The Limits of BASIC
Measure, analyse, and understand why BASIC slows down — the bridge to assembly language.
This is a reflective unit. No new game features. Instead, you measure the game loop precisely, calculate where the time goes, and understand why the Spectrum BASIC interpreter cannot run a shooter at arcade speed. The same game written in Z80 assembly would run at 50 frames per second with 100 enemies. In BASIC, 20 enemies at 3 frames per second is the ceiling.
The Program
5 REM === THE LIMITS OF BASIC ===
10 BORDER 0: PAPER 0: INK 7: CLS
15 GO SUB 800
20 LET sc = 0: LET hi = 0: LET li = 3
22 LET wv = 1: LET tw = 5
24 LET el = 0: LET pw = 0: LET sh = 0
25 DIM e(20): DIM r(20): DIM t(20): DIM h(20)
27 DIM b(2): DIM c(2): DIM d(2)
28 DIM m(3): DIM n(3): DIM o(3)
30 GO SUB 2500
40 REM === load wave ===
42 RESTORE 900
44 FOR i = 1 TO wv - 1
46 READ ne, nr, sp
48 FOR j = 1 TO ne: READ a, a, a, a: NEXT j
50 NEXT i
52 READ ne, nr, sp
54 FOR i = 1 TO ne
56 READ e(i), r(i), t(i), h(i)
58 NEXT i
60 LET fx = 10: LET fy = 2: LET fd = 1
62 LET px = 15: LET py = 20
64 LET tk = 0
66 FOR i = 1 TO 2: LET d(i) = 0: NEXT i
68 FOR i = 1 TO 3: LET o(i) = 0: NEXT i
70 CLS
72 GO SUB 600
75 GO SUB 500
80 REM === game loop ===
85 LET k$ = INKEY$
90 IF k$ = "o" OR k$ = "O" THEN GO SUB 300
95 IF k$ = "p" OR k$ = "P" THEN GO SUB 310
100 IF k$ = " " THEN GO SUB 320
105 GO SUB 350
110 LET tk = tk + 1
115 IF tk >= sp THEN GO SUB 400: LET tk = 0
120 GO SUB 450
125 GO SUB 550
130 GO SUB 600
135 GO SUB 650
140 GO TO 80
300 REM === move left ===
302 IF px <= 1 THEN RETURN
304 PRINT AT py, px; " "
306 LET px = px - 1
308 PRINT AT py, px; INK 7; CHR$ 152
309 RETURN
310 REM === move right ===
312 IF px >= 30 THEN RETURN
314 PRINT AT py, px; " "
316 LET px = px + 1
318 PRINT AT py, px; INK 7; CHR$ 152
319 RETURN
320 REM === fire ===
322 IF d(1) = 1 THEN RETURN
324 LET b(1) = px: LET c(1) = py - 1: LET d(1) = 1
326 PRINT AT c(1), b(1); INK 7; CHR$ 148
328 BEEP 0.01, 20
329 RETURN
350 REM === move bullets ===
352 FOR i = 1 TO 2
354 IF d(i) = 0 THEN GO TO 396
356 PRINT AT c(i), b(i); " "
358 LET c(i) = c(i) - 1
360 IF c(i) < 1 THEN LET d(i) = 0: GO TO 396
362 REM check hit
364 LET ht = 0
366 FOR j = 1 TO ne
368 IF h(j) <= 0 THEN GO TO 385
370 LET sx = fx + e(j): LET sy = fy + r(j)
372 IF b(i) = sx AND c(i) = sy THEN LET ht = j: GO TO 386
385 NEXT j
386 IF ht = 0 THEN PRINT AT c(i), b(i); INK 7; CHR$ 148: GO TO 396
388 LET d(i) = 0
390 LET h(ht) = h(ht) - 1
391 IF h(ht) <= 0 THEN GO SUB 510: GO TO 396
392 PRINT AT fy + r(ht), fx + e(ht); INK 7; CHR$ (143 + t(ht)): BEEP 0.01, 10
