elficpu1

Another one simple 4-bit CPU written in VHDL

Features

• 4-bit word CPU
• Up to 65536 words of instruction memory
• Up to 256 I/O ports
• Two general purpose registers A and B
• One 16-bit counter register CNT

Reset behaviour

Program counter is set to address 0x0000. General purpose registers are set to 0.

Instructions

• nop. No operation :)
• mov const, a. Load register A with a 4-bit constant data. Opcode: 1h X.
• mov const, b. Load register B with a 4-bit constant data. Opcode: 2h X.
• halt. Halts CPU. Opcode: Fh.
• out a, portaddr. Write register A to port. Opcode: 3h, X, Y.
• out b, portaddr. Write register A to port. Opcode: 4h, X, Y.
• init cnt, xxxx. Load register CNT with a 16-bit constant value. Opcode: 5h, A, B, C, D
• dec cnt. Decrement CNT register. Opcode: 6h.
• jnz addr. Jump on CNT register is not zero. Opcode: 7h, A, B, C, D.
• jmp addr. Jump to address. Opcode: 7h, A, B, C, D.

FPGA resources

On Sparten 3e board: 147 slices. Max. clock frequency: 118 MHz.

RTL schematics