Code Probe (VIC-20)

A machine language monitor for the Commodore VIC-20 and VIC-1211A Super Expander.

• Inspect and modify memory with a hex dump and interactive alter mode.
• View the CPU registers and processor status flags captured at the end of the last executed routine.
• Save and load machine language programs to and from tape, in PRG and SEQ formats.
• Loads at $0480 inside the VIC-1211A Super Expander's 3 KiB expansion RAM and is invoked from BASIC with SYS 1152.

See also: Code Probe (C64) — For the Commodore 64 verison of Code Probe.

🚀 Quick Start

Want to just run Code Probe? Download what you need from the v1.1 release:

File	Download	Use case
code-probe.prg	download	Run on the VICE emulator
code-probe.tap	download	Load on a real VIC-20 via Datasette / SD2IEC
code-probe-user-manual-vic-20.pdf	download	Read the manual

You'll also need either a physical VIC-1211A Super Expander cartridge if you're using a physical VIC-20, or a VIC-1211A Super Expander ROM if you're using an emulator. In the case of a ROM for emulation, See Super Expander ROM for the download link and setup.

Run on VICE:

xvic -memory 3k -cartA super-expander-a000.prg -autostart code-probe.prg

Run on real hardware (Super Expander cartridge inserted, code-probe.tap on Datasette or SD2IEC):

LOAD "CODEPROBE",1,1
RUN

📑 Contents

• 🚀 Quick Start
• 🔎 Overview
• 🕒 History
• 💾 Loading and Starting
• 📝 Command Reference
• ⚖️ Compared to Other VIC-20 Monitors
• 💻 Building From Source
• 👪 Code Probe Family
• 🙋‍♂️ Acknowledgements
• 📄 License

🔎 Overview

Code Probe is a software-based machine language monitor that runs in the 3 KiB expansion RAM of the VIC-1211A Super Expander cartridge and produces machine language programs that run on a stock unexpanded VIC-20 with no cartridge. The Super Expander must be inserted at boot; without it, there is no RAM at $0480 for Code Probe to live in.

The design of Code Probe was inspired by the DOS DEBUG utility, and presents a similar terminal-style user interface and commands. All numeric input is hexadecimal. Addresses are 4 digits, byte values are 2 digits, file types are 2 digits.

Features

• Memory inspection - Hex dump with PETSCII character display.
• Memory editing - Interactive alter mode with cursor navigation, auto-space between bytes, and auto-commit at five bytes per line.
• Register display - View A, X, Y, and P (with expanded flag bits) captured at the moment the most recent G command returned.
• Program execution - Run machine language programs via a JSR trampoline; the user routine returns to the monitor with RTS.
• Tape I/O - Save and load PRG and SEQ files to and from a Datasette. PRG files round-trip with the original load address; SEQ files carry raw bytes only.
• Screen control - Clear the display with a single command.
• Exit to BASIC - Return to BASIC's READY. prompt; re-enter Code Probe with SYS 1152.

🕒 History

I originally wrote Code Probe in 1988 on a Commodore VIC-20 with the VIC-1211A Super Expander cartridge. As a teenager I always wanted a cartridge with a monitor. I did end up getting a cartridge, the Super Expander, but it never came with a monitor. It did, however, have 3K of extra RAM, which at the time got me thinking, how hard could it be to make my own machine language monitor?

It turns out monitors aren't super complicated, especially a minimal one like Code Probe. You're basically just building a simple CLI that lets a user poke values into RAM, except using load and store (LDA, STA) instead of POKE. The monitor itself can be poked in via a BASIC machine language loader, which is how I built the very first version of Code Probe, using HEX-Loader.

I no longer have the original physical VIC-20 today, but I did end up getting a Commodore 64 in the early 1990s, which I used to copy Code Probe's PRG from tape to disk so I could use it to make a C64 version of Code Probe. From there, both the VIC-20 and C64 versions eventually made their way onto SD card in the early 2000s via SD2IEC. And then from SD card onto PC hard drive, where they remained dormant for many decades. ...Until now!

In 2026, just to see if it could be done, I fished out the PRG binaries and tried using Claude Code to disassemble them and generate modern Kick Assembler source listings for both. I'm pleased to report, it worked! Really well, in fact. I was able to use Claude Code together with Kick Assembler to fix a handful of long-standing bugs and make one or two small improvements.

Although the rebuilt versions aren't byte-for-byte identical to the originals, for the sake of nostalgia I kept the original attribution lines exactly as they appeared on the 1988 and 1990 builds: ROHIN GOSLING (1988) for the VIC-20 version, and ROHIN GOSLING (1990) for the C64 version. But they both do have a fresh coat of 2026 paint, courtesy of Claude Code and Kick Assembler.

