Popup Menu

This program implements a joystick-driven popup menu system intended as a user interface framework for a drawing or graphics application. The menu bar displays five options (MODE, ATTR, SCRN, FILE, QUIT) across the bottom status line, with the active menu highlighted using inverse video via OVER Z. Selecting a menu item causes a popup window to appear, drawn using custom UDG characters that form box-drawing borders, with menu entries listed between top and bottom border strips built from the strings T$ and B$. The STICK keyword is used exclusively for navigation — left/right moves between menus, up/down scrolls through items within an open popup, and the fire button confirms a selection. UDG “o” through approximately “o”+47 are loaded from DATA at line 50 to provide the box-corner and border graphics used in rendering the popup frames.

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Program Structure

The program divides cleanly into several functional regions:

1. Initialization (lines 10–40): Sets up constants, UDGs, and string templates, then jumps to the main loop.
2. UDG Data (lines 50–60): 48 bytes of pixel data for box-drawing characters loaded into UDG “o” onward.
3. Input Subroutine (line 100–102): Polls INKEY$, STICK fire, and directions; returns as soon as any input is detected.
4. Menu Input Handler (line 105–155): Draws the highlighted menu item, waits for joystick, and routes navigation accordingly.
5. Popup Draw Subroutine (lines 160–165): READs positional and content data to render a bordered popup window.
6. Popup Erase Subroutine (line 170): Clears the popup column by overprinting spaces.
7. Menu DATA (lines 200–230): Packed strings encoding position, width, and item labels for each of the four active menus.
8. Main Loop / Menu Dispatcher (lines 1003–1420): Renders the menu bar, dispatches to the correct DATA block per menu, and handles the QUIT menu specially.

Constants and Abbreviations

A notable idiom throughout is the use of Z= SGN PI (equals 1) and Y= NOT PI (equals 0) as compact substitutes for the literals 1 and 0. This saves a byte or two per use in the tokenized file. All GO TO and GO SUB targets are expressed as VAL "number" rather than bare literals, which is a known memory-saving technique in Sinclair BASIC. Similarly, INP, PRMN, and CINP hold the line numbers 100, 160, and 105 respectively, allowing GO SUB INP and GO TO CINP as shorthand.

UDG Usage

Lines 40–60 load 48 bytes of data into UDG “o” (and subsequent characters) to provide custom box-corner and border tile graphics. The DATA values contain the token k, which in context represents the BASIC keyword INK — this is likely a listing artifact where the tokenizer has stored the INK keyword byte (value 25 decimal / 0x19) in place of the intended numeric constant. This would be a data corruption bug in practice, causing incorrect UDG shapes unless the DATA were entered correctly with numeric values.

Popup Rendering

The popup window is built from two string templates. T$ (set in line 20) encodes a top border row, and B$ encodes a bottom border row, each using UDG block graphic characters. The subroutine at line 160 READs the row, column, width, and item count from the current DATA position, then prints the top border clipped to width, iterates item labels one per row with side borders, and finally prints the bottom border. Line 165 handles the bottom border print, and line 170 erases the popup by printing spaces over the same column range.

Navigation Logic

Within an open menu (lines 105–155), STICK directions are interpreted as follows:

• ST=2 — down: advances highlighted item, wraps from last to first.
• ST=1 — up: retreats highlighted item, wraps from first to last.
• ST=4 — left: decrements MEN (active menu index), wraps at 1.
• ST=8 — right: increments MEN, wraps at 5.
• Fire button (PB): jumps to OBJ, a line number set per menu to a stub REM where application-specific handling would be added.

Left and right navigation also erases the current popup (via GO SUB VAL "170") before returning to the main dispatcher at line 1003.

Menu Bar Highlight

The menu bar is printed on the lower status line (PRINT #Z) in line 1003. The active menu label is highlighted by overprinting a block of inverse video using OVER Z and the string C$ (six solid block characters), positioned at column (6*MEN)-5. This neat arithmetic places the highlight exactly over each six-character menu label slot.

QUIT Menu Handling

Menu 5 (QUIT) does not call the popup draw subroutine. Instead, lines 1400–1420 simply wait for joystick input: fire jumps to line 9999 (a non-existent line, causing the program to halt or error gracefully), left moves to menu 4, right wraps back to menu 1, and any other input loops.

Extensibility Design

Each menu’s selection handler is a stub REM line (1010, 1110, 1210, 1310) labeled with <><> comments indicating where application logic should be inserted. The variable OBJ holds the target line number, and the pattern LET OBJ= VAL "nnnn":GO TO CINP makes it straightforward to wire up real functionality by replacing each stub.

Anomalies

• Line 1003 tests MEN <> Z but the only value never dispatched to its own block is MEN=4 (FILE menu, line 1300 falls through only when MEN is not 3 and not 5), so menu 4 is handled at line 1305–1310 by exclusion rather than explicit equality.
• The variable T (current highlighted row) is set locally inside the CINP block at line 105 but also modified at lines 125 and 130; its relationship to W and T2 means the wrap logic depends on W being correctly set by the popup draw subroutine, which reads it from DATA.