A short key press can begin and end while BASIC is printing a lane. Keep one recognised press until the next movement opportunity.
Replace lines 180, 291, 292.
180 GO SUB 1000: GO SUB 3000: LET dx=0: LET dy=0: POKE 23560,0: LET tick=PEEK 23672
291 LET k$=INKEY$: IF k$="" THEN LET k$=CHR$ (PEEK 23560)
292 POKE 23560,0: LET dx=0: LET dy=0RUN the changed program.
Complete source at this checkpoint
10 GO SUB 7000
100 DIM p(6): DIM d(6): DIM t(6): DIM v(6): DIM r(6): DIM a$(6,30): DIM b$(6,30)
110 LET x=7: LET y=8: LET steps=0: RESTORE 7100
120 FOR i=1 TO 6: READ p(i),d(i),v(i),r(i): LET t(i)=v(i): NEXT i
140 FOR i=1 TO 6: FOR j=0 TO 2: FOR k=0 TO 3
150 LET c=2 *p(i)+10 *j+k: LET c=c-30 *INT (c/30)+1
160 LET a$(i,c)=CHR$ (144 +k): LET b$(i,c)=CHR$ (148 +k)
170 NEXT k: NEXT j: NEXT i
180 GO SUB 1000: GO SUB 3000: LET dx=0: LET dy=0: POKE 23560,0: LET tick=PEEK 23672
200 LET k$=INKEY$
220 IF k$="q" OR k$="Q" THEN GO TO 9000
230 IF k$="r" OR k$="R" THEN GO TO 100
280 LET elapsed=PEEK 23672 -tick: IF elapsed<0 THEN LET elapsed=elapsed+256
290 IF elapsed<32 THEN GO TO 200
291 LET k$=INKEY$: IF k$="" THEN LET k$=CHR$ (PEEK 23560)
292 POKE 23560,0: LET dx=0: LET dy=0
293 IF k$="q" OR k$="Q" THEN GO TO 9000
294 IF k$="r" OR k$="R" THEN GO TO 100
295 IF k$="i" OR k$="I" THEN LET dy=-1
296 IF k$="k" OR k$="K" THEN LET dy=1
297 IF k$="j" OR k$="J" THEN LET dx=-1
298 IF k$="l" OR k$="L" THEN LET dx=1
300 LET tick=PEEK 23672: LET nx=x+dx: LET ny=y+dy
310 IF nx<0 OR nx>14 OR ny<0 OR ny>8 THEN LET nx=x: LET ny=y
320 GO SUB 2000
330 IF hit=1 THEN LET e$="That gap was not clear.": GO TO 4000
340 IF nx=x AND ny=y THEN GO TO 350
341 GO SUB 3100: LET x=nx: LET y=ny: GO SUB 3000
350 FOR i=1 TO 6: LET t(i)=t(i)-1
360 IF t(i)>0 THEN GO TO 430
370 LET t(i)=v(i): LET p(i)=p(i)+d(i)
380 IF p(i)=15 THEN LET p(i)=0
390 IF p(i)=-1 THEN LET p(i)=14
400 IF d(i)=1 THEN LET a$(i)=a$(i,29 TO 30)+a$(i,1 TO 28): LET b$(i)=b$(i,29 TO 30)+b$(i,1 TO 28)
410 IF d(i)=-1 THEN LET a$(i)=a$(i,3 TO 30)+a$(i,1 TO 2): LET b$(i)=b$(i,3 TO 30)+b$(i,1 TO 2)
420 GO SUB 1200: IF y=r(i) THEN GO SUB 3000
430 NEXT i
440 GO SUB 2000: LET steps=steps+1
450 IF hit=1 THEN LET e$="The lane caught you.": GO TO 4000
460 IF x=7 AND y=0 THEN LET e$="Across! A well-timed journey.": GO TO 4000
470 LET dx=0: LET dy=0: GO TO 200
1000 BORDER 0: PAPER 0: INK 7: CLS
1010 PRINT AT 0,1; INK 5;"QUICKSTEP";AT 0,21; INK 4;"EXIT ABOVE"
1030 FOR j=0 TO 8 STEP 4
1040 PRINT AT 2+2*j,1; PAPER 1; INK 5;s$;AT 3+2*j,1;s$
1060 NEXT j
1070 PRINT AT 2,15; PAPER 4; INK 7;" ";AT 3,15;" "
1080 FOR i=1 TO 6: GO SUB 1200: LET z$=">": IF d(i)=-1 THEN LET z$="<"
1081 PRINT AT row,0; PAPER 0; INK colour;z$;AT row,31;z$
1082 NEXT i
1090 PRINT AT 20,2; PAPER 0; INK 7;"I up J left K down L right"
1110 PRINT AT 21,2; INK 7;"R retry Q quit";
1120 RETURN
1200 LET row=2 +2 *r(i): LET colour=6: IF d(i)=-1 THEN LET colour=5
1210 PRINT AT row,1; PAPER 0; INK colour;a$(i);AT row+1,1;b$(i)
1240 RETURN
2000 LET hit=0: IF ny=0 OR ny=4 OR ny=8 THEN RETURN
2010 LET lane=ny: IF ny>4 THEN LET lane=ny-1
2015 LET delta=nx-p(lane): LET delta=delta-5 *INT (delta/5)
2020 IF delta<2 THEN LET hit=1
2030 RETURN
3000 LET shade=0: IF y=0 OR y=4 OR y=8 THEN LET shade=1
3010 IF y=0 AND x=7 THEN LET shade=4
3020 PRINT AT 2+2*y,1+2*x; PAPER shade; INK 7;h$;AT 3+2*y,1+2*x;f$
3030 RETURN
3100 IF y<>0 AND y<>4 AND y<>8 THEN PRINT AT 2+2*y,1+2*x; PAPER 0;" ";AT 3+2*y,1+2*x;" ": RETURN
3110 PRINT AT 2+2*y,1+2*x; PAPER 1; INK 5;g$;AT 3+2*y,1+2*x;g$: RETURN
4000 PRINT AT 1,1; PAPER 0; INK 6;e$
4010 PRINT AT 20,2; PAPER 0; INK 7;"R plays again. Q quits. "
4020 IF INKEY$<>"" THEN GO TO 4020
4030 LET k$=INKEY$
4040 IF k$="q" OR k$="Q" THEN GO TO 9000
4050 IF k$="r" OR k$="R" THEN GO TO 100
4060 GO TO 4030
7000 RESTORE 7200: FOR j=0 TO 103: READ n: POKE USR "a"+j,n: NEXT j
7010 LET h$=CHR$ 152 +CHR$ 153: LET f$=CHR$ 154 +CHR$ 155
7020 LET g$=CHR$ 156 +CHR$ 156: LET s$="": FOR j=1 TO 15: LET s$=s$+g$: NEXT j: RETURN