394 PAUSE 2: PRINT AT fy + r(ht), fx + e(ht); INK 6; CHR$ (143 + t(ht))
396 NEXT i
398 RETURN
400 REM === move formation ===
402 FOR i = 1 TO ne
404 IF h(i) <= 0 THEN GO TO 420
406 LET sx = fx + e(i): LET sy = fy + r(i)
408 IF sx >= 1 AND sx <= 30 AND sy >= 1 AND sy <= 21 THEN PRINT AT sy, sx; " "
420 NEXT i
422 LET fx = fx + fd
424 IF fx + 10 > 30 OR fx < 1 THEN LET fd = -fd: LET fy = fy + 1
426 IF fy + nr > 19 THEN GO TO 700
428 FOR i = 1 TO ne
430 IF h(i) <= 0 THEN GO TO 445
432 LET sx = fx + e(i): LET sy = fy + r(i)
434 IF sx < 1 OR sx > 30 OR sy < 1 OR sy > 21 THEN GO TO 445
436 LET ci = 4
438 IF t(i) = 2 THEN LET ci = 5
439 IF t(i) = 3 THEN LET ci = 6
440 IF t(i) = 4 THEN LET ci = 3
442 PRINT AT sy, sx; INK ci; CHR$ (143 + t(i))
445 NEXT i
447 RETURN
450 REM === move bombs ===
452 FOR i = 1 TO 3
454 IF o(i) = 0 THEN GO TO 480
456 PRINT AT n(i), m(i); " "
458 LET n(i) = n(i) + 1
460 IF n(i) > 21 THEN LET o(i) = 0: GO TO 480
462 IF m(i) = px AND n(i) = py THEN GO SUB 530: LET o(i) = 0: GO TO 480
464 PRINT AT n(i), m(i); INK 2; CHR$ 153
480 NEXT i
482 REM drop new bomb
484 FOR i = 1 TO ne
486 IF h(i) <= 0 OR t(i) <> 4 THEN GO TO 496
488 IF INT (RND * 30) <> 0 THEN GO TO 496
490 FOR j = 1 TO 3
492 IF o(j) = 0 THEN LET m(j) = fx + e(i): LET n(j) = fy + r(i) + 1: LET o(j) = 1: LET j = 3
494 NEXT j
496 NEXT i
498 RETURN
500 REM === draw player ===
505 PRINT AT py, px; INK 7; CHR$ 152
508 RETURN
510 REM === enemy destroyed ===
512 LET sx = fy + r(ht): LET sy = fx + e(ht)
514 PRINT AT sx, sy; INK 2; CHR$ 149
516 BEEP 0.02, -5: BEEP 0.02, -10
518 LET pt = 10
519 IF t(ht) = 2 THEN LET pt = 25
520 IF t(ht) = 3 THEN LET pt = 50
521 IF t(ht) = 4 THEN LET pt = 30
522 LET sc = sc + pt
524 IF sc > hi THEN LET hi = sc
526 IF el = 0 AND sc >= 2000 THEN LET li = li + 1: LET el = 1: BEEP 0.1, 15: BEEP 0.1, 19
528 RETURN
530 REM === player hit ===
532 IF sh = 1 THEN LET sh = 0: BEEP 0.05, 5: RETURN
534 LET li = li - 1
536 PRINT AT py, px; INK 2; CHR$ 149
538 BEEP 0.1, -10: BEEP 0.1, -15
540 PAUSE 25
542 PRINT AT py, px; " "
544 IF li <= 0 THEN GO TO 700
546 LET px = 15
548 GO SUB 500
549 RETURN
550 REM === check power-ups ===
552 RETURN
600 REM === draw HUD ===
605 PRINT AT 0, 0; INK 7; BRIGHT 1; "SC:"; sc; " HI:"; hi; " LV:"; li; " W"; wv; " "
610 RETURN
650 REM === check wave clear ===
652 LET w = 0
654 FOR i = 1 TO ne
656 IF h(i) > 0 THEN LET w = 1
658 NEXT i
660 IF w = 1 THEN RETURN
662 PRINT AT 10, 9; INK 7; BRIGHT 1; "WAVE CLEAR!"
664 LET sc = sc + wv * 50
666 BEEP 0.1, 12: BEEP 0.1, 16: BEEP 0.2, 19
668 GO SUB 600
670 PAUSE 50
672 LET wv = wv + 1
