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Notes on a novel in-game CPU: the dcpu-16

The hacker behind the Minecraft phenomena, Notch, is working on his next game, most likely another hit. This one is interesting in that it includes an in-game 16-bit processor called the dcpu-16. Details are sparse, but it seems as though gamers will use this processor to control spacecraft and play in-game games. The dcpu-16 spec is currently available at, and in the few days since its release there are already many community produced assemblers and emulators.

Like moxie, it's a load-store architecture with variable width instructions (16 to 48 bits long). But the dcpu-16's 16-bitness is pervasive. There are 8 16-bit registers, and the smallest addressable unit of memory is a 16-bit word. There are only about 16 unique opcodes in the 16-bit instruction, which means there's room for 2 6-bit operands. With only 8 registers, a 6-bit operand can encode multiple addressing modes (direct, indirect, offset, etc) and still have room for small literal values.

If you poke around github you'll find the start of a llvm backend as well as a tcc port. I haven't looked into these compilers, but a C ABI for the dcpu-16 would certainly be unusual to most developers. You would likely have a 32-bit long, but char, short and int would all be 16 bits.

As far as GNU tools go, a binutils port would be pretty straight forward. I created a branch in moxiedev to try my hand at a dcpu-16 binutils port. It's not very well tested, but gas, ld, objdump, etc all appear to work as advertised. All instructions with immediate operands, whether literal values or computed by linker relocations, are encoded in their long form. Taking advantage of the smaller encodings will require linker relaxation work. It's not rocket science, but more work than the couple of hours I was willing to put into it. There appears to be one bug in GNU ld related to handling relocations for ELF targets where the smallest addressable memory value is 16 bits vs 8. I worked around it by making one small non-portable change to the target independent linker code.

I think GDB should be fairly straight forward as well. For most real targets GDB will want to insert breakpoint instructions in the text of a program, and it wants that instruction to be the same size as the smallest instruction available on the target. Alas, the dcpu-16 has no breakpoint instruction, 16-bit or othwerwise, so the simulator will have to include special hardware breakpoint emulation logic. My suggestion is to repurpose some of the 16-bit illegal instruction encodings. For instance, the ISA allows for nonsensical instruction like this:

  SET 5, 6

This means set the literal value 5 to 6. Setting a literal value makes no sense, and the spec currently says that these instructions are silently ignored. Rather than ignore them, you could use this class of instruction as special software interrupt/breakpoint/trap instructions like moxie's swi.

A GCC port would be more challenging. It's definitely possible, but would stretch GCC outside of its comfort zone. You'd end up excercising bits of the compiler that aren't normally tested, and I imagine would end up spending a lot of time debugging some of the darker recesses of the compiler code. Best of luck to the brave soul who tries this!

I'm very curious to see how this all plays out. Given the massive success of Minecraft, I wouldn't be surprised if we see an app store for in-game dcpu-16 based games. Good luck to Notch and the team at Mojang.

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