💾 Loading and Starting

Code Probe loads at address $0480 (1152 decimal) and occupies 2614 bytes of RAM in the $0480-$0EB5 region of the Super Expander's expansion RAM. The full PRG file is 2743 bytes: a 13-byte BASIC stub at $0401-$040D, a zero-fill gap, the monitor body at $0480-$0EB5, plus the standard 2-byte PRG header. The native VIC-20 RAM at $1001-$1DFF is left free for user machine language programs.

From Tape

LOAD "CODEPROBE",1,1
RUN

The ,1,1 parameter loads the program to the address stored in its PRG header ($0401), rather than to the default BASIC area on the expanded machine. RUN invokes the BASIC stub, which executes SYS 1152 and transfers control to Code Probe. Alternatively, SYS 1152 typed by hand at the BASIC prompt has the same effect.

From the VICE Emulator

To launch xvic with the Super Expander cartridge attached, the 3 KiB Block 0 RAM enabled, and build/code-probe.prg autostarted into the monitor:

xvic -memory 3k -cartA roms/super-expander-a000.prg -autostart build/code-probe.prg

To boot a bare Super-Expanded VIC-20 without autostart, then attach a tape image inside VICE and load Code Probe by hand (LOAD "CODEPROBE",1,1 followed by RUN):

xvic -memory 3k -cartA roms/super-expander-a000.prg

Run both commands from the repository root so the relative paths to roms/ and build/ resolve. xvic must be on PATH, or substitute the full path to your VICE install.

What happens at startup

1. The screen border and background are set to black via VIC-I register $900F.

2. The KERNAL text colour at $0286 is set to white.

3. The screen is cleared via CHROUT $93.

4. The title banner is displayed:

   CODE PROBE      (v1.1)
   ROHIN GOSLING   (1988)
   

5. A blank line separates the banner from the first prompt.

6. The monitor prompt loop begins.

   CODE PROBE      (v1.1)
   ROHIN GOSLING   (1988)
   
   : █
   

📝 Command Reference

All address and count values are hexadecimal. Addresses are 4 digits, byte values are 2 digits, file types are 2 digits. Command dispatch is single-character: C, CLS, and CLEAR all route to the clear-screen command; E, EXIT, and END all route to the exit-to-BASIC command.

Command	Syntax	Description
A	A <address>	Enter alter mode to write hex bytes to RAM.
D	D <start> [<end>]	Hex dump memory from start to end (inclusive).
R	R	Display A, X, Y, and P (with expanded flag bits).
G	G <address>	Execute machine code at address; capture post-RTS regs.
S	S <start> <end> <type>	Save tape file. <type> = 01 PRG, 00 SEQ.
L	L <filename>	Load PRG file (uses file's load address).
L	L <filename> <address>	Load SEQ file to specified address.
CLS	CLS	Clear the screen.
EXIT	EXIT	Exit to BASIC. Re-enter with SYS 1152.

The S command splits across two lines: the first line names the address range and the file type, and the second line is an auto-prompted FILE: entry for the filename (up to 16 characters, unquoted). Splitting the filename off the command line means long names do not have to fit alongside three other tokens on the 22-column screen.

See docs/code-probe-user-manual-vic-20.pdf for the full user manual, including worked tutorials, the memory map, error messages, a quick reference card, and an appendix on attaching the Super Expander cartridge under VICE.

⚖️ Compared to Other VIC-20 Monitors

The VIC-20 has had several machine language monitors over the years, from full-featured commercial cartridges to minimal type-in listings published in magazines. Code Probe sits at the smaller, more focused end of that spectrum. Originally a type-in-style program that runs inside the Super Expander's existing 3 KiB RAM rather than requiring its own cartridge. Now converted into a modern open source assembly listing for Kick Assembler.

Monitor	Year	Form & Size	Notable Features
Code Probe (this project)	1988	Tape PRG, ~2 KB at $0480 (in Super Expander RAM)	Hex dump, alter mode, registers, JSR run, PRG and SEQ tape I/O. MIT-licensed modern 2026 assembly rebuild from the original 1988 binaries.
VICMON (VIC-1213)	1982	Cartridge, 4 KB ROM at $6000	Commodore's first-party monitor. Assembler, disassembler, breakpoints, single-step. The full-featured option.
HESMON (HES C302)	1982	Cartridge, 4 KB ROM	Commercial cartridge from HES (Terry Peterson). Assembler and disassembler.
Super VICMON	early 1980s	Type-in PRG, ~3 KB	Jim Butterfield's SuperMon ported to VIC-20 by David A. Hook. Hunt, transfer, mini-assembler, disassembler.
TINYMON1	1982	Type-in PRG, ~760 bytes	Jim Butterfield (COMPUTE! issue 20). M, R, G, S, L, X — the closest scope analogue to Code Probe in this list.

