HELP

HELP is an interactive command-reference utility that provides two browsing modes for a built-in list of option commands: a scrollable list mode and a keyword query mode. In list mode, the program displays paired primary and secondary DATA entries side by side using the lower screen areas (PRINT #0 and PRINT #1), advancing on a keypress. In query mode, the user types a command name, and the program searches the primary DATA entries by comparing the first three characters, then retrieves the matching secondary description from a parallel DATA block offset by 100 line numbers (e.g., RESTORE data+100). The DATA is split into two separate blocks starting at lines 3800 and 3900, with the primary block terminated by a dummy space-filled string at line 3815 to mark the end of the list.

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Program Analysis

Program Structure

The program is organized into a set of clearly delineated routines, all residing in the 3000–3999 line range, with data from 3800 onward and a save statement at 9999. The top-level menu loop runs from lines 3001–3007, dispatching to one of two help modes or exiting.

Line Range	Purpose
3001–3007	Main menu: L = List, Q = Query, X = Exit
3010–3085	List mode — scrollable primary/secondary pair display
3110–3175	Query mode — keyword search and description retrieval
3690–3720	Screen-clear subroutine (PRINT #0 blanking)
3800–3815	Primary DATA block (command names)
3900–3914	Secondary DATA block (command descriptions)

Dual-Screen Output

The program makes deliberate use of both the upper screen (PRINT #1) and the lower two-line area (PRINT #0) to separate status/prompt information from content. The clear-screen subroutine at line 3700 uses PRINT #0; AT 0,0,,,, — printing four empty items — to blank the lower area without disturbing the main display.

List Mode Technique

List mode (lines 3010–3085) uses a sliding window approach: it pre-reads two adjacent DATA items into p$ and b$, displaying them as a pair. When the user presses S to scroll, p$ is assigned the value of b$ and a new b$ is read, effectively advancing the window by one entry without needing to re-RESTORE from the beginning. The loop bound is computed as (endata-data), i.e., 3815-3800 = 15 iterations.

Query Mode and the DATA Offset Trick

Query mode (lines 3110–3175) performs a linear search through the primary DATA block by incrementing a variable data from 3800 to 3815 and using RESTORE data before each READ. Only the first three characters of both the input and each DATA entry are compared (via string slicing v$( TO 3) and p$( TO 3)), making the match prefix-based rather than exact.

Once a match is found at line data, the secondary description is retrieved with RESTORE data+100. This works because the secondary DATA block starts exactly 100 lines after the primary block (line 3800 → 3900, 3801 → 3901, etc.), forming a parallel structure that is navigated purely by arithmetic on line numbers.

Key BASIC Idioms

• PAUSE 0 followed by INKEY$ checks — used in list mode (lines 3045–3060) as a keypress-wait loop.
• RESTORE data with a variable — unconventional use of RESTORE with a numeric variable holding a line number, enabling dynamic repositioning within the DATA stream.
• GO SUB 3700 before mode transitions — consistently called to clear the lower screen before returning to the menu.
• Line 3003 uses the standard IF INKEY$="" THEN GO TO 3003 wait idiom before lines 3004–3006 re-read INKEY$ for the actual keypress value. This introduces a potential race condition: a very fast keypress could be missed between lines 3003 and 3004.

Bugs and Anomalies

• INKEY$ race condition: Lines 3003–3006 each call INKEY$ independently. After the wait at 3003 detects a non-empty key, lines 3004, 3005, and 3006 each re-sample INKEY$. If the key has been released by then, all three conditions fail and the program falls through to GO TO 3001, effectively ignoring the keypress.
• Line 3035 syntax: PRINT #0; AT 0,15,, AT 0,15;p$ — the double comma after AT 0,15 prints two null items (effectively two spaces or separators) before repositioning, which is an unusual construction likely intended to clear a portion of the line before reprinting.
• Line 3160 capitalization: PRINT #0; AT 0,0;"OPTION QUERY>";P$ uses uppercase P$, which in Spectrum BASIC refers to the same variable as lowercase p$ — not a bug, but inconsistent style with the rest of the program.
• Secondary DATA misalignment: The secondary block has 15 entries (lines 3900–3914) but the primary block’s loop limit is endata-data = 15. Line 3902 describes “MOVES n PIXELS LEAVING NO LINE” but corresponds to primary entry “turn d” at line 3803 — the secondary descriptions appear shifted by one relative to the primary entries, suggesting a DATA entry was inserted or deleted inconsistently between the two blocks.
• Unused line 9900: STOP at line 9900 is never reached by normal program flow and serves no functional purpose.

Data Design

The primary DATA block documents commands for what appears to be a turtle-graphics or Logo-like interpreted language (commands such as turn d, draw n, go to x y d, object name-end, repeat n(options)). The HELP utility is therefore designed as an embedded reference system for a separate application, not a standalone program — it is intended to be merged into or called from that application’s line number space.