Chase H.Q. for ZX Spectrum — reverse-engineered and reconstructed in portable C

by David Thomas, 2023-2026

This is an in-progress disassembly and rebuild of the ZX Spectrum conversion of Chase H.Q. by Ocean Software.

A SkoolKit disassembly of the original game: control files, skool files and a Makefile that fetches the tape, makes a pristine snapshot and builds assembly listings, TAP/Z80 files and a cross-referenced HTML disassembly. Just the 128K version is covered, with a control/skool file for each paged bank.

The current disassembly output is available here. POKEs for infinite credits, time, turbos and more are listed in that README too.

A mostly hand-written, human-readable C reimplementation of the game engine, translated function by function from the disassembly and keeping the original game's Z80 addresses in the comments. It builds with CMake, links against SDL3 and plays through all five stages.

The two feed each other: the disassembly explains the game, the C port proves the explanation is right. They get out of sync sometimes.

The repository also builds a self-contained 32-bit !ChaseHQ desktop application. It runs one 48K or 128K game in a Wimp window or in single-tasking fullscreen mode. This first native release is deliberately silent; AY and beeper audio are deferred.

The target requires a 32-bit RISC OS system with the Wimp, ColourTrans and SpriteExtend. A 26-bit system is not supported. To build it, use a RISC OS cross-build environment containing AMU, Norcroft C, RISC_OSLib and riscos-build-run, then run these commands from the repository root:

riscos-amu test
riscos-amu install INSTDIR=install

The test should print ChaseHQ native tests passed. The install command creates install/!ChaseHQ; copy that application directory to the desired RISC OS filer location, or run it from there. Generated o32, aif32, install and .rotransform content is disposable and must not be committed.

An experimental 64-bit build is also available for local testing. It avoids RISC_OSLib and the Wimp application shell, starts a silent 128K game directly in single-tasking fullscreen mode, and exits back to the saved desktop when Escape is pressed:

riscos64-amu install INSTDIR=install64

This creates install64/!ChaseHQ. The fullscreen host has been exercised on a real 64-bit system, but remains deliberately outside CI.

Select the iconbar icon to open the game. The native host shortcuts are:

Gameplay keys are accepted only while the game window has the input caret. The original Spectrum keyboard is available, and the arrow keys emulate a Kempston joystick with keypad Enter as fire. Closing the game window stops that game but leaves the iconbar application running.

Fullscreen mode prefers a suitable 4-bpp mode, then falls back to 8-bpp or 32-bpp output. Set ChaseHQ$ScreenMode to a RISC OS mode number or mode string to override it, for example X800 Y600 C16. Only Escape is handled as a host shortcut while fullscreen because the application deliberately does not poll the Wimp in that mode.

• Project videos: https://www.youtube.com/watch?v=hWz7eTfn2l4&list=PLAn5dfRl63Oc
• Spectrum Computing forums thread: https://spectrumcomputing.co.uk/forums/viewtopic.php?p=113633
• Original artist Will Harbison's video about the game: https://www.youtube.com/watch?v=1pT6KOJFgMg
• Play the original Chase H.Q. online: http://torinak.com/qaop#!chasehq
• Lou's Pseudo 3D Page: https://www.extentofthejam.com/pseudo/index.html

• My previous Speccy disassembly project: https://github.com/dpt/The-Great-Escape
• ...and rebuild: https://github.com/dpt/The-Great-Escape-in-C