Spectrum Draw

This program is a BASIC loader and menu shell for a machine code drawing application called “DRAW,” converted from a Spectrum original (published in Your Computer, June 1984) to the Timex/Sinclair 2068. The BASIC sets up system variables via POKE, loads the machine code block at address 30000 (2610 bytes), and enters a tight loop at line 40 that repeatedly calls a machine code routine at USR 31967 to handle user input and drawing. Subroutines at lines 1000–1210 handle saving/loading screen data and overlaying text, while the circle-drawing subroutine at lines 3000–3130 computes a parametric circle in BASIC using SIN/COS, calling USR 31526 to execute each DRAW step in machine code. Line 9000 provides a SAVE routine to re-save both the BASIC loader and the machine code block.

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Program Structure

The program is organised as a thin BASIC shell around a substantial machine code block. Execution flows as follows:

1. Lines 10–30: Initialisation — set border/paper/ink via POKEs to system variables, CLEAR 29999 to protect the machine code area, then LOAD "DRAW" CODE to bring in 2610 bytes at address 30000.
2. Line 30: RANDOMIZE USR 30017 — one-shot machine code initialisation entry point.
3. Line 40: GO SUB USR 31967: GO TO 40 — the main event loop; the machine code routine handles input and calls back into BASIC subroutines as needed.
4. Lines 1000–1210: BASIC subroutines for saving a screenshot (SAVE … SCREEN$), loading a screenshot, and printing text onto the drawing.
5. Lines 3000–3130: A parametric circle-drawing subroutine, selectable with or without recentring (flag F).
6. Line 9000: Development utility to re-save both the BASIC program and the machine code block.

Machine Code Interface

Three distinct machine code entry points are visible in the BASIC:

Address	Role
30017	Redraw/refresh the screen or reset display state; called after most user-visible operations.
31967	Main interactive loop entry — reads joystick/keyboard and dispatches to BASIC subroutines via GO SUB USR.
31526	Execute a single DRAW step using delta coordinates held in BASIC variables X, IX, Y, IY; the REM comments on lines 3100 and 3120 document the intended arguments.
31176	Render the text string held in A$ onto the screen.

The pattern RANDOMIZE USR addr is the standard idiom for calling machine code while discarding any return value; the Z80 routine returns 0 in BC, which RANDOMIZE accepts without error.

The GO SUB USR 31967 idiom (line 40) is less common: the machine code routine must itself execute a Z80 RET that maps to a BASIC RETURN, causing the BASIC interpreter to treat execution as if a GOSUB had returned, then GO TO 40 re-enters it. This gives the machine code full control while still allowing it to invoke BASIC subroutines.

Memory Map and POKEs

Lines 10 and 20 configure both Spectrum system variables and custom machine code parameters:

Address	Value	Purpose
23693	7	System variable ATTR P — permanent attribute (white paper, white ink)
23694	0	System variable MASK P
23697	0	System variable P FLAG
32578	128	Machine code variable: current X position (low byte)
32579	88	Machine code variable: current Y position
32582	1	Machine code flag (likely draw colour/mode)
32583	1	Machine code flag
23681	0	System variable COORDS X (reset graphics cursor)

Parametric Circle Subroutine (Lines 3000–3130)

The circle routine demonstrates an interesting BASIC/machine-code hybrid approach. It computes the circle radius from two stored points (current position and an “input” point read from addresses 32578–32581), then steps around the circumference using COS and SIN:

• R = SQR((CX-IX)²+(CY-IY)²) — Euclidean radius in screen pixels.
• Step size S is chosen adaptively: 2*INT(INT(PI*SQR(R)+0.5)/2)+2 — this rounds up to the nearest even integer, ensuring the loop closes cleanly. It is clamped to 252 to avoid excessively long loops for large circles.
• Each vertex is passed to the machine code DRAW routine at USR 31526 as a delta from the previous point (X-IX, Y-IY), matching the Spectrum’s native DRAW dx,dy semantics.
• Lines 3000 and 3010 set flag F: when F=1 (entered via line 3010), the centre is moved to the previous input point before drawing, enabling concentric or chained circles.

Text Overlay (Lines 1200–1210)

Line 1200 uses an INPUT AT 0,0; PAPER 6; INK 0; prompt that dynamically calculates the maximum number of characters that fit on the remaining canvas width: INT((256 - PEEK 32578)/8 / PEEK 23728). Here PEEK 32578 gives the current X pixel position and PEEK 23728 is the system variable DF SZ (lower screen size). The trailing PRINT ; after the INPUT anchors the cursor, and then USR 31176 renders A$ using the machine code text renderer.

The REM A$ at the end of line 1200 is a documentation comment; in Sinclair BASIC a REM following a colon is never executed, so it serves purely as an in-line note to indicate that A$ is the operative variable.

Save/Load Subroutines

Lines 1000 and 1100 validate the filename (1–10 characters) before performing SAVE/LOAD … SCREEN$ operations. The validation loop IF NOT LEN A$ OR LEN A$>10 THEN GO TO 1000 re-prompts on empty or over-length names. After each save or load, RANDOMIZE USR 30017 is called to refresh the display from the machine code side.

Development Save Line

Line 9000 re-saves the full package: SAVE "specdraw" LINE 1 stores the BASIC with auto-run at line 1, followed by SAVE "DRAW" CODE 30000,2610 to dump exactly the 2610-byte machine code block. The subsequent STOP prevents accidental fall-through.