Graphics Toolkit

Graphics Toolkit is a machine code utility for the ZX81/TS1000 that provides a suite of display manipulation routines accessible via RAND USR calls. The first program segment installs the toolkit by copying machine code stored in a REM statement (line 10) into high memory using an LDIR block-copy routine, then saves it to tape. The demonstration program (second segment) showcases the toolkit’s capabilities: scrolling in four directions (up, down, left, right via USR 30696, 30699, 31666, and 30656), screen on/off toggling (USR 30702/30705), background fill with a selectable character (USR 31351), search-and-replace of screen characters (USR 31398), shape-behind-others (priority drawing, USR 30946/30954), and rectangle/square drawing (USR 31329). The banner in the first program is rendered entirely using ZX81 block graphics characters to spell out “GRAPHICS TOOLKIT” in large pixel-art lettering across six print lines. A guard check at line 250 uses PEEK 27394 to verify the system state before proceeding.

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Program Analysis

Overview and Structure

The listing contains two independent programs. The first (lines 10–260) is an installer/loader that embeds machine code in a REM statement, copies it into high RAM, and saves the result to tape. The second (lines 10–1020) is a demonstration program that exercises the installed toolkit’s routines one by one, with explanatory text and animated effects.

Program 1: The Installer

Line 10 is a REM statement whose body is raw Z80 machine code. The bytes decode as follows:

• 2A 10 40 — LD HL,(4010H): load the source address
• 11 06 00 — LD DE,0006H: offset
• 19 — ADD HL,DE: advance past the REM header to the payload
• 01 40 04 — LD BC,0440H: byte count for the block copy
• 11 C0 77 — LD DE,77C0H (decimal 30656): destination in high RAM
• ED B0 — LDIR: block copy machine code to destination
• 3A 25 40 — LD A,(4025H): read a system variable
• FE F7 — CP F7H: compare
• 20 F9 — JR NZ,-7: loop if not equal
• 21 C0 77 — LD HL,77C0H: reload destination
• 22 04 40 — LD (4004H),HL: poke address into system variable
• C9 — RET

Lines 20–30 initialize two toolkit parameters via POKE before the installer runs. Line 225 triggers the copy routine with RAND USR 31152, pointing into the REM body. Line 230 saves the configured system to tape. The guard at line 250 (PEEK 27394 <> 179) detects whether the toolkit is properly installed and calls NEW if not, preventing the demo from running on a bare system.

Program 2: The Demonstration

The demo introduces each toolkit feature in sequence, using delay loops and PRINT AT captions. The structure is largely linear with a single subroutine at line 1000 used as a short busy-wait delay (10 iterations of an empty FOR/NEXT).

Toolkit Entry Points

USR Address	Function
30656	Scroll up
30696	Scroll left
30699	Scroll right
31666	Scroll down
30702	Screen off
30705	Screen on
30946	Draw shape (with priority/behind)
30954	Erase shape
30962	Background fill setup
31017	Initialization
31152	Main installer entry point
31329	Draw rectangle/square
31351	Fill with background character
31398	Search and replace screen characters

Toolkit Parameter Passing

Parameters are passed to machine code routines by POKEing values into fixed RAM locations immediately before calling RAND USR. For example, the background fill routine reads the target character from address 31740/31741, the rectangle drawer reads coordinates from 31732/31733, and search-and-replace uses 31730/31731 for the find/replace character pair. The shape-drawing routines use 31738 for a position parameter, and PRINT AT is used to set the cursor before some calls, suggesting the routines read the system cursor position.

Block Graphics Banner

Lines 100–150 of program 1 print a large six-line banner spelling “GRAPHICS TOOLKIT” using only ZX81 2×2 block graphic characters. Each character cell is constructed from combinations of quarter-block elements (▘▝▀▖▌▞▛▄▟▙█▜▐▚▗), forming pixel-art letterforms without any UDG or PLOT operations. This is a common space-efficient technique for titles on systems with limited graphics modes.

REM as Data Store

Storing machine code in a REM statement (line 10 of program 1) is a well-established technique. The LDIR-based copy then relocates the code to a fixed high-memory address where it persists through subsequent RUN commands. The block size of 0x0440 (1088 bytes) indicates a substantial machine code library.

Delay Loops

Timing throughout the demo relies entirely on empty FOR/NEXT loops. The subroutine at line 1000 provides a short 10-iteration delay reused at multiple points, while inline loops of 100 or 200 iterations provide longer pauses between demonstration sections. There is no use of PAUSE.

REM Strings as Shape Data

Lines 10 and 20 of the demo program contain REM statements labeled SNAKE and CACTUS with sequences of characters that appear to encode shape data (using #, >, <, V, ", and colon sequences). These are likely sprite/shape definitions read by the machine code shape-drawing routines, which would scan the program text for these named records.

Anomalies and Notes

• Line 970 and 990 in program 1 both contain RUN, creating a path where the program restarts after saving — a common loader idiom to re-enter the demo automatically after tape operations.
• The RAND USR idiom is used throughout rather than USR in a LET statement, discarding the return value as is conventional when calling machine code for side effects only.
• Line 40 of the demo POKEs 151 into address 31742 and line 50 POKEs 22 into 30989 before the initialization call, suggesting these are configuration registers for the toolkit (possibly screen dimensions or feature flags).
• Lines 755–785 demonstrate search-and-replace by iterating I from 0 to 63, using RND*63 for the replacement character — effectively randomizing a range of character values across the screen.