A simple 8-bit CPU designed to be built in Minecraft.
The CPU simulation is found in cpu.dig, where .dig is a file format for Digital. In order to run the simulation, you need to generate the microcode via the command below.
% python3 opcodes.py
The ROM chip in the simulation is configured to load the bootloader. The source is found in boot.asm. It must be assembled via customasm.
% mkdir bin/
% customasm asm/boot.asm -o bin/boot.bin
The emulator is found in emu/ and should be compatible with most C++ compilers. With GCC:
% mkdir bin/
% g++ emu/main.cpp emu/mc8.cpp emu/dis.cpp emu/bus.cpp -o bin/emulator
The ROM file is provided via a command line argument:
% ./bin/emulator bin/boot.bin
For systems with GCC available, the mc8.py script can be used to perform builds and runs: python mc8.py run rom_file or
python mc8.py build.
Points to the location of the next instruction to be executed. During normal operation, this increments transparently to the programmer. Its value can be changed via a jump operation.
The left hand operand and destination for all mathematical operations with full memory operation support.
The right hand operand for many mathematical operations with full memory operation support.
- Carry Flag serves as the 9th bit for mathemtical operations. For shifts, this represents the bit which fell off as a result of the operation. For arithmetic operations, this denotes that the result is greater than 255 or less than 0. It is undefined for logical operations.
- Zero Flag denotes whether the result of the previous mathematical operation was zero. Subtracting two values and checking the zero flag can be used to determine equality.
The mc8 instruction set uses opcodes encoded within five bits in order to simplify decoding. The top three bits are unused except for an add with carry and small immediate instruction, where they encode the immediate. Opcodes are followed by an optional 8-bit operand, which is either an immediate for mathematical operations or an address for memory and jump operations.
Do nothing. This instruction is uses as a placeholder and is used to provide delays within a program.
| Mnemonic | Address Mode | Carry Flag | Zero Flag |
|---|---|---|---|
nop |
Implied | Unchanged | Unchanged |
Swap the values in the A and B registers.
| Mnemonic | Address Mode | Carry Flag | Zero Flag |
|---|---|---|---|
swap |
Implied | Unchanged | Unchanged |
Set the A and B registers to zero.
| Mnemonic | Address Mode | Carry Flag | Zero Flag |
|---|---|---|---|
zero |
Implied | Unchanged | Unchanged |
Set the carry flag to zero.
| Mnemonic | Address Mode | Carry Flag | Zero Flag |
|---|---|---|---|
set |
Implied | 1 | TODO |
Set the carry flag to one.
| Mnemonic | Address Mode | Carry Flag | Zero Flag |
|---|---|---|---|
clear |
Implied | 0 | TODO |
Jump to the specified address if the specified condition is true.
| Mnemonic | Address Mode | Carry Flag | Zero Flag | Note |
|---|---|---|---|---|
jmp.c {addr: u8} |
Absolute | Unchanged | Unchanged | Jump to addr if the Carry Flag is set. |
jmp.nc {addr: u8} |
Absolute | Unchanged | Unchanged | Jump to addr if the Carry Flag is not set. |
jmp.z {addr: u8} |
Absolute | Unchanged | Unchanged | Jump to addr if the Zero Flag is set. |
jmp.nz {addr: u8} |
Absolute | Unchanged | Unchanged | Jump to addr if the Zero Flag is not set. |
Add with carry the value in the A register and a specified operand.
| Mnemonic | Address Mode | Carry Flag | Zero Flag |
|---|---|---|---|
addc a, b |
Accumulator | Updated | Updated |
addc a, {imm: i3} |
Immediate | Updated | Updated |
addc a, {imm: i8} |
Immediate | Updated | Updated |
addc a, [{addr: u8}] |
Indirect | Updated | Updated |
Subtract with borrow the specified operand from the value in the A register.
| Mnemonic | Address Mode | Carry Flag | Zero Flag |
|---|---|---|---|
subb a, b |
Accumulator | Updated | Updated |
Shift the specified value by 0 - 7 bits to the left, filling the open bits on the right with zeroes.
Shift the value in the A register by 0 -7 bits to the right, filling the open bits on the left with zeroes.
Compare the value in the A register with a specified operand by subtracting them and discarding the result.
Test the value of the specified operand by bitwise ANDind it with itself and discarding the result.
Bitwise AND the value in the A register and a specified operand and store the result in the A register.
Bitwise OR the value in the A register and a specified operand and store the result in the A register.
Bitwise XOR the value in the A register and a specified operand and store the result in the A register.
Bitwise NOT the value in the A register and store the result in the A register.
Load the specified register with a specified value.
Store the specified register to a specified location.