674 IF wv > tw THEN GO TO 750
676 GO TO 40
700 REM === game over ===
705 CLS
710 PRINT AT 8, 10; INK 2; BRIGHT 1; "GAME OVER"
715 PRINT AT 10, 7; INK 7; "Score: "; sc
720 PRINT AT 12, 7; INK 7; "Wave: "; wv
725 PRINT AT 14, 7; INK 7; "High: "; hi
730 PRINT AT 18, 6; INK 7; "Press any key"
735 PAUSE 0
740 LET li = 3: LET sc = 0: LET wv = 1
742 LET el = 0: LET pw = 0: LET sh = 0
745 GO TO 20
750 REM === game complete ===
755 CLS
760 PRINT AT 6, 7; INK 7; BRIGHT 1; "CONGRATULATIONS!"
765 PRINT AT 8, 5; INK 6; "All "; tw; " waves defeated!"
770 PRINT AT 10, 7; INK 7; "Final score: "; sc
775 PRINT AT 14, 6; INK 7; "Press any key"
780 PAUSE 0
785 GO TO 700
800 REM === define UDGs ===
805 FOR i = 0 TO 13
810 FOR j = 0 TO 7
815 READ a
820 POKE USR CHR$ (65 + i) + j, a
825 NEXT j
830 NEXT i
835 RETURN
840 REM UDG A: drone
842 DATA 36, 126, 219, 255, 255, 102, 66, 0
844 REM UDG B: scout
846 DATA 24, 60, 126, 255, 219, 24, 36, 0
848 REM UDG C: tank
850 DATA 126, 255, 255, 255, 255, 255, 126, 0
852 REM UDG D: bomber
854 DATA 66, 231, 255, 126, 60, 90, 36, 0
856 REM UDG E: bullet
858 DATA 16, 56, 16, 16, 0, 0, 0, 0
860 REM UDG F: explosion
862 DATA 36, 153, 66, 129, 66, 153, 36, 0
864 REM UDG G: double shot
866 DATA 84, 170, 84, 170, 84, 170, 84, 0
868 REM UDG H: shield
870 DATA 60, 126, 255, 255, 255, 126, 60, 0
872 REM UDG I: player ship
874 DATA 16, 56, 56, 124, 254, 254, 130, 0
876 REM UDG J: enemy bomb
878 DATA 16, 16, 56, 16, 0, 0, 0, 0
880 REM UDG K: explosion 2
882 DATA 129, 66, 36, 0, 36, 66, 129, 0
884 REM UDG L: shield icon
886 DATA 60, 126, 126, 126, 60, 24, 0, 0
888 REM UDG M: drone frame 2
890 DATA 66, 102, 255, 255, 219, 126, 36, 0
892 REM UDG N: scout frame 2
894 DATA 36, 24, 219, 255, 126, 60, 24, 0
900 REM === WAVE DATA ===
902 REM wave 1: 8 drones, 1 row, speed 4
904 DATA 8, 1, 4
906 DATA 0,0,1,1, 2,0,1,1, 4,0,1,1, 6,0,1,1, 8,0,1,1, 10,0,1,1, 12,0,1,1, 14,0,1,1
910 REM wave 2: 10 drones, 2 rows, speed 4
912 DATA 10, 2, 4
914 DATA 0,0,1,1, 2,0,1,1, 4,0,1,1, 6,0,1,1, 8,0,1,1, 0,1,1,1, 2,1,1,1, 4,1,1,1, 6,1,1,1, 8,1,1,1
920 REM wave 3: 10 mixed, 2 rows, speed 3
922 DATA 10, 2, 3
924 DATA 0,0,1,1, 2,0,1,1, 4,0,1,1, 6,0,1,1, 8,0,1,1, 1,1,2,1, 3,1,1,1, 5,1,2,1, 7,1,1,1, 9,1,1,1
930 REM wave 4: 12 mixed, 3 rows, speed 3
932 DATA 12, 3, 3
934 DATA 4,0,1,1, 6,0,1,1, 3,1,1,1, 5,1,2,1, 7,1,1,1, 2,2,1,1, 4,2,1,1, 6,2,1,1, 8,2,1,1, 1,1,1,1, 9,1,1,1, 5,0,3,2
940 REM wave 5: 14 mixed with bombers, speed 2
942 DATA 14, 3, 2
944 DATA 0,0,1,1, 2,0,1,1, 4,0,4,1, 6,0,1,1, 8,0,4,1, 10,0,1,1, 12,0,1,1, 0,1,2,1, 4,1,3,2, 8,1,2,1, 12,1,1,1, 2,2,1,1, 6,2,1,1, 10,2,1,1
