Four Stroke

Four Stroke Engine is an animated educational demonstration that illustrates the four stages of an internal combustion engine — induction, compression, ignition, and exhaust — using user-defined graphics (UDGs) to render a cylinder, piston, valves, and spark plug. Eleven UDGs are defined at runtime by reading DATA values and POKEing them into memory via USR. The animation loop cycles through the four strokes continuously, with the piston position controlled by a FOR-NEXT loop iterating rows 9 to 15 and back, and a PAUSE 15 delay between frames to regulate speed. A pause routine triggered by the “P” key allows the user to freeze the animation, continue with “C”, or quit with “S”. Labels for the engine components (spark plug, inlet manifold, exhaust manifold, cylinder, piston) are printed as static text around the animated diagram.

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Program Structure

The program is organized into clearly delineated sections, each preceded by a REM comment using decorative block-graphic borders. The main flow is:

1. Lines 10–60: Title screen with flashing text, then call UDG setup subroutine.
2. Lines 70–180: Draw the static cylinder outline using PLOT/DRAW, print static labels (via subroutine at 540), and display the control hint bar.
3. Lines 190–470: Main animation loop cycling through four strokes — Induction (200–250), Compression (270–310), Ignition (330–380), Exhaust (400–460) — then GO TO 190 to repeat.
4. Lines 480–530: Pause/stop subroutine.
5. Lines 540–610: Static label printing subroutine.
6. Lines 615–830: UDG definition subroutine with DATA.
7. Line 840: SAVE with autorun.

User-Defined Graphics

Eleven UDGs are defined by the subroutine starting at line 620. The character string a$ at line 630 lists the UDG characters in the order their DATA rows appear: \\e\\f\\g\\h\\i\\j form the piston body (top-left, top-middle×3, top-right, bottom-left, bottom-middle×3, bottom-right), \\a\\b are the connecting rod or valve graphics, \\k is the spark flash, \\d is the intake/exhaust flow indicator, and \\c is a valve graphic. DATA is read sequentially (8 bytes per UDG) and POKEd via USR b$, which resolves to the base address of that UDG’s bitmap in the system font area.

Notably, many DATA lines contain c as a literal value (lines 740–830). Within the DATA context this is the numeric variable c, which is uninitialized and defaults to 0 in Sinclair BASIC. This is an intentional technique to embed zero bytes without typing “0” repeatedly — it reads as valid BASIC and evaluates to 0 at runtime.

Animation Technique

Each of the four stroke phases uses a FOR-NEXT loop on variable n ranging from 9 to 15 (piston descending) or 15 to 9 STEP -1 (piston ascending). On each iteration, the previous piston row is erased with spaces and the piston UDG block is reprinted at the new row, creating smooth vertical movement within the cylinder. PAUSE 15 (roughly half a second) paces the animation; the hint line tells the user any keypress speeds it up (via INKEY$ shortcutting the PAUSE).

Cylinder Drawing with PLOT/DRAW

The cylinder walls are drawn using pixel-level PLOT and DRAW commands rather than character graphics, giving a precise outline independent of the character grid. The left wall is drawn from (96,130) and the right from (152,130), with short horizontal segments representing valve apertures. The values NOT PI evaluate to 0 (since PI is non-zero, NOT PI = 0 in Sinclair BASIC), so DRAW NOT PI, -84 draws a vertical line of 84 pixels — a common idiom for embedding zero without the digit.

Key BASIC Idioms

• NOT PI evaluates to 0; used extensively in DRAW, PRINT AT column, and INK arguments as a compact zero literal.
• SGN PI evaluates to 1 (since PI > 0); used in BRIGHT SGN PI and INK SGN PI to set attributes to 1 without the literal digit.
• POKE 23658,8 at line 30 enables CAPS LOCK (sets the caps-shift flag), ensuring consistent keyboard input behavior.
• RESTORE 620 at line 620 resets the DATA pointer to line 620 — since there is no DATA before line 730, this effectively resets to the first DATA line. It ensures the UDG routine works correctly if called after any prior READ.

Pause and Stop Subroutine

The pause subroutine (lines 480–530) uses PAUSE 0 to wait indefinitely for a keypress, then checks INKEY$ for “C” (continue) or “S” (stop). The loop at line 530 (GO TO 490) re-enters the pause and re-issues PAUSE 0 if neither key is pressed, preventing spurious returns. The “P” key check in the main animation loops (lines 240, 440) is a simple IF INKEY$="P" THEN GO SUB 480; it is only polled once per animation frame rather than continuously.

Stroke Phase Details

Stroke	Lines	Piston Direction	Valve/Effect
Induction	200–250	Down (9→15)	Inlet valve open (\\c), fuel symbol (\\d\\d)
Compression	270–310	Up (15→9)	Valve closed, label “Compression”
Ignition	330–380	Down (9→15)	Spark UDG (\\k) with FLASH 1
Exhaust	400–450	Up (15→9)	Exhaust valve indicator (\\d\\d), label “Exhaust”

Potential Anomalies

• Line 740 contains DATA 255,0,255,0,255,0,c,c — the trailing c values rely on the variable c being 0 (from the POKE loop at line 700 where c is the READ target, last assigned the final byte of the previous UDG). This is fragile but works in practice because after the first UDG is fully read, c holds the last byte (128 from line 730), not 0. However the DATA for UDG \\f (line 740) ends with two c bytes which would be 128, not 0 — this may result in slightly incorrect bitmaps for those UDGs, giving unexpected pixel rows, though the visual effect may be minor or intentional.
• The “PRESS P TO PAUSE / ANY KEY TO SPEED UP” hint at line 180 is only 64 characters and wraps onto two screen rows as intended, but the P-key check is absent in the Compression and Ignition loops (lines 280–310, 340–380), meaning pausing is only possible during Induction and Exhaust strokes.