Zebra Light Pen

Zebra Light Pen is a two-part ZX81/TS1000 program package consisting of a light pen driver and a Concentration-style card-matching game. The first listing (the game) uses a 6×7 grid stored in arrays P and C, assigning pairs of characters from the ZX81 character set (CHR$ values starting at 166) to random board positions, then using a light pen scanning routine to detect which cell the player selects. The second listing is a standalone test/demonstration routine that also POKEs 17 bytes of Z80 machine code starting at address 16514; this machine code reads the EAR port (IN A,(254)), tests bit 3, and returns a non-zero value if the light pen is detected, which BASIC then calls via USR 16514. Sound effects are produced by POKEing values to address 9000, which is also used as the BLIP variable. The game awards 10 points per match and deducts 5 per mismatch, running until all 21 pairs are found.

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Program Analysis

Overview and Structure

The package comprises two separate BASIC programs. The first is the Concentration game (LPEN), and the second is a light pen test routine (LPTEST). Both share the same Z80 machine code driver POKEd into memory starting at address 16514. The game program embeds the machine code implicitly inside its REM line 1, while the test program re-POKEs it explicitly at lines 80–96.

Machine Code Light Pen Driver

Lines 80–96 of the test program POKE 17 bytes of Z80 machine code into addresses 16514–16530. These bytes implement the light pen polling routine called via USR PEN (where PEN=16514). The disassembly is approximately:

1. XOR A — clear accumulator
2. LD C,A — zero C register
3. LD B,A — zero B register (BC = return value candidate)
4. LD A,(0FEh) / IN A,(254) — read EAR/MIC port
5. Test bit 3 of the result; if clear (light detected), jump forward
6. Load a count and loop, returning 1 if light pen pulse found, 0 otherwise
7. RET — return to BASIC

The routine reads port 254 (the keyboard/EAR port), isolating bit 3 which corresponds to the EAR input — the standard method for interfacing a light pen on this hardware. USR PEN returns a non-zero value when the pen is detected over a bright screen area.

Game Board Initialization

The board is a 6×7 grid (42 cells) managed by two arrays: C(6,7) stores character codes for each cell’s hidden value, and P(6,7) tracks whether a cell has been matched (1) or not (0). Fourteen pairs are placed (covering 28 of 42 cells), with character codes starting at 167 (Y+166 for Y=1..14). Pairs are placed by random selection with collision checking at line 2093.

An unusual loop controls how many times each pair value is placed:

FOR Z=1 TO 2+(Y/2=INT(Y/2))*2

This iterates 2 times for odd Y and 4 times for even Y, meaning even-numbered card types get placed four times (two pairs) while odd ones get two. This gives 7×2 + 7×4 = 42 placements — exactly filling the board. However, the win condition checks MATCHED<21, implying 21 pairs, which is inconsistent with only 14 distinct card values across 42 cells.

Light Pen Scanning Routine

The scan is a two-phase process. First, a row scan (lines 2130–2200) iterates Y from 1 to 6, printing a solid bar (O$, 29 block characters) across each row, sampling USR PEN before and after, then restoring the row. A hit is detected by Q AND NOT R — the pen fired while the bar was displayed but not after it was removed. The column scan (lines 2300–2370) then similarly sweeps across the 7 columns of the identified row, printing a 2-character block at each column position.

The variables DELAY and DELAY1 are set to line numbers 3010 and 3030 respectively and used as GOSUB DELAY and GOSUB DELAY1. Line 3010 contains only a REM and a RETURN, making it a near-zero delay. Line 3030 runs a FOR J=1 TO D: NEXT J loop for a variable delay. This is a clever use of numeric variables as GOSUB targets.

Sound Effects

Audio feedback is generated by POKEing values to address 9000, stored in variable BLIP. This directly manipulates the cassette/speaker output bit. Three distinct sound patterns are used:

• Match sound (subroutine 3100): ascending POKE sequence 1..16
• Select beep (subroutine 3060): brief POKE 7, short loop, POKE 0
• Mismatch buzz (subroutine 3300): POKE 15, short delay, POKE 0
• Setup chime (subroutine 3200): POKE 3, loop, then random POKE

Subroutine 3250 (lines 3250–3280) implements a toggling sound effect using variable W as a flag, but this routine is never called from the main game flow — it appears to be an unused utility.

Screen Layout

The board occupies even rows 2–14 (for the 6 card rows, addressed as 2*Y+1). Each column occupies 4 character positions (4*ZC-3). The string O$ (29 solid block characters) is used both as a “curtain” during scanning and to blank status lines. X$ (built as 7 repetitions of " ██") forms the hidden-card display pattern.

Test Routine Structure

The second program provides a standalone light pen verification tool. It displays a fixed trivia question about New York state capitals and scans four answer rows. The scan subroutine at line 300 flashes each cell on and off twice, accumulating a score in N: +10 if pen detected when cell is blank, +1 if detected when cell is lit. A score of exactly 20 (N=20) indicates a confident pen detection at that row. Only answer W=1 (“ALBANY”) triggers the “CORRECT” response.

Bugs and Anomalies

• Line 2590 prints “CONGRATIONS” — a typo for “CONGRATULATIONS.”
• Line 2120 prints “MAKE Y0UR FIRST CHOICE” with a zero instead of the letter O, but line 2490 corrects this with “MAKE YOUR FIRST CHOICE.”
• The win condition MATCHED<21 at line 2600 requires 21 matches, but only 14 distinct values are placed (yielding at most 14 or 7 pairs depending on interpretation) — the game may never reach the win state as written.
• Subroutines at lines 3250–3280 are defined but never called from any reachable code path.
• Line 2341 contains POKE 9000,0 using a literal address rather than the BLIP variable, breaking the abstraction used elsewhere.
• The score variable SC is initialized at line 2024 but MATCHED is initialized at line 105 in a different section; a RUN 2000 or mid-program restart would not reset MATCHED.

Key Variables Summary

Variable	Purpose
PEN	Address of machine code USR routine (16514)
DELAY	Line number used as GOSUB target for near-zero delay (3010)
DELAY1	Line number used as GOSUB target for variable delay (3030)
BLIP	Address for speaker POKE sound output (9000)
C(6,7)	Board cell character codes
P(6,7)	Board cell matched flags
ZC, ZD	Column and row of pen-selected cell
XC, XD	Column and row of first chosen cell
SC	Current score
MATCHED	Count of matched pairs found