Computed

Originally a port of my own cursed ISA, Computed introduces a 32-bit custom ISA for embedded systems... in Minecraft!

What does it offer?

The system used by Computed (mippi2) is more 'traditional' compared to other mods. Instead of writing pure Lua, or relying on custom ROMs with fixed resource requirements that might eat up more of your performance, you get to do whatever you want and leave it at that.

Basically, you can write a subset of assembly language to make more efficient devices.

- All Computed devices are virtual machines, however machine code is executed without JIT-style compilation to keep things a bit safer, and with the assumption that the JVM's own JIT can improve performance by using the fastest native instruction where possible. This hopefully should reduce the attack surface to JVM buffer handling, which if compromised would also mean that Minecraft Multiplayer in general is screwed. (TL:DR, the emulator aims to be safe instead)

- Expansion cards, however, are effectively written in Java for the sake of performance and being able to 'bridge' between the game and VM. This code execution is why cards can only be added via user-installed mods.

The best comparison I could make is Mojang's own DCPU-16 architecture on the ship computer in their cancelled Minecraft-like space sim, 0x10ᶜ, which has a very similar concept.

In terms of resource use, mippi2 is tested with the following configurations:

• Frequency: 50kHz 150MHz
• RAM: 4kB 2GiB (can be raised further later)
• Virtual Java TTY and Redstone Cards.

In this alpha release, the default system configuration is 64K of RAM alongside a 1MHz clock, with a Redstone Card installed. This will be changed once more expansion hardware is made.

mippi2 operates off of a simplified design with support for (currently) one IRQ at a time. Technologically, you're effectively working with hardware equivalent to the late 1980s/early 1990s.

A PDF with slightly more tech info is available here.

Computers

While the survival features of Computed aren't fully implemented, the recipe for a Computer is as follows:

When placed, you'll notice it doesn't really... do much. This is because it has to be programmed! This is done using any kind of text book, including signed ones.

Remember to back-up your code!

Instruction Table

Registers are written as r[number] in the assembler. Addresses use regular numbers OR a label name.

Instruction · Arguments · Description
NOP · [] · Does nothing.
LOAD · [register, address] · Loads two bytes from the address.
LOADI · [register, value] · Loads the immediate value you specify.
LOADS · [register, offset] · like LOAD, but with relative offset.
STORE · [register, address] · Store register to address.
STORI · [register] · Writes 2 bytes from a register at current address.
STORS · [register, offset] · Stores register with relative offset.
ADD · [register, register] · Effectively does A += B (Signed).
ADDU · [register, register] · Effectively does A += B (Unsigned).
SUBU · [register, register] · Effectively does A -= B. (Unsigned).
XOR · [register, register] · Effectively does A ^= B.
OR · [register, register] · Effectively does A \ · = B.
AND · [register, register] · Effectively does A &= B.
JMP · [address] · Jump to address/label
JMPR · [register, register] · Jump to address formed by (integer) `(A B
HALT · [value] · If > 0, halt for value's cycles. If 0, stop machine.

A common hack for systems like this is to do a B XOR A style operation, as this effectively copies a register, and has been situationally used to zero a register (A XOR A) faster than an x86 mov.

Signed subtraction can easily be done via ADD using values with existing sign bits. SUBU exists only to allow for unsigned subtraction.

Device Expansion

Computed machines are very configurable, especially if you want to create new hardware!

A simple ExpansionCard class can be extended to implement behaviours when the card bus is read from, written to, or is just doing its own thing!

In the future, an example 3D video card will be added. These can be provided as items to insert into machines once the API is fully complete.

Redstone support

Redstone support is currently provided by the mod itself, although redstone updates are a messy for now.

These can be set by writing a register to set a direction (starting from address 56565) high/low. To turn the east channel to full (15) strength, write a snippet like:

; Make a label for jumping
SETHIGH:

; set r1 to 15
LOADI r1 15

; set east to strength 15
STORE r1 56570

Note for admins: assembly files are temporarily stored before compilation (with the JVM instructed to delete after) in whatever the available temporary directory is. This means that if not on a true tmpfs, this could be spammed by someone to use up storage. This only really occurs if your server repeatedly crashes with a JVM error, and 99% of servers won't have to worry about this.

The device emulation is entirely performed within Java, so all architectures will function without separate compilation. However there may be potential for how the JVM treats the byte buffer to cause conflicts regarding endianness, however I have not tested this myself.