Game Killer

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Gamekiller Screenshot
Game Killer Screenshot
Technical Info
ROM 8 kbyte
RAM None
Hardware features 1 Button Freeze
Manual JPG: 01 page
CRT ID 42
Control Registers $DE00/$DF00 (likely all IO1/2)
Programming See Tech Section

The adverts found for this rather rare cartridge which was only available in the UK as of current knowledge contain the words:

WARNING: It is possible that some of the games you own will not have all the levels and screens promised by the software houses. This could mean that you could send them back due to false information under the Trade Description Act.

and is somehow funny wording since the rest of the advert just mentions that the cartridge is a revolutionary survival kit, disables sprite collision and for the interesting part start your game where you want!.

The company behind this advert just renamed from Robcom to ROBTEK as the advert states. Robcom was previously selling modems mainly. The change on the company name came with the introduction of the Game Killer cartridges if we can trust the adverts. Publisher Information at Amiga Hall of Light also notes ROBTEK as the company behind a few games.
Game Killer Cartridge

On powerup a real intro pops up. It is packed with MC CRACKEN COMPRESSOR 1.3 and includes a scroller reading:

Copyright (c) M.F.Zonnefeld/P.A.Share - ROBTEK 1986. Graphics by J.W.A.Kampstra. Music by R.Hubbard. Exclusively licensed to MASTERTRONIC Int. for America, Canada and Australia under the name 'INFINITY MACHINE'. Manufactured by RAM ELECTRONICS....GAME KILLER gives you the advantage over you computer. Break through your game with ROBTEKS GAME KILLER.....HIT SPACE TO CONTINUE....

This article from Zzap! also notes the tune by Rob Hubbard and this excellent article at Commodore Zone about Rob reads:

In 1986 the company Robtek marketed a new product called Game Killer for the Commodore 64. It was a cartridge-based device that allowed gamers to cheat when playing certain games that contained sprite collision code. The cartridge would allow the sprite collision to be switched off, which in effect meant that you could complete a game because the in-game enemies could never kill your character!

Unfortunately it did not work that well as it was only useful for certain games due to the software method used.

Robtek asked Hubbard to compose the theme music for the software. It was the only commercial software utility that contained a Hubbard soundtrack and what a soundtrack it is! It’s actually a remix of some music he did for the Human Race (Mastertronic 1986).

It has a memorable chorus line, which you won’t easily forget, it’s one of those tunes that just seems to stay in your head even after all these years - I can still hear the tune now! As usual it complements the software well and is most appropriate.


Game Killer Funtions
Commodore Horizons Issue 27, March 1986
Game Killer Cartridge
Game Killer Package Inlay


The Zzap! article and the package are explaining the only functionality of the cartridge being achieved by pressing the cartridge button alone to disable all collisions or while holding the joystick pushed up or down to disable just sprite/sprite or sprite/background collision. To enable all collisions again holding down the fire button while pressing the cartridge button is probably working fairly well. Commodore Horizons Issue 27 published some quick docs (see right) which slightly differ from the Zzap! article. They are also wrong on the last part since the cartridge button does not serve as a cheat toggle.

All Accu, X and Y reads on the sprite/sprite and sprite/background registers are properly replaced by LDA/X/Y #$00 ; NOOP where the NOOP has $1a for sprite/sprite and $7a for sprite/background. NOOP is a "stable" illegal opcode and additionally ensures that likely the correct code is replaced on restauration attempts since these code combinations are rarely used. One has to seriously admit that trying to apply these cheats very often leads to crashes and the press and people did not seem to like that either.

Game Killer Advert March 1986
Game Killer Advert May 1986

Tech

Game Killer _just_ made it into this wiki and the category Freezers due to it really applying the techniques required - on a very very basic level. A quick disassembly on the routine taking over the collision hunting and disabling shows a first weakness:

  • Zeropage address $44 will be trashed by the cartridge - it is being modified before some work area is freed and moved to the stack (including $44 again).
  • Kernal ROM routines are often hit hard and taken to a crash when trying to use Game Killer. Normal program routines are not that easily being crashed but Game Killers success rate is quite high in overall.

The following is cited from gamekiller.c found at the VICE project page:

ROBTEK "Game Killer"

- 1 8k ROM
- when cartridge is active, ultimax is enabled when addr>=e000, so the
  rom is visible at e000, below is normal c64 ram
- the code writes 0 to both de00 and df00 to disable the cartridge. we
  assume the cart uses the full io1 and io2 range
- when the freezer button is pressed the cartridge will be enabled and
  an NMI will be triggered

Binaries

Game_Killer_binaries_rr.c64.org_2010-04.rar contains just the C64 binaries:

Game_Killer_all_rr.c64.org_2010-04.rar contains:

Many thanks to TNT/Beyond Force for this special ROM dump and the scans!

Trivia

  • As it seems the cartridge was not an overwhelming success and sales were quickly coming to an end. Advertising apparently was stopped after a very short time and likely only cartridges of a first and final production run were sold. A quick look at the market before said production run might have saved ROBTEK from doing it at all since the competition was already going strong at that time.

Weblinks