Laurie

Laurie is an interactive frame-by-frame animation drawing tool that uses the joystick (STICK function) to move a cursor around the screen and plot pixels or filled squares. The user can toggle drawing on or off, adjust the brush block size with ‘+’/’-‘ keys, and save completed frames as raw CODE at address 16384 with a length of 2048 bytes. A small machine code routine is poked into RAM at address 65000 and called via RANDOMIZE USR to copy display memory between two frame buffers located at 32000 and 65005/65002 address parameters. Line 410 bootstraps the machine code by parsing a packed decimal string into 12 bytes starting at address 65000, then re-runs the program.

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Program Analysis

Program Structure

The program is organized into several distinct phases:

1. Initialization / intro (lines 10–80): Clears memory above address 32000, displays a two-screen instruction sequence with BEEP/PAUSE 0 keypresses between pages.
2. Drawing loop setup (lines 90–130): Draws a reference line, seeds cursor position at x=125, y=165, sets draw flag a=1 and block size b=0.
3. Machine code call (lines 140–140): Calls the routine at 65000 via RANDOMIZE USR 65000 to perform a frame-buffer operation using parameters POKEd at 65002 and 65005.
4. Main drawing loop (lines 150–330): Reads joystick and keyboard, moves cursor, and plots pixels or inverse-video squares.
5. Frame save (lines 340–400): Copies the frame buffer back via the machine code, saves the display as CODE 16384,2048, then loops back to drawing.
6. Bootstrap (line 410): Installs the machine code routine from a packed decimal string, then re-runs the program.

Machine Code Bootstrap

Line 410 is the entry point when the program is first loaded. It clears memory to 65000, then decodes a 12-byte machine code routine from the string literal "033000125017000064001000008237176201". The string is consumed three characters at a time using VAL a$( TO PI) — since PI truncates to 3 in integer contexts here — extracting each byte value and POKEing it sequentially from address 65000. After installation the program re-runs from line 10 with RUN.

The 12 bytes decode as follows:

Address	Byte (decimal)	Z80 Mnemonic (approximate)
65000	033	LD HL, nn (low byte follows)
65001	000	(low byte of address)
65002	125	(high byte — POKEd to 125=32000 high)
65003	017	LD DE, nn
65004	000	(low byte)
65005	064	(high byte — POKEd to 64=16384 high)
65006	001	LD BC, nn
65007	000	(low byte = 0)
65008	008	(high byte = 8, so BC=2048)
65009	237	ED prefix
65010	176	LDIR — block copy HL→DE, BC bytes
65011	201	RET

This is a simple Z80 LDIR block copy. By changing the high bytes at 65002 and 65005 before calling the routine, the program can copy either from the display (16384) to the frame buffer (32000) or vice versa, effectively implementing a double-buffer flip for animation.

STICK-Based Joystick Input

Lines 210–280 use the TS2068 STICK function (rendered as |) to read joystick port 1. The return values follow the standard numeric encoding: 1=up, 2=down, 4=left, 8=right, with diagonals as sums (9=up-right, 10=down-right, 5=up-left, 6=down-left). Line 210 uses |(1,1)=0 to detect the neutral (no movement) state and loop back to line 150 without moving or drawing.

Line 220 checks |(2,1)=1 (fire button) and inverts the draw flag a, toggling between draw and erase mode.

Drawing Logic and Block Size

The cursor is represented by a single pixel plotted with PLOT OVER 1 (XOR mode) twice in succession at line 150, which acts as a visible cursor that does not permanently alter the canvas. The block size variable b is adjusted by pressing ‘k’ (increase) or ‘j’ (decrease). When drawing (a=1), lines 300–310 plot the anchor pixel and then draw a square of side b using four DRAW calls. When erasing (a<>1), lines 290 and 320 use INVERSE 1 to erase the same square.

Key BASIC Idioms

• LET x=x+(|(1,1)=8)-(|(1,1)=4) — Boolean expression used as ±1 delta, a compact idiom replacing IF/THEN chains for cardinal movement.
• VAL a$( TO PI) — Using the constant PI (≈3.14159) as a slice index; BASIC truncates it to 3, extracting three-digit substrings for byte decoding.
• RANDOMIZE USR 65000 — Standard idiom for calling a machine code subroutine; the return value from Z80 RET is discarded by RANDOMIZE.
• PAUSE 0 at lines 50 and 80 — Waits indefinitely until any key is pressed, gating the instruction screens.

Bugs and Anomalies

• Line 320 executes unconditionally when a<>1, but line 290 already branches with GO TO 320, meaning line 320’s IF a<>1 guard is redundant — though harmless.
• Cursor boundary checking is absent; moving beyond x=0, x=255, y=0, or y=175 will cause an “Out of screen” error from PLOT/DRAW.
• The ‘+’/’-‘ keys mentioned in the instructions (line 60) are not actually tested in the main loop; instead lines 190–200 use ‘k’ and ‘j’. This is a discrepancy between the documentation and the code.
• Line 340 uses PRINT AT 10,0; FLASH 1; BRIGHT 1;" " with a single space — it prints only one flashing space, likely intended as a visual indicator but has minimal visible effect.
• The SAVE "p"CODE 16384,2048 at line 380 saves only the first third of the display file (2048 bytes covers the pixel area but not the attribute area, which starts at 22528).