💻 Building From Source

Code Probe is a single-file assembly project built with Kick Assembler. Java is required.

Assemble:

java -jar KickAss.jar src/code-probe-vic-20.asm -odir build

The build produces build/code-probe.prg — a 2743-byte PRG that loads at $0401 (the BASIC start of an unexpanded VIC-20). The same PRG runs on a physical VIC-20 with the VIC-1211A Super Expander cartridge inserted, and on the VICE emulator with the Super Expander ROM image attached.

Running on a Physical VIC-20

Hardware required:

• A Commodore VIC-20 (PAL or NTSC).
• A VIC-1211A Super Expander cartridge inserted at boot. Code Probe loads at $0480, inside the cartridge's 3 KiB expansion RAM, and will not run without it.
• A means of transferring build/code-probe.prg from the build host to the VIC-20 — for example a Datasette with a PRG-to-WAV / PRG-to-TAP converter, an SD-card storage cartridge (SD2IEC, Penultimate Cartridge, Ultimate II), or a 1541 / 1541-II disk drive with a PRG-to-D64 toolchain.

With the Super Expander cartridge inserted and the PRG on tape, disk, or SD:

LOAD "CODEPROBE",1,1
RUN

The ,1,1 parameter forces the file to load at the address in its PRG header ($0401) rather than the default expanded-machine BASIC area. RUN invokes the embedded BASIC stub, which executes SYS 1152 and transfers control to the monitor. SYS 1152 typed by hand has the same effect.

Running in VICE

xvic -memory 3k -cartA roms/super-expander-a000.prg -autostart build/code-probe.prg

Run from the repository root so the relative paths resolve. xvic must be on PATH, or substitute the full path to your VICE install (e.g. C:\Programs\GTK3VICE-3.10-win64\bin\xvic.exe on Windows, /usr/bin/xvic on most Linux distributions).

Super Expander ROM

The VIC-1211A Super Expander cartridge image is not redistributed in this repository. Download it from the Zimmers archive and place it at roms/super-expander-a000.prg:

http://www.zimmers.net/anonftp/pub/cbm/vic20/roms/tools/4k/

The expected file is a 4098-byte PRG with a $A000 load-address header (4096 bytes of ROM plus a 2-byte header).

Note:
Two versions of the VIC-1211 Super Expander are available on zimmers.net. The VIC1211m, originally targeting the Japanese market in the 1980s, and the standard VIC-1211A, listed as Super Expander.prg on the download page. For Code Probe, download the standard VIC-1211A (Super Expander.prg) and rename it to super-expander-a000.prg so the xvic -cartA argument resolves. The -a000 filename suffix is a VICE smart-attach hint.

Tape Images

code-probe.tap is the project's primary tape image — a Datasette-format archive containing the compiled CODEPROBE PRG (the v1.1 monitor binary).

Additional tape images live in dist/examples/: cube.tap (a 3D rotating wireframe cube targeting the unexpanded VIC-20), and hello.tap / hello2.tap (minimal "hello world" greeters demonstrated in the user manual's tutorial chapter). These are round-trip test fixtures and tutorial subjects, not part of Code Probe itself.

👪 Code Probe Family

There are two main versions of Code Probe. The original 1988 VIC-20 version, and then the 1990 port to C64. Both were rebuilt in 2026 from their original PRG binaries using Kick Assembler and Claude Code.

Machine	Repository	Original	Status
Commodore VIC-20	code-probe-vic-20	1988	v1.1 — This repo
Commodore 64	code-probe-c64	1990	v1.1 — Commodore 64 version remo

🙋‍♂️ Acknowledgements

Tool	Author / Maintainer	Role in this project
Kick Assembler	Mads Nielsen	6502 cross-assembler. Builds codeprobe.prg from code-probe-vic-20.asm.
VICE	The VICE Team	Commodore emulator suite. xvic and x64sc for development and testing.
C64 TrueType	STYLE	TrueType C64 font set. Used to typeset the user manual in an authentic Commodore style.
Claude Code	Anthropic	AI coding assistant. Constructed the Kick Assembler listings from the original PRG binaries.

📄 License

Copyright © 2026 Rohin Gosling.

Code Probe and its accompanying user manual are distributed under the MIT License — a permissive, free-software licence that allows use, modification, and redistribution (including commercial use), provided the copyright notice and licence text are preserved.

This is a personal retrocomputing project shared for historical and educational purposes.