Iceberg - From logic gates to 16-bit OS

Iceberg is a virtual computer built from scratch, built starting from the logic gates and CPU to an assembler and compiler, whilst at the very high level Java is implemented.

I was inspired by the Nand2Tetris course where they used a simpler HDL, an easy assembly dialect and a Java-like language to teach various notions about Computer Architecture. I thought that it would be fun to recreate the same thing but using languages that are commonly used in the industry.

Architecture

Logic Gates (path: /gates)

Logic gates are the lowest level point from which this project is implemented. They are written in Verilog and then simulated using iverilog on a dedicated testbench, and any FPGA simulator will actually work. See Resources for an online 16-bit FPGA simulator. Most of the gates also have a 16-bit version and eventually a n-way version (some even n-way-16-bit).

Ex: a 4-way mux gate:

module mux4way(out, i0, i1, i2, i3, sel0, sel1);
    input i0, i1, i2, i3;
    input [3:0]sel;
    output [3:0]out;
    wire tmp0, tmp1, tmp2, tmp3;

    wire [3:0] [1:0] sel0 = sel;
    wire [0:3] [1:0] sel1 = sel;

    not(notSel0, sel0);
    not(notSel1, sel1);

    and(tmp0, i0, notSel0);
    and(tmp1, i1, sel0);
    and(tmp2, i2, notSel1);
    and(tmp3, i3, sel1);

    or4way(out, tmp0, tmp1, tmp2, tmp3);
endmodule

Predefined gates (path: /gates/predefined)

The predefined gates are by default implemented on Verilog/Systemverilog. Thus they are not strictly needed for building more advanced gates or chipset, but I still decided to add them as reference and learning purposes.

Processor (path: /cpu)

Arithmetic Logic Unit (path: /cpu/alu)

The ALU is written is Systemverilog (such as the CPU and other complex components). It handles two 16-bit inputs, a 6-bit opcode and a 16-bit output and a 2-bit sign flag. It computes the following instructions:

x+y
x-y
y-x
0
1
-1
x
y
-x
-y
!x
!y
x+1
y+1
x-1
y-1
x&y
x|y

The 6-bit opcode gets divided in single bits and refered to as:

za
na
zb
nb
f
no

The ALU logic manipulates the a and b inputs and operates on the resulting values, and in the exact order as it follows, thus permitting to make all of the 16 operations described before:

• if (za == 1) set x = 0 16-bit constant
• if (na == 1) set x = !x bitwise not
• if (zb == 1) set y = 0 16-bit constant
• if (nb == 1) set y = !y bitwise not
• if (f == 1) set out = x + y integer 2's complement addition
• if (f == 0) set out = x & y bitwise and
• if (no == 1) set out = !out bitwise not

And the 2-bit sign flag is: 0 (if output < 0) 1 (if output = 0) 2 (if output > 0)

Memory

The memory is also created from scratch, starting from a single-bit register that helps in the building of several types of flipflops, that will then be used to create defferent sizes of RAMs, a program counter, and a ROM. The final volatile memory will be a 24Kb RAM: 16Kb of general purpose memory, 8Kb allocated for screen memory and 16-bit for keyboard input.

Assembler

Compiler

Usage

Verilog/Systemverilog logic gates

To run any Verilog (.v) or Systemverilog (.sv) file, use iverilog (available for Windows, Linux and MacOS) using the command:

>> iverilog filename.v

To visually see the functioning of a gate and it's testbench you need to use gtkwave and have a .vcd file of the gate (plus eventually the testbench, in the same file):

>> gtkwave filename.vcd

Resources

• Online Verilog Compiler: https://www.tutorialspoint.com/compile_verilog_online.php
• Online FPGA Simulator: https://simulator.nirajmmenon.com/