Phasor

PHASOR is a space shooter game in which the player fires phasors at aliens moving across a 26-column play field, managing both a shield count and a phasor count across three difficulty levels. The player fires by pressing the key whose column code matches the alien’s position, detected via CODE INKEY$-38 compared to R6+1. Difficulty controls the inner loop count INP, calculated as (1/INT X)*25, tightening reaction time at higher levels; mid-game rank promotions (Falcon, Hawk, Eagle) are triggered at score thresholds 1050, 2100, and 2850. The fire-explosion sequence in the GOSUB 1000 routine uses layered PRINT AT statements with block graphics to animate an expanding blast, and FAST/SLOW toggling inside the loop creates a flash effect. Reaching zero shields triggers the game-over starfield routine at line 8000, which randomly scatters dot and block-graphic characters across a cleared screen.

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Program Analysis

Program Structure

The program is divided into clearly separated functional blocks:

1. Lines 1–18: Initialisation, difficulty selection, and rank-string assignment.
2. Lines 20–180: Main screen setup — starfield rendering, score/status panel drawing.
3. Lines 200–290: Main game loop — alien placement, firing detection, alien movement.
4. Lines 300–380: Hit subroutine — score update and hit animation.
5. Lines 500–600: Miss/alien-fire subroutine — shield deduction and hit reporting.
6. Lines 1000–1240: Explosion graphics subroutine with FAST/SLOW flash effect.
7. Lines 2010–2030: Game-over restart prompt.
8. Lines 7000–7030: Rank promotion congratulations display.
9. Lines 8000–8100: Game-over starfield clear and redraw routine.

Difficulty and Timing Mechanics

Difficulty level X (1, 2, or 3) drives the inner loop count INP via LET INP=(1/INT X)*25, yielding 25, 12, or 8 iterations respectively. Each iteration gives the player one polling opportunity, so higher difficulty dramatically shrinks reaction time. At line 205, once I reaches 150, INP is decremented by 1 each cycle (unless already at 1), providing an escalating challenge even within a chosen difficulty level.

Firing Detection Idiom

The core shooting mechanic at line 250 uses:

IF CODE INKEY$-38=R6+1 AND P<>0 THEN GOSUB 300

On the ZX81/TS1000 keyboard layout, the digit keys ‘0’–’9′ have character codes 28–37, and the letter keys begin at 38 (‘A’=38, ‘B’=39, …). Subtracting 38 maps key ‘A’ to 0, ‘B’ to 1, and so on across the 26-column display. Since R6 is an alien column 0–23 and the comparison is against R6+1, there is a consistent off-by-one offset built into the mapping, meaning the player must press the key one position to the right of the alien’s column.

Rank Promotion System

Three rank titles are assigned in lines 16–18 using cascading IF statements. The ranks escalate with both score and difficulty:

Rank	Conditions
Falcon	X=1 (any score)
Hawk	X=2, or X=1 and S≥2100
Eagle	X=3, or X=2 and S≥2100, or X=1 and S≥2850

The congratulations subroutine at line 7000 is triggered at line 20 when the score equals exactly 1050, 2100, or 2850 — relying on score increments of exactly 10 per hit, so these thresholds are always reachable.

Screen Layout and Block Graphics

The play field occupies columns 0–25 across rows 0–20, while columns 26–31 form a status panel. The starfield is built in two passes: lines 45–90 scatter 80 inverse-space characters ("%.") as background stars, then lines 95–130 place 10 block-graphic "\:'" (▛) characters as larger star objects. The alien is displayed as "%(%0%)" (inverse parentheses around an inverse zero). The explosion subroutine (lines 1000–1240) prints expanding rings of block graphics across rows 10–20 at fixed columns, producing a symmetrical blast pattern centred at column 11.

FAST/SLOW Flash Effect

Inside the explosion loop (lines 1015–1240), FAST and SLOW are toggled on consecutive lines within a FOR L=1 TO 5 loop. This causes the display to flicker between the two rendering modes, producing a visible flash effect that accompanies the explosion animation without requiring any additional graphics work.

Alien Random Fire

A random value R8 = INT(RND*30)+1 is rolled each inner-loop iteration. If INKEY$="" (no key pressed) and R8=1, the alien fires via GOSUB 525. The 1-in-30 probability makes alien fire relatively rare but unpredictable. The alien-hit subroutine further rolls R7 = INT(RND*3)+1; only when R7=1 does the alien shot actually hit the player and reduce SH (shields).

Game-Over Starfield Routine

When shields reach zero, subroutine 8000 clears the screen row by row in FAST mode, then in SLOW mode randomly populates it with either a period (B$=".") or a block-graphic dot (B$="\. ", i.e. ▖) at 80 random positions, generating a sparse star-debris effect as the final display.

Notable Bugs and Anomalies

• Line 241: IF I=150 THEN GOTO 11 — line 11 does not exist. This causes the program to jump to the next higher line, which is line 12 (LET P=20). This appears to be an intentional or accidental partial game reset when the escalation counter reaches 150, resetting phasors but not score or shields.
• Line 580: IF SH<0 THEN GOTO 1000 — the condition is less than zero, but SH is decremented at line 570 after a shield check. If SH was 0 before decrement, it becomes -1, which correctly triggers the game-over path. However, the display at line 590 (PRINT AT 12,30;SH) is skipped in this case, leaving the shield count display unchanged.
• Line 170: The shield and phasor count labels are printed character-by-character with individual AT clauses, and the initial counts (5 and 20) are hardcoded as string literals rather than read from SH and P. This means if the initial values were changed, the display would not reflect them.
• Line 8: LET INP=(1/INT X)*25 — if X=0 were allowed, this would cause a division-by-zero error. The guard at line 7 uses X<0 rather than X<1, so entering 0 would pass the check and crash at line 8.