Magazine Index

This program implements a simple flat-file database for cataloguing projects with associated magazine references, storing up to five records each containing a project name (25 chars), magazine (16 chars), month/year (6 chars), page number (4 chars), and remarks (30 chars). The main menu uses `GOTO 1000*VAL I$` to dispatch between four options — data entry, viewing/editing, file clear, and tape save — using arithmetic on the menu choice character. Record entry uses five parallel string arrays (`P$`, `M$`, `D$`, `R$`, `N$`) dimensioned with a common variable `T=5`, and a counter `R1` tracks how many records have been entered. The editing sub-menu at line 2700 uses `GOTO 2800+(VAL Y$*10)` to jump to field-specific edit routines via computed branching. The display alternates between `FAST` and `SLOW` modes throughout to balance rendering speed with input responsiveness.

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Program Analysis

Program Structure

The program is organised around a main menu (lines 100–230) that dispatches to four functional sections using a computed GOTO. Lines 1000–1920 handle data entry, lines 2000–2999 cover viewing and editing, line 3000 performs a RUN (file clear by restart), and lines 4000–4080 manage tape saving.

Line Range	Function
4–90	Initialisation: array dimensions, counter reset
100–230	Main menu display and dispatch
1000–1920	Data entry loop
2000–2999	View, search, and edit records
3000	“Clear file” via RUN
4000–4080	Tape save routine

Data Model

Five parallel string arrays hold the database fields, all dimensioned to T=5 rows at startup:

• P$(T,25) — Project name (25 characters)
• M$(T,16) — Magazine name (16 characters)
• D$(T,6) — Month/Year (6 characters)
• N$(T,4) — Page number (4 characters)
• R$(T,30) — Remarks (30 characters)

The variable R1 tracks how many records have been populated. Entry begins at N=R1+1, so re-entering the entry routine appends rather than overwrites. Pressing Enter on an empty project field at line 1055 exits the loop and sets R1=N-1.

Computed GOTO Dispatch

The main menu validates that CODE I$ falls between 29 and 32 (the ZX81 key codes for digits 1–4) then executes GOTO 1000*VAL I$ at line 230. This cleanly maps menu choices to section start lines without an IF-chain. A similar technique appears in the edit sub-menu at line 2750: GOTO 2800+(VAL Y$*10) routes choices 1–5 to lines 2810, 2820, 2830, 2840, and 2880 respectively — note that 2850–2870 are absent, so choice 5 (Remarks) lands at 2880 rather than 2850, which is intentional.

FAST/SLOW Mode Alternation

The program switches between FAST and SLOW modes frequently. FAST is used for screen-clearing and rendering to avoid flicker, while SLOW is engaged immediately before INPUT or INKEY$ polling loops to allow the display to remain stable during user interaction. This pattern repeats throughout all four functional sections.

Key BASIC Idioms

• Inverse-video banner: Lines 110–115 use block graphic escapes to draw a bordered title box with a solid top bar, inverse-video text, and a bottom border made of ▄ characters.
• INKEY$ polling loop: Lines 200–210 and 2020–2060 implement a spin-wait on INKEY$ with validation against allowed key codes, a standard ZX81 idiom for menu selection.
• SCROLL for output management: The entry routine (lines 1010–1800) uses repeated SCROLL calls rather than CLS to create a rolling display of field prompts and entered values.
• Substring comparison for search: Line 2170 uses X$( TO LEN I$) to match only the leading characters of a stored project name against the search string, providing a simple prefix search.
• File clear via RUN: Option 3 jumps to line 3000 which contains only RUN, restarting the program and thus reinitialising all arrays — a common minimal “clear” technique.

Notable Anomalies and Observations

• Line 155 prints a warning in inverse video: “NEVER ENTER RUN-ENTER GOTO 102”, instructing the user to use GOTO 102 instead of RUN to return to the menu without losing data — since RUN would reinitialise all arrays.
• Line 180 immediately overwrites the normal text printed at line 170 with an inverse-video version of the same string (“ENTER ONE OF ABOVE”), creating an inverse prompt effect without clearing the screen.
• The edit field dispatch via GOTO 2800+(VAL Y$*10) accepts codes 29–36 (digits 1–8) but only defines handlers at 2810–2880 for five fields. Choices 6, 7, or 8 would jump to non-existent lines 2860, 2870, or 2880 — where 2880 happens to be the Remarks handler. Choices 6 and 7 would cause an error.
• Lines 4060–4080 appear after the tape save routine and are unreachable in normal execution: CLEAR followed by SAVE "1032%9" (saving under an inverse-character filename) and RUN. These lines appear to be a bootstrap or master save remnant left in the listing.
• The database capacity is fixed at T=5 records. Increasing T at line 5 is the only way to expand capacity, constrained by available RAM.