Single-Cycle CPU

This project is a simple verilog project to implement a single-cycle CPU. The CPU is based on the 32-bit MIPS instruction set under Harvard architecture. The CPU is designed to be able to run on FPGA board.

For course report, see docs/course-report.pdf.

Instruction Set

Type	Instruction	Description
R	add	Add
R	addu	Add Unsigned
R	sub	Subtract
R	subu	Subtract Unsigned
R	and	Bitwise AND
R	or	Bitwise OR
R	xor	Bitwise XOR
R	sll	Shift Left Logical
R	srl	Shift Right Logical
R	sra	Shift Right Arithmetic
I	addi	Add Immediate
I	addiu	Add Immediate Unsigned
I	andi	Bitwise AND Immediate
I	ori	Bitwise OR Immediate
I	lw	Load Word
I	sw	Store Word
I	beq	Branch on Equal
I	bne	Branch on Not Equal
J	j	Jump
...	...	...

Usage

Edit the instruction file inst.asm in directory of test/.

Note

The first line of instruction may not be run. The duration of the first PC, which is initialized with 0x00000000, is shorter than one clock cycle time. During this short time caused by the positive edge of clock given at the very beginning, other components would finished initializing, and then wait for the next positive edge of clock and to trigger PC adding 4.

The command make inst will generate the hexadecimal instruction file inst.hex from inst.asm at the same directory. A derived online tool asm.tampoo.io is developed for this purpose.

As the code snippet in io/InstMem.v shows, the instruction memory is initialized with the hexadecimal instruction file inst.hex.

initial begin
    $readmemh("test/inst.hex", MEM_DATA);
end

Similarly, the data memory is initialized with the hexadecimal data file data.hex, as the code snippet in io/DataMem.v will load the data file data.hex into the data memory.

initial begin
    $readmemh("test/data.hex", MEM_DATA);
end

The example instruction file offered in test directory solves the problem of whether a year is a leap year. The year number is loaded from the address of 0x00000000 in data memory, and the result (0 for no, 1 for yes) will be saved at the address of 0x00000008 in data memory.

Development

This project offers a Makefile. Run make help for usage.

Usage:
  make              - Transform instruction file and run test
  make help         - Show this help message
  make inst         - Analyze and transform instruction file only
  make test         - Run test only
  make clean        - Remove build artifacts (*.vvp, *.vcd, *.out)
  make <file>       - Simulate specified verilog file
       <file>.vvp

For simulation, make sure the following packages are installed on your system:

• iverilog
• GTKWave

On Ubuntu/Debian-like linux, they can be easily installed:

apt-get install -y iverilog gtkwave

The make inst runs a JS script. Make sure you have any JavaScript runtime installed. node is taken default in Makefile.

Devcontainer is provided with recommended environment configured. You can launch a container on your IDE or create a codespace.

Simulation

Run make test to simulate the CPU. If the wave cannot meet the needs, modify the following fields at the top of the file top_sim_testbench.v.

`define N_CLOCKS  10'd500
`define N_OPERATE $stop

For example, you can modify N_CLOCKS to 10'd100 to simulate the CPU per 100 clocks, then enter cont in the terminal prompt to continue next 100 clocks of simulation.

You can modify N_OPERATE to $finish to finish and exit the simulation after N_CLOCKS clocks. Or you can set environment variable DEBIAN_FRONTEND=noninteractive to add -n to VVP_FLAGS, which means non-interactive ($stop = $finish). For example:

DEBIAN_FRONTEND=noninteractive make

Hardware Testing

Top module TOP_TouchDisplay (at top_touch_display.v), which depends on constraints.xdc as the constraint file and lcd_module.dcp as its display IP core, is designed and test on LOONGSON FPGA Board (FPGA-A7-PRJ-UDB XC7A200T-FBG676-2).

For hardware testing, instance cpu_rf in module Regfile has exposed test_addr and test_data, in order that we can see the register file's content on the touch screen. To remove these two ports, just remove all the lines or blocks in this project with comment ? TEST INTERFACE.

File top_touch_display.v allows you to input value into data memory, and meanwhile to read the data memory's content. At the top of the file, edit the following fields to change the address of input and output.

`define INPUT1_ADDR  32'b0000
`define INPUT2_ADDR  32'b0100
`define OUTPUT1_ADDR 32'b1000
`define OUTPUT2_ADDR 32'b1100

The first input value will be saved at the address of INPUT1_ADDR, and the second input value will be saved at the address of INPUT2_ADDR. When you touch OK on the touch screen, the value will be automatically saved into the corresponding address of data memory. The content of the address of OUTPUT1_ADDR and OUTPUT2_ADDR will be displayed on the touch screen.

The top module for this FPGA board also implements button control.

Button	Variable	Description
SW18	input_sel	0 to input for INPUT1_ADDR, 1 to input for INPUT2_ADDR
SW20	cont_clk	0 to use button single trigger, 1 to use continuous trigger
SW_STEP0	btn_clkn	Press to trigger CPU clock (disabled when cont_clk is 1) (Button Debounced)
SW_STEP1	cpu_rstn	Press to reset the CPU (PC and Register File) (Button Debounced)
FPGA_RST	/	Reset the touch screen (clear input, but not mean to clear Data Memory)

As well as LED display.

LED	Variable	Description
LED1	led_cout_rst	The reset signal (high level active)
LED2	led_cout_clk	The clock signal of CPU (high level active)

The basic clock frequency is 100 MHz. The CPU clock is 1 MHz. Debounce time is 10 ms.

References

• FPGA board product by Loongson
• Vivado Design Suite by Xilinx
• Video MIPS单周期CPU设计 | Verilog

License

CopyRight (c) Cnily03. All rights reserved.

Licensed under the MIT License.