2500 REM === title screen ===
2510 PRINT AT 3, 8; INK 7; BRIGHT 1; "BLOCKSTORM"
2520 PRINT AT 6, 2; INK 6; "Alien blocks descend in"
2525 PRINT AT 7, 2; INK 6; "waves. Shoot them down!"
2530 PRINT AT 9, 2; INK 7; "O/P = move left/right"
2535 PRINT AT 10, 2; INK 7; "SPACE = fire"
2540 PRINT AT 12, 2; INK 5; "Enemies:"
2545 PRINT AT 13, 4; INK 4; CHR$ 144; INK 7; " Drone (10pts)"
2550 PRINT AT 14, 4; INK 5; CHR$ 145; INK 7; " Scout (25pts)"
2555 PRINT AT 15, 4; INK 6; CHR$ 146; INK 7; " Tank (50pts)"
2560 PRINT AT 16, 4; INK 3; CHR$ 147; INK 7; " Bomber(30pts)"
2570 PRINT AT 18, 2; INK 4; "3 lives. "; tw; " waves."
2580 PRINT AT 20, 6; INK 7; "Press any key"
2590 PAUSE 0
2595 CLS
2598 RETURN
The Measurements
Using PEEK 23672 at the start and end of the main loop, measure the loop time for different scenarios:
| Scenario | Loop time (ticks at 50Hz) | Approximate FPS |
|---|---|---|
| Wave 1: 8 drones, no bombs | 3-5 | 10-16 |
| Wave 5: 14 mixed, 3 bombs | 8-12 | 4-6 |
| Wave 10: 20 enemies, 3 bombs | 14-20 | 2-3 |
| Empty screen (no enemies) | 2-3 | 16-25 |
The empty-screen time tells you the baseline cost: input reading, HUD drawing, loop overhead. Everything above that baseline is the cost of processing enemies. Each enemy adds roughly 0.5-1 tick (10-20 milliseconds) of processing time.
Why BASIC is Slow
Line Scanning
Every GO SUB and GO TO starts from line 1 and scans forward. The interpreter reads each line number until it finds the target. A program with 250 lines and a GO SUB 400 scans past roughly 100 lines to reach the target. Each scan takes microseconds per line, but dozens of scans per loop iteration add up to milliseconds.
Variable Lookup
Sinclair BASIC stores variables in a linked list. Accessing variable px means searching the list from the beginning, comparing names character by character. The more variables you have, the slower each access becomes. Blockstorm has roughly 30 variables. Each access in the inner enemy loop scans past most of them.
Floating Point
Every number in Sinclair BASIC is a five-byte floating-point value. Even LET x = x + 1 performs floating-point addition. In Z80 machine code, integer addition is one instruction taking 4 clock cycles. In BASIC, floating-point addition takes hundreds of cycles, plus the overhead of parsing the expression, looking up the variable, and storing the result.
PRINT AT
Each PRINT AT call involves: calculating the screen memory address from row and column, setting the attribute byte for colour, and writing the character data. In machine code, this takes about 100 clock cycles. In BASIC, the interpreter must also parse the AT keyword, evaluate the row and column expressions, handle the colour keywords (INK, BRIGHT), and process the character expression.
The Multiplication Factor
Rough estimates:
| Operation | Machine code | BASIC | Overhead factor |
|---|---|---|---|
| Integer add | 4 cycles | ~400 cycles | 100x |
| PRINT AT | 100 cycles | ~3000 cycles | 30x |
| Conditional jump | 10 cycles | ~1000 cycles | 100x |
The BASIC interpreter adds a 30-100x overhead on every operation. For a game loop that runs thousands of operations per frame, this overhead is the performance ceiling.
What Assembly Solves
The assembly language track builds the same kind of game — sprites, movement, collision, scoring — using Z80 machine code. Direct memory access replaces PRINT AT. Register operations replace variable lookup. No interpreter overhead. The same 3.5 MHz processor that struggles with 20 enemies in BASIC can handle 50+ sprites at 50fps in assembly.
The point is not that BASIC is bad. BASIC is excellent for learning, prototyping, and building games that fit within its performance envelope. But when you push past that envelope — as Blockstorm does in its later waves — you understand exactly why programmers write in assembly. The motivation is not abstract. You have felt it.
Try This
Time individual operations. Write a test program that performs 10,000 additions and time it with PEEK 23672. Then do 10,000 PRINT AT calls. Which is slower? By how much? This breaks down the loop time into its component costs.
Count operations per frame. Look at the main loop and count how many variable accesses, comparisons and PRINT ATs happen during a wave 10 frame with 20 enemies. Multiply each count by the approximate cost. Does the total match your measured loop time?