Howard Mao

EZ8 Soft Microcontroller

EZ8 is an 8-bit processor architecture I designed to have a very simple encoding. The goal was to make the hardware implementation very easy and understandable. I implemented an EZ8 microcontroller in Verilog and ran it on an Arrow SoCKit Cyclone V development board.

Architecture Overview

The EZ8 architecture has an 8-bit address space and datapath with 16-bit instructions and a 12-bit instruction space. There is only one register, which is an accumulator register. Computational instructions like add, subtract, and bitwise operations take the accumulator and a memory address as arguments. There are also immediate versions of these instructions which take the accumulator and a literal as arguments.

The control flow instructions take the form of unconditional jumps, like goto, call, ret, and retint, or conditional skip instructions, which advance the program counter by +2 instead of +1 if a certain condition is met. The call instruction is an unconditional jump which saves the next instruction address on a stack before jumping. The ret instruction does the inverse, popping an instruction off the stack and jumping to that instruction. The EZ8’s stack has space for 15 instructions.

Microarchitecture Overview

The microarchitecture of the processor consists of a 3-stage in-order pipeline. The stages of the pipeline are instruction decode, memory read / execute, and memory writeback.

Here’s a simplified block diagram.

The pipeline is mostly pretty straightforward. Bypassing the output of the ALU to the input of the memory controller resolves read-after-write hazards without having to stall or insert NOPs into the pipeline.

The most complicated logic is built into the program counter controller. This unit resolves the control hazards that arise. It does this by sending the kill instruction, which disables writeback to the data memory, effectively turning a previously issued instruction into a NOP. If an instruction is an unconditional jump, the controller is not aware of this until after instruction decode, 1 cycle after issue. When an unconditional jump is detected, the controller kills the instruction issued after the jump instruction and then changes the program counter to the correct address. If a skip occurs, the controller does not know of it until 2 cycles after issuing the instruction. When a skip is detected, the controller kills the instruction after the skip and then continues as normal.

Functionality

The microcontroller can interface with simple peripherals like leds, keys, switches, and a millisecond-precision timer through memory-mapped IO. I was able to program the microcontroller onto the Cyclone V FPGA and run an example program that swept LEDs back and forth either at regular intervals or on the press of a button, depending on the setting of a switch.

I also implemented a software emulator and an assembler for EZ8 assembly. The Verilog descriptions, software source code, and documentation can be found on Github.