A (currently) 4 bit Minecraft computer.
The Halvin is a 4 bit computer build using redstone in Minecraft. It was build for educational purposes, and is hence a very simple computer with no intention of being practical. The Halvin provides a very basic instruction set, which includes simple arithmetic (addition and subtraction), RAM operations and conditional branching. It has 2 general purpose registers, 8 x 4bit memory cells (4 bytes), an ALU with a built-in comparator and an instruction board to program on.
The PC has multiple parts which are available separately. The following World-Edit schematics are available:
parts/4bit_decoder- A 4 bit decoder used in the instructions and the 7 segment display.parts/7seg_dual_display- A 7 segment dual digit display. Does not include decoder.parts/ALU_comparator- ALU capable of subtracting and adding two 4 numbers. The ALU is also able to compare two numbers for equality and whether one number is greater than the other.parts/Bus_piece- Piece of the bus used to connect all components. The bus can extend redstone pulses bidirectionally.parts/Instruction_board- Board that is used for programming. Includes a decoder for a 4 bit program counter value.parts/PC_overwritable- Program counter that can either be overwritten with a custom value, or incremented by 1 tick.parts/RAM_XX- RAM modules that are addressable and can be read from and written to.parts/Register- R/W Register used in different parts of the computer.
The instructions are programmed through the instruction board, which uses levers to store an instruction. Currently, there are 9 working instructions in the Halvin with some OP codes left open for future instructions. In the current full computer build, a program can consist of 16 instructions.
Instructions in the Halvin are 12 bits wide.
Bits used to encode what instruction should be executed. The instructions with their corresponding op codes are explained in the next section.
Bits used to select what general purpose registers should be used in the instructions. There are two general purpose registers in the Halvin, so a low value selects register 1 and a high value selects register 2. For operations where two general purpose registers are needed (for instance: addition) there is a second register select bit.
Bits used in instructions that include memory operations. These bits are connected straight to the address decoder of the ram module, and are thus not used in instructions without memory operations. An exception is the first address bit, which doubles as the second register select bit. This is possible because an instruction never involves two values from general purpose registers and one from RAM.
Bits connected to the bus and the program counter. The constant bits should only be used when executing an instruction involving a constant, or a jump. As these bits are connected to the bus, they CAN NOT be used in other instructions. This is because the constant bits will remain on the bus until the next instruction executes, meaning it will meddle with instructions that expect the bus to be empty.
0b0001 - Moves a constant to a register. The first register selection bit is used to determine what register is used, and the constant is programmed in the constant bits.
0b0010 - Moves a constant to an address in memory. The memory address bits are used to select a ram cell, and the constant is put in the constant bits.
0b0011 - Moves value from register to an address in memory. The first register selection bit is used to determine what register is used, the memory address bits are used to select a ram cell.
0b0100 - Moves value from an address in memory to register. The same bits are used as in the register -> ram instruction.
0b0101 - Adds the value of one register to another. The first value is taken from the register selected in the first register selection bit, the second value is taken from the second register selection bit. The resulting value is loaded into the register selected in the first register selection bit.
0b0110 - Subtracts the value of one register to another. The bit layout is the same as the register + register instruction. The value in the second selected register is subtracted from the first selected register.
0b1000 - Compares the value of the first selected register with the one in the second selected register. The ALU tests whether the first value is greater than the second, and whether they are equal. The results of this instruction are stored in temporary 1 bit flag registers inside the ALU and are not accessible to a program. This instruction is comparable to how the x86 architecture handles branching.
0b1001 - Updates the program counter to point to an address in program memory if the equal flag is raised in the ALU. The address the program counter jumps to is given in the instruction the constant bits. Because this instruction is dependent on the compare instruction, compare should always be called before this instruction. Not doing so is considered undefined behaviour.
0b1010 - Updates the program counter to point to an address in program memory if the greater than flag is raised in the ALU. The bit layout is the same as the jump if equal instruction. This instruction too should be preceeded by the compare instruction.
Below are some screenshots of the computer from different angles.
