Pix-FX

Pix-FX is a multi-function screen image editor that applies special effects to saved SCREEN$ files, including planet/globe wrapping, perspective transformation, magnify, reduce, copy and paste, merge, invert, and horizontal flip.

The program operates as a loader stub that pulls in a title screen and two machine code blocks (“fxa” and “fxb”) from tape, with nearly all processing delegated to those routines via RANDOMIZE USR calls at addresses such as 52480, 62869, 60398, 61217, and others.

Two separate screen buffers are maintained—a “Final Screen” (FS) and a “Working Screen” (WS)—with dedicated machine code swap, copy, and load routines for moving image data between them.

A compressed SCREEN$ save option is included, using a machine code compressor at address 36612 and reporting the byte count before saving as CODE from address 40246. Menu navigation is handled by a machine code routine at address 33467 that returns the selected item index in the BASIC variable X, driving GO TO arithmetic such as GO TO X*100+1000 and GO SUB X*500.

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Program Analysis

Program Structure

The BASIC listing is essentially a shell loader and menu dispatcher. Line 9990 handles initialization: it CLEARs memory, patches the system, loads a title screen CODE block, two main machine code files (“fxa” at 52480 and “fxb” at 32768), sets up system variables, and then RUNs from line 20. Line 9999 provides the master SAVE sequence for the whole package. The runtime program (lines 20–5500) is a layered menu system with all heavy computation offloaded to machine code.

Initialization and Machine Code Setup

Line 20 establishes a set of LET constants that are addresses of machine code entry points, then immediately calls RANDOMIZE USR C (C=32959) to run a startup routine. Line 30 sets border and system variables before calling the “menu display” routine at O=33595. The variables are:

Variable	Address	Purpose
C	32959	Startup/init routine
M	33467	Menu selection routine (returns choice in USR result → X)
O	33595	Display/overlay routine
RF	59987	Restore Final Screen to display
RW	60069	Restore Working Screen to display
SF	59975	Save display to Final Screen buffer
SW	60081	Save display to Working Screen buffer

Menu Navigation Idiom

The program uses a consistent pattern: display a menu with PRINT, call USR M (the menu selection machine code), store the result in X, then dispatch with computed GO TO or GO SUB arithmetic. Examples include GO SUB X*500 (main menu, line 60), GO TO X*100+1000 (storage sub-menu, line 1030), and GO SUB X*10+500 (help sub-menu, line 505). This is an efficient way to build multi-level menus without large IF/THEN chains. A return value of 0 from the menu routine consistently signals “exit this menu.”

Dual Screen Buffer Architecture

Two image buffers are maintained in RAM: a Final Screen (FS) and a Working Screen (WS). Machine code routines SF/RF copy between the display area and FS, while SW/RW do the same for WS. Lines 1325 and 1335 implement FS↔WS copy operations by chaining these routines. Line 1365 at address 61420 performs a full swap of FS and WS. This dual-buffer design allows non-destructive editing and comparison of two images.

Special Effects Dispatch

Each main menu item maps to a distinct line range and machine code entry point:

• Planet Maker (line 2000): RANDOMIZE USR 52480 — wraps the screen image around a globe.
• Perspective (lines 2500/3000/3500/3510): POKEs a mode byte into address Y=65535, then calls RANDOMIZE USR 62869 with three sub-modes (1, 2, 3).
• Magnify/Reduce/Copy&Paste (line 1500): calls RANDOMIZE USR 36721 after setting up screen and border conditions.
• Merge SCREEN$ (line 4000): RANDOMIZE USR 60398 — merges FS and WS, result stored in FS.
• Invert SCREEN$ (line 4500): chains RF → USR 60423 → SF.
• Horizontal Flip (line 5000): chains RF → USR 61217 → SF.

Compressed SCREEN$ Save

Lines 1200–1240 offer an optional compressed save. Machine code at address 36612 compresses the screen data, and USR 40192 returns the compressed length into L. After subtracting the base offset (40246), if the compressed size exceeds 6912 bytes (the raw SCREEN$ size), the program falls back to a normal SAVE and notifies the user. Otherwise it prints the exact SAVE command (including byte count) before executing SAVE N$ CODE 40246,L, letting the user see what will be saved.

Help System with Encoded Page References

Lines 510–560 store help page data as floating-point constants in Z, encoding two 16-bit addresses in a single BASIC number: the integer part is the low address (RANDOMIZE A seeds it so PEEK of system variables L=23670 and H=23671 gives the two bytes), and the fractional part scaled by 1000 gives the second address. Line 570 unpacks these into a six-byte structure at address V=61276 and calls USR V to display the relevant help page. This is a compact technique for passing two 16-bit pointer pairs through a single BASIC variable.

Loader and Save Sequence

Line 9990 is the bootstrap: it patches a LOAD address in the system area (PEEK 23631 + 256*PEEK 23632 + 5) to byte 101, loads the title graphic CODE block to 40246 (4750 bytes), then loads “fxa” (13056 bytes at 52480) and “fxb” (7424 bytes at 32768). System variables 23728–23607 are then set up before running. Line 9999 SAVEs all four components (BASIC loader with auto-run at 9990, title CODE, fxa CODE, fxb CODE) in sequence, which is the standard multi-block tape distribution method.

Notable Techniques and Anomalies

• LET Y=65535 in line 30 is used as a parameter-passing address — a single byte is POKEd there before calling the perspective machine code to select the transformation mode.
• Line 1100 uses PAUSE 0 followed by INPUT ; and CODE INKEY$ to detect whether the user pressed key “2” (ASCII 50) to choose the working screen view, a common keypress-detection idiom.
• Line 1357/1358 re-use the menu choice value X (still holding 4 or 5 from the sub-menu) after the LOAD to decide which buffer to move the freshly loaded image into — a tidy reuse of state.
• The POKE 23624,15 in line 1500 sets the border/paper color to white before the magnify/reduce/copy operation begins, ensuring a clean display state.
• Line 1301 issues OUT 254,7 (white border) and calls USR 61804 repeatedly across several routines as a standard “clear and reset display” helper, effectively factored into a reusable subroutine.