The BBC Tetris Companion
The BBC Tetris Companion
"Ever done dumb stunts for your friends on a dare?"
I was recently spending time with a group of vintage computing enthusiasts when the conversation shifted toward the BBC Bridge Companion. The consensus among the group—many of whom had owned one—was that it was a fundamentally useless piece of hardware.
Naturally, this was a challenge. I know how to play Tetris and I believe I remember how to write Z80 assembly.
Project Overview: Bridgetris
This post serves as a high-level summary of the "Bridgetris" project. I intend to publish more granular, technical deep-dives in the coming weeks, but for now, here is the "why" and "how" of this ridiculous endeavor.
What is the BBC Bridge Companion?
Essentially, it was a dedicated game console released by the British Broadcasting Corporation. Its sole purpose was to teach users how to play the card game bridge via their television. It was a tie-in for the BBC Bridge Club television series, where experts sought a digital medium to onboard beginners.
The Economics of 1985
The machine was prohibitively expensive. According to Wikipedia, the launch price in 1985 was £200. To put that in perspective:
| Device | Approx. Price | Year | Notes |
|---|---|---|---|
| BBC Bridge Companion | £200 | 1985 | Dedicated Bridge Machine |
| ZX Spectrum (48k) | £175 | 1982 | General Purpose Computer |
By 1985, a Spectrum would have been even cheaper, leaving the buyer with plenty of cash for actual bridge software. I suspect the price was either slashed later or was simply an unrealistic MSRP.
The Software Library
Games were delivered via ROM cartridges, each housed in a massive plastic "shock box" accompanied by a comprehensive coil-bound manual. There were only nine known releases:
- Advanced Bidding
- Advanced Defence
- Bridge Builder
- Club Play 1
- Club Play 2
- Club Play 3
- Conventions 1
- Duplicate 1
- Master Play 1
The Technical Guts
The hardware is surprisingly robust for a niche educational tool. It utilizes a Z80 CPU paired with a Texas Instruments Video Display Processor (VDP).
Specifically, it uses the TMS9129 (the PAL version of the TMS9128, which is the RGB sibling of the TMS9118). This is essentially the same power found in a ColecoVision.
Despite this "beefy" hardware, the original bridge games barely scratched the surface of the VDP's capabilities. I felt it was a waste of potential. Plus, I was overdue for another "Coleco-adjacent" homebrew project. If it fails, I can always port it to the actual ColecoVision so at least one other person can play it.
Designing "Bridgetris"
I needed a game that was timeless, challenging, and playable using the console's clunky, ATM-style buttons.
The Legal Hurdle:
The Tetris Company is protective of their IP. Since my version doesn't follow every strict rule and I'm not paying royalties, I've named it Bridgetris. Totally different name, totally different game.
The Mental Block: For years, I avoided making a block-puzzle game because I assumed the rotation logic was a mathematical nightmare. However, while building a similar game in Rust during the pandemic, I realized that rotation is typically just hardcoded.
In mathematical terms, instead of complex trigonometry, I can simply map the coordinates:
The To-Do List
- Figure out how to execute custom code on a console I don't physically possess.
- Write a full Tetris clone in Z80 assembly (my first major assembly project since high school TI-83 Plus coding).
- Document the process in a blog post.
Tricking the System
On simple systems like the Sega SG-1000, you just place your code at memory address $0000 and the CPU runs it upon reset. The BBC Bridge Companion is more sophisticated; it has an internal BIOS that mounts at $0000.
If the BIOS doesn't detect a valid cartridge, it displays a message telling the user to insert one. To bypass this, I used the MAME debugger to perform a step-by-step analysis of the boot sequence.
; Conceptual BIOS check
LD A, (CART_HEADER)
CP VALID_SIGNATURE
JR NZ, SHOW_ERROR_MESSAGE
JP GAME_START
After several nights of comparing official ROMs with my own, I successfully engineered a custom header. This "fooled" the BIOS into believing my homebrew code was a legitimate BBC product.
And from there... there was nothing left to stop me.