Last month's surprise announcement that pixelated open-world game Retro City Rampage will be ported to MS-DOS left us with a lot of questions. For one: Why? For another: How? Gamasutra talked to Retro City Rampage creator Brian Provinciano recently to get answers to those and other questions about the quixotic port.

In the interview, Provinciano says that a DOS version of the game was a "burning ambition that had been on my mind for at least half a decade, that I finally got to scratch." The porting took a month or two that Provinciano treated as a "vacation" after years of work porting the game to pretty much every modern platform.

Much of the optimization necessary to squeeze Retro City Rampage into a DOS-friendly format had already been done while doing a 3DS port, Provinciano told Gamasutra. Making that version required changing the HUD and cropping the screen to account for the lower resolution. It also meant "playing Tetris" with limited memory allocations, rather than just loading everything into RAM at once as in the other console versions.

Those memory issues were even more extreme in a DOS environment. "DOS machines have the 640 kilobytes of base RAM, and then they've got the extended RAM," Provinciano said. "So you've got to do separate allocators, so that if you want to take full advantage of all the RAM, you can't just use your normal allocation, because it won't give you access to the bottom 640K."

Provinciano was able to pare down inefficiencies to the point where the game's original 128MB or so footprint now fits inside 4MB of RAM for the DOS port. All those space savings were found with minimal impact on the game's features; the DOS version is only missing a few secret characters and pseudo-3D minigames.

"I got down to where the game was really close to being 4 megs of RAM, and also where it was really close to fitting on a single floppy disk as an executable installer file," he said. "So that's where I faced the biggest challenges: when you've got to get your game down to 1.38 megabytes to fit it on a floppy, and you've got it down to 1.52, well... let's just say it took a very long time to get rid of that last few hundred kilobytes."

Provinciano, who has had experience with DOS programming since the '90s, said he also ran into issues finding good documentation for DOS-specific issues; resources found online were often inaccurate or incomplete.

"I ended up having to go through this old, old book that I'd saved—I'd thrown away most of my old, obsolete programming books—and I was actually able to use that to figure out how to do things like trigger interrupts to play sounds on the PC speaker," he said.

While the DOS version of the game is all set for a digital release to owners of the PC/Mac versions (through DOSBox or as flat files that can run from a USB bootstick version of DOS), Provinciano is holding off release until he can manufacture an actual floppy disc version of the game. Good news if you've been holding on to an old 486 in the hopes of someday playing a new game on the system.