parser, or the whole emulator.
Original references
Commodore, Commodore 64 Programmer’s Reference Guide, BASIC statement reference (INPUT, GOSUB, RETURN) and variable names.