7100 DATA 0,1,3,1,9,-1,4,2,11,1,2,3
7110 DATA 7,-1,3,5,1,1,4,6,10,-1,2,7
7200 DATA 0,0,63,127,120,120,120,120
7210 DATA 0,0,255,255,24,24,24,24
7220 DATA 0,0,255,255,24,24,24,24
7230 DATA 0,0,252,254,30,30,30,30
7240 DATA 127,127,127,127,63,7,7,0
7250 DATA 255,255,255,255,255,192,192,0
7260 DATA 255,255,255,255,255,3,3,0
7270 DATA 254,254,254,254,252,224,224,0
7280 DATA 0,7,15,13,15,7,3,31
7290 DATA 0,224,240,176,240,224,192,248
7300 DATA 63,55,7,7,6,6,14,0
7310 DATA 252,236,224,224,96,96,112,0
7320 DATA 0,0,0,0,0,16,0,0
9000 PAPER 0: INK 7: PRINT AT 21,0;"Finished. RUN to try again. ";: STOPHold a direction and the controls behave as before. A recognised short tap during drawing can now survive after you release the key.
Read the present, then the remembered key
INKEY$ answers the current-key question. The ROM also keeps LAST K at memory address 23560, recording a newly recognised key. PEEK reads that byte; CHR$ turns the code into a string that our existing direction tests can compare.
Line 291 first reads INKEY$. Only if it is empty does the program use the remembered byte. A held key therefore has priority at the sampling point.
Line 292 clears the latch with POKE 23560,0 after consuming the command. Line 180 clears it when resetting the crossing, so a command from the previous attempt is not carried into the new start. These changes leave the collision queries, lane periods and drawing order alone.
One remembered key is not a queue
The byte has room for one recognised key code. A newer key can replace it before the game reads it; the program does not replay an entire sequence of taps. Very short presses can still evade keyboard scanning, and simultaneous keys are not guaranteed to behave like successive commands.
Holding a key can also produce further recognised repeats. A move may remain buffered after release. Releasing means you are no longer holding a direction; it does not undo a command already waiting in the latch.
You tap J and then L while the game is drawing, releasing both before the next update. Must the player make a left step followed by a right step?
Show the explanation
No. There is one remembered key, not two queued events. L can replace J before the update reads the byte, leaving only a rightward command. The program consumes at most one movement command at that update.
Compare the same interruption
Stay on the bottom strip so traffic cannot end the experiment. Tap J briefly while a lane redraws, then release. Compare this with the previous checkpoint, where only a key still held at line 291 could affect that update.
The recorded execution checks used a four-video-frame tap during lane-update work. It was missed in the polling-only version and retained here. Your manual tap is not a four-frame measuring instrument, so repeat the comparison and judge whether short presses are more dependable, rather than expecting every attempt to align identically.
If old movement repeats indefinitely, check that line 292 clears the latch. If a previous attempt’s key appears at the next start, check line 180. If a held direction stops repeating, check that INKEY$ still has priority and that the old press-and-release gate has not returned.
Keyboard note: PEEK and POKE
In the Spectrum’s original keyword entry, POKE is on O in keyword mode. For PEEK, enter extended mode with CAPS SHIFT and SYMBOL SHIFT, then press O. CHR$ is extended U. Use your emulator’s documented keyboard mode when entering the listing.
Sources
Steven Vickers, edited by Robin Bradbeer, ZX Spectrum BASIC Programming, second edition (Sinclair Research, 1983), chapter 14, chapter 18, chapter 25. These chapters describe character conversion, INKEY$ and LAST K at 23560. The one-command consumption and reset behaviour are implemented by the three changed lines.
Run it here
This unit's program, built for you.
- Program
- quickstep.tap
- Machine
- ZX Spectrum 48K (PAL)