CPU 16

Simple load-store 16-bit CPU ISA, along with a Rust simulator and Veryl design.

Instruction Set

Instruction	Description	Opcode	Flags	Arguments
CTL	Control flags	0b00	Zero, Carry	<padding: 11> <ctrl_op: 3>
SET	Set register	0b01	-	<dst: 3> <value: 11>
MEM	Load/Store memory	0b10	-	<dst: 3> <addr: 3> <load/store: 1> <offset: 7>
ALU	ALU operation	0b11	Zero, Carry	<dst: 3> <src1: 3> <src2: 3> <alu_op: 5>

Registers

Register	Description	Index
Z	Zero register (always 0, cannot be written to)	0
R1	General purpose register	1
R2	General purpose register	2
R3	General purpose register	3
R4	General purpose register	4
R5	General purpose register	5
SP	Stack pointer	6
PC	Program counter	7

CtrlOps

Opcode	Operation
0b000	Halt
0b001	SetZero
0b010	ClearZero
0b011	SetCarry
0b100	ClearCarry
0b101	RestoreRegsAndFlags

AluOps

Opcode	Operation
0b000_00	Add
0b000_01	Sub
0b000_10	Adc (Add with carry)
0b000_11	Sbc (Subtract with carry)
0b001_00	AddIfZero
0b001_01	SubIfZero
0b001_10	AdcIfZero
0b001_11	SbcIfZero
0b010_00	AddIfNotZero
0b010_01	SubIfNotZero
0b010_10	AdcIfNotZero
0b010_11	SbcIfNotZero
0b011_00	AddIfCarry

The assembler defines common operations using these primitive instructions.

Generating bin files

Nexys A7 (Xilinx Artix 7 XC7A100T)

1. Install Vivado
2. Run make build-vivado in the fpga directory
3. The bitstream will be in fpga/build/cpu16_Top.bin
4. Run openFPGALoader or any other compatible tool to program the FPGA