用LLM辅助将1993年Amiga游戏移植至Godot引擎的工程复盘

In 1993, in Baghdad, I built a game called Babylonian Twins on an Amiga 500: 512KB of RAM, no hard drive, plugged into a TV. I was an engineering student in my twenties. Pure 68000 assembly, every sprite and every scanline by hand. Murtadha Salman drew the art and Mahir AlSalman composed the music. We were under sanctions. No internet, no game development resources, just one copy of the Amiga Hardware Reference Manual, which I used to program the hardware directly, and electricity a few hours a day. The constant floppy disk swapping (because of the small memory) and the 50°C summers killed my disk drive three times. Left: 1993, on the Amiga. Right: 2026, the same gateway. On the Amiga, “by hand” means the game doesn’t ask the operating system for anything while it runs. At startup it saves the interrupt vectors, switches the OS interrupts off and takes the whole machine: ``` move.l #$dff000,a0 ;Base for hardware registers lea save(pc),a1 ;Get the system move.w #$4000,intena(A0) ;from the AMIGA ``` “Get the system from the AMIGA” is my comment, from 1993. From that point on the display is the game’s own copper list (the Amiga’s programmable video coprocessor), rewritten on the fly for sprites and sky colours. Tiles move by writing the blitter’s registers directly and waiting on its done flag. The joystick is read straight from the hardware port, and the fire button is one pin on a CIA chip. The OS comes back only between levels, to load the next level’s files from the disk, and then it’s switched off again. It was the first commercial game made in Iraq, and for a long time a game very few people got to play. Commodore collapsed and sanctions scared off publishers, so the finished game sat on a shelf. An Amiga forum found it in 2008 from my brother’s YouTube uploads and hunted me down for the disks; the thread is still there. That game is finally out. The original 1993 disks are free on itch.io, and the rebuilt version this post is about, the Definitive Edition, is on iOS and Android now and comes to Steam this fall. The game has been ported once before, by hand, in 2010. The same team rebuilt it for the iPhone on an engine written from scratch, about 34,000 lines of C++, over months of nights and weekends. Apple and Google featured it, and it reached over two million downloads. That story is here. I didn’t do this port. I asked for it, played the result every night, said what felt wrong, and made the few decisions that needed somebody who was there in 1993. The file formats and the assembly reading were the AI’s work, and so were the decisions about how to carry thirty-year-old code across, and it went faster than I could follow. This post is what I found when I sat down weeks later and read what had been done to my own game. Some of it was wrong, and I didn’t notice for weeks. ## Why I tried again I’d tried this before. About a year ago I gave an earlier model the same Amiga material and asked it to make sense of my binary level maps. It got there in the end, but it took several rounds and a lot of hints from me. Then Claude Fable 5 shipped, and I gave it the same files. The test was deliberate. My guess was that there is little Amiga assembly code in LLM training sets. If the model was better at working things out rather than recalling them, this is where it would show. The July 4th weekend was coming up, so I planned three steps, each one conditional on the previous working. Step one, the safe ask: my own 2010 engine, the 34,000 lines of C++, moved into Godot 4. This was the control. Step two, the unfair ask: the original 72,758 lines of 68000 assembly, for a machine that had gone out of production, with no comments to speak of and nothing in common with the C++. Rebuild that in Godot too, at the Amiga’s original 50 Hz. Step three, the greedy ask: put the second one inside the first, so buying the modern game gets you the 1993 original as a second thing you can launch. All three worked. The level format that had taken several rounds and my corrections a year earlier came out in a single pass, with no hints from me. ## How it was run I ran it in Claude Code, so it had a terminal and my filesystem. It could edit files, run the assembler, build the game, launch the game and read what came back. When I say below that it rebuilt my 1993 binaries and checked them, it did that by running vasm and diffing the output. Early on it added a set of command-line flags to the game so it could play without me: ``` --level= load a level directly --pose= put the twins at exact positions --drive= press buttons on a script, frame by frame --probe dump switch / gate / door / key state --screenshot= render a frame and quit ``` Which turns “does the jump feel right” into something a machine can read: ``` drive[btw_jump:2.2] pos=(25.44, 24.04) vel=(0.00, -14.51) ground=false apex_y=22.48 ``` It also had two headless checks it could run before showing me anything: one that compiles every script, and one that builds every level and reports failures. On the Amiga side it drove the real toolchain, vasm to assemble and FS-UAE to boot the result. What wasn’t automated: there was no image comparison on the modern port (it took screenshots, I looked at them), and nothing checked whether the game felt right. ## Step one: 34,000 lines of C++ in an evening Wednesday night, the safe ask. Timestamps, unedited: ``` 22:23 Godot 4 project scaffold, asset sync, TMX level pipeline 22:44 both twins playable — collision, physics, camera, switching 23:19 all 38 entity types ported — full object roster live 00:35 full screen flow — menus, map, story, save, game flows 02:15 exporting to macOS, iOS and Android ``` Twenty-one minutes from empty project to a playable character. Every line it moved that night was a line I’d written, over months, in 2010. I went to bed confused. Getting it to feel right took about three days after that: jump arcs and trampoline timing, and hit detection that rewards mashing, fixed in batches on July 2nd, 3rd and 4th. I wasn’t testing alone. My thirteen-year-old son played every build with me. He’s always known I made this game, it’s a fact about his father he grew up with, but he’d never seen me working on it. The testing turned into a father-and-son thing I didn’t plan, and it’s one of my favourite parts of the whole project. ## Same units, same tick All the gameplay state lives in tile units (1.0 = one 48px tile), and the update runs at a fixed 60 Hz, because the 2010 iOS build ran at 60 Hz. That matters because the original applies drag multiplicatively, every frame: ``` static const float GROUND_DRAG_FACTOR = 0.85f; this->velocity.x *= GROUND_DRAG_FACTOR; // every tick! ``` Multiply by 0.85 sixty times a second and you get one amount of friction; multiply fifty times a second and you get another. Port it to a different tick rate and every acceleration curve in the game changes. Nothing crashes, it just feels wrong forever, and you won’t find it by reading the diff. At 60 Hz the constant transplants verbatim. This is also why the 1993 rebuild runs at 50 Hz and the modern one at 60: two sets of hand-tuned numbers, each only correct at its own tick. It kept both clocks. I’d have been tempted to tidy them into one. ## It didn’t use CharacterBody2D Godot ships CharacterBody2D and move_and_slide(), and every tutorial tells you to use them. The port used neither for the player. The original has its own hand-written movement code, and rebuilding that on somebody else’s physics would feel slightly wrong in ways that are miserable to track down. The player is a plain Node2D, and the 150-line collision routine came across line for line, including the fudge numbers I picked by feel fifteen years ago and the comments I wrote to my future self: