Life

This program implements Conway’s Game of Life cellular automaton, using a machine-code routine loaded from tape (via LOAD “”CODE at line 2) stored from address 30000 onward and occupying 410 bytes. The BASIC shell provides a menu for random or programmed colony starts, with a cursor-driven cell editor that allows placing or erasing cells using numeric keys mapped to directions and actions. Screen memory is directly manipulated via POKEs to address 22527+(32*r)+c for cursor display, and system variable 23659 (REPDEL) is toggled to control keyboard repeat behaviour. The machine-code entry points at USR 30088, USR 30139, and USR 30351 handle the random initialisation, Life evolution loop, and a third function respectively. Line 9998 saves both the BASIC and machine-code segments back to tape with LINE 1 autostart.

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Program Structure

The program is divided into clearly demarcated sections:

1. Lines 1–80: Initialisation — clears memory to 29999, loads machine code from tape, sets colours, and shows an introduction screen.
2. Lines 90–180: Main menu — offers random start, programmed start, or end.
3. Lines 190–320: Cursor-driven cell editor for programmed starts, using numeric keypad-style input.
4. Lines 400–610: Action handlers for returning to menu (400), placing a live cell (500), and erasing a cell (600).
5. Line 700: Clean exit restoring REPDEL and stopping.
6. Lines 1000–1020: Subroutines for screen/system setup and status-bar printing.
7. Lines 9998–9999: Save/verify routines for both BASIC and machine-code segments.

Machine Code Usage

The machine-code block is loaded to address 30000 and is 410 bytes long. Three distinct entry points are used:

Address	Call site	Inferred function
USR 30088	Line 170	Random colony initialisation
USR 30139	Line 410	Life evolution loop (run simulation)
USR 30351	Line 190	Programmed-start evolution / entry point

The result of each USR call is assigned to a variable (l or x) but the return values are unused; the calls are purely for their side effects on the display.

Screen and System Variable Manipulation

The cursor is tracked with row r and column c variables. The display-file address is computed with a DEF FN at line 210:

DEF FN p()=22527+(32*r)+c

This gives the address of character cell (r,c) in the Spectrum display file. The cursor character is written as POKE 30 (char code 18, a Spectrum block graphic), and the original cell value z is saved and restored on cursor movement.

System variable 23659 (REPDEL) is set to 2 during menus to enable key repeat, and to 0 during the cell editor to suppress it. System variables 23684–23687 are poked in subroutine 1000; these correspond to ATTR_P, MASK_P, P_FLAG, and the start of the print buffer area — effectively resetting permanent paper/ink attributes and cursor state.

Cell Editor Key Mapping

The editor reads INKEY$ and converts to a numeric digit with CODE INKEY$-48, valid range 1–9:

Key	Action
5	Move cursor left
8	Move cursor right
6	Move cursor down
7	Move cursor up
4	Place live cell (POKE p,170; z=48)
9	Erase cell (POKE p,170; z=54)
3	Return to menu

Movement uses the idiom c+(d=8)-(d=5), exploiting Boolean expressions evaluating to 1 or 0 to add or subtract without IF statements. Wrap-around at screen edges is handled by lines 310–313, providing a toroidal grid in the editor.

Notable Techniques and Anomalies

• The cursor display writes char code 18 (POKE p,18) directly to display RAM — a fast but attribute-ignorant method.
• Lines 500 and 600 branch back to line 245 (GO TO 245), but line 245 does not exist; the interpreter will fall through to line 250. This is a minor anomaly that works correctly in practice because Spectrum BASIC performs a sequential search and executes the next available line.
• The OVER 1 print at lines 20 and 100 draws underscores over the title text to simulate an underline effect.
• POKE 23659,2 sets REPDEL to 2 frames (very fast repeat) at menus; POKE 23659,0 disables repeat in the editor — a deliberate and correct use.
• The save routine at line 9998 uses SAVE "Life" LINE 1 to autostart the program, and saves the machine code separately as a CODE file starting at 30000, length 410.
• CLEAR 29999 at line 1 sets RAMTOP just below the machine-code load address, protecting the routine from being overwritten by BASIC’s memory management.