ZX81 Quiz Builder

This program is an interactive quiz-creation and quiz-taking tool that allows a teacher or user to author quizzes of three types: multiple choice, true/false, or direct answer. All question text, answers, explanations, and multiple-choice options are packed sequentially into a single large string array A$(10505,4) using a pointer table stored in the numeric array A(A+1,D), a compact storage technique that avoids wasting memory on fixed-length string slots. Six POKE statements at lines 283–288 directly manipulate memory at a calculated address (derived via PEEK 16400/16401) to set up what appears to be a machine-code or system-variable patch. The quiz engine supports optional wrong-answer explanations, inverse-video prompts built from the B$ banner string, and a final percentage score display; results can be saved to tape under the filename “QUIZ-64K”.

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Program Analysis

Program Structure

The program divides cleanly into three phases: configuration (lines 10–270), question authoring (lines 280–760), and quiz playback (lines 770–1260). During configuration the user selects quiz type, number of questions, whether wrong answers should be explained, and (for multiple choice) the maximum number of choices. Authoring collects questions, answers, explanations, and choices one at a time. Playback presents each question, accepts input, scores it, and finally reports a percentage grade.

Packed String Storage

Rather than declaring a 2-D string array with fixed-length rows (which wastes memory), the program stores all text in a single flat string array A$(10505,4) and maintains a pointer table in numeric array A(A+1,D). Each row F of A holds character offsets into A$:

• A(F,1) — start of question text
• A(F,2) — start of answer text
• A(F,3) — start of explanation text
• A(F,4) onward — start of each multiple-choice option

Slices are written with A$(A(F,n) TO A(F,n)+H)=E$ and read back with A$(A(C,n) TO A(C,n+1)-1). The sentinel value A(F+1,1)=A(F,D) (line 750) chains each question’s last offset to the next question’s start, making the whole array self-indexing.

Memory Manipulation (POKEs)

Lines 282–288 perform direct memory writes after computing a base address from the system variable at addresses 16400–16401:

L = PEEK 16400 + 256*PEEK 16401

The six bytes written starting at L+1 are:

Offset	Value	Decimal
L+1	41	0x29 — INC HL (Z80)
L+2	164	0xA4 — AND H
L+3	1	0x01 — LD BC,nn (first byte)
L+4	38	0x26 — LD H,n (or low byte of BC)
L+5	164	0xA4 — AND H
L+6	0	0x00 — NOP

The exact intent is unclear without knowing the system variable pointed to, but this is a known technique for patching the ZX81/TS1000 BASIC interpreter or I/O routines at run time to extend available memory or modify behaviour before the large DIM statements at lines 280–281 consume RAM.

Quiz Type Logic

The variable C$ encodes quiz type: "1" = multiple choice, "2" = true/false, "3" = direct answer. Branching is handled throughout with inline IF C$="x" THEN … guards rather than a structured subroutine, keeping the code flat but repetitive. For multiple choice, the correct-answer byte is overwritten in A$ at line 735 with CHR$(C+28), encoding the choice number as a low ASCII code to allow a single-character comparison at answer time (line 955).

Input Validation Idiom

Throughout the program, user input is validated by checking CODE of the first character against expected ranges. For example:

• Line 70: IF CODE C$<=28 OR CODE C$>=32 THEN GOTO 40 — accepts only characters with codes 29, 30, or 31 (digits “1”, “2”, “3” on the ZX81 character set).
• Line 140: accepts codes 28 or 29 only (“0” or “1”).
• Lines 270, 380, 490, etc.: similar range checks.

This avoids the overhead of string comparison for single-character inputs and is a common ZX81 BASIC idiom.

Banner String B$

Line 30 builds a reusable prompt banner B$ consisting of a long row of block-graphic characters followed by the inverse-video text ENTER ANSWER. It is printed at the bottom of most input screens as a consistent UI element. At line 950 only the first 32 characters of B$ are printed (B$( TO 32)), omitting the text portion when only a visual divider is needed.

Score and Save

The score accumulator S is reset to 0 at line 1200 (after display) so the quiz can be repeated without restarting. If the user opts to save, line 1230 executes SAVE "QUIZ-64K" which saves the entire program including the populated data arrays, allowing the completed quiz to be reloaded and retaken. Line 1250 (SAVE "1018%9") and line 1260 (RUN) appear to be a secondary save/restart block that would only be reached by a direct GO TO 1250; they are unreachable in normal flow.

Notable Bugs and Anomalies

• Line 530: IF CODE D$<=28 OR CODE D$>=3 THEN GOTO 600 — the condition CODE D$>=3 is always true for any printable character, so the explanation-entry block (lines 540–590) is never reached. The intended condition was probably CODE D$>=30 (mirroring line 140), meaning “skip if D$≠’1′”.
• Line 400/410: A(F,2)=A(F,1)+H and A$(A(F,1) TO A(F,1)+H)=E$ — the slice written is H+1 characters wide (inclusive both ends), but A(F,2) is set to A(F,1)+H, so when reading back at line 940 the slice A$(A(C,1) TO A(C,2)-1) is only H characters. This off-by-one means the last character of each stored string is effectively ignored during playback.
• Line 250: LET E=42022 is a hard-coded constant labelled as “total character input”; it bears no computed relationship to A (number of questions) but is used to display an “average characters per question” figure, so this is cosmetic only.
• Line 170: serves as a re-entry point for the choices-count screen; jumping here from line 215 on invalid input is correct, but it also re-runs CLS and skips re-printing the number-of-questions prompt, which could confuse a user who enters 10 or more choices.