Language Constructs

Packages

Packages group related top-level declarations under a common namespace.

package VGA {
    const SCREEN_WIDTH = 640
    const SCREEN_HEIGHT = 480

    module SyncPulses(
        clk: clock,
    ) -> (
        hsync: bool,
        vsync: bool,
        row: uint[clog2!(SCREEN_HEIGHT)],
        col: uint[clog2!(SCREEN_WIDTH)],
    ) {
        // ...
    }
}

module Top(
    clk: clock,
) -> (
    hsync: bool,
    vsync: bool,
    vga_color: (r: u8, g: u8, b: u8),
) {
    let pulses = VGA::SyncPulses(clk, hsync, vsync)
    vga_color = (
        r: pulses.row[7:0],
        g: pulses.col[7:0],
        b: 8'd127,
    )
}

Members declared within a package are accessed using the :: operator.
Imported files are implicitly wrapped in a package with the same name as the file.

Constant Declarations

Constants are named fixed values known at compile-time.

Yodl / constant declarations

const BAUD_RATE = 115200
const CLOCK_FREQ = 100 * 1_000_000 // 100 MHz
const CYCLES_PER_BIT = CLOCK_FREQ / BAUD_RATE

Type Alias Declarations

Type aliases are used to give a name to a commonly used type.

Yodl / type alias declarations

type U8 = u8
type RGB = (r: U8, g: U8, b: U8)

Parameterised type aliases

Type aliases can be parameterised by providing a list of type parameters.

Yodl / parameterised type aliases

type Triplet[T] = (T, T, T)

module Top() -> () {
    let triplet: Triplet[bool] = (true, false, false)
}

Module Declarations

Modules enable hierarchical design by encapsulating reusable functionality.
A module is defined by a name, an optional list of parameters, and a list of input and output ports.

Yodl / module declarations

module FullAdder(
    a: bool,
    b: bool,
    carry_in: bool,
) -> (
    sum: bool,
    carry_out: bool,
) {
    let xor1 = a xor b
    sum = carry_in xor xor1
    carry_out = (carry_in and xor1) or (a and b)
}

Parameterised Modules

Modules can be parameterised by specifying a list of parameters surrounded by [ and ] after the module name.

Yodl / parameterised modules

module Adder[N: Nat](
    a: uint[N],
    b: uint[N],
    carry_in: bool,
) -> (
    sum: uint[N],
    carry_out: bool,
) {
    let carry_chain: [N + 1]bool
    carry_chain[0] = carry_in
    let bits: [N]bool

    for i in 0..<N {
        FullAdder(
            a: a[i],
            b: b[i],
            carry_in: carry_chain[i],
            carry_out: carry_chain[i + 1],
            sum: bits[i],
        )
    }

    carry_out = carry_chain[N]
    sum = uint!(bits)
}

Module instances

Module instances are first-class values that can be assigned to variables.
If a module is parameterised, all the parameters must be specified when creating an instance.
Parameter names can be omitted if the parameters are specified in the same order as the declaration.

Yodl / module instances

module Adder[N: Nat](
    a: uint[N],
    b: uint[N],
    carry_in: bool,
) -> (
    sum: uint[N],
    carry_out: bool,
) {
    let total: uint[N + 1] = a + b + carry_in
    sum = total[N - 1 : 0]
    carry_out = total[N]
}

module Top(clk: clock) -> () {
    let counter = Reg[u32](clk)
    counter.d = counter.q + 1

    let adder = Adder[32]( // or Adder[N: 32](..)
        a: counter.q,
        b: 32'1,
        carry_in: 1'0,
    )
}

A module instance need not assign all ports as the module's ports are accessible as fields of the instance using the . operator.
Ports whose values are local variables with the same name can omit the right-hand side of the assignment.

All three Reg instances in the following example are equivalent.

Yodl / module instances 2

module Top(clk: clock, rst: bool) -> () {
    let reg1 = Reg[u32](clk: clk, rst: rst)
    let reg2 = Reg[u32](clk, rst)
    let reg3 = Reg[u32]()

    reg3.clk = clk
    reg3.rst = rst
}

Let Bindings

A let binding introduces a named value in the current scope.
Let bindings must be defined inside a module declaration.
The type of the binding is inferred from the right-hand side of the assignment if present otherwise, the type must be explicitly specified.

Yodl / let bindings

let message1: [3]u8
message1 = "Yo!"
let message2 = "Hi!"
let message3: [3]u8 = "Hey"

Assignments

If a binding is assigned multiple times, only the last assignment is used.

Yodl / assignments

let a = true // this first assignment is ignored
a = false

Block Expressions

Block expressions group multiple statements in a new lexical scope and optionally return a value:

Yodl / block expressions

let result = {
    let a = 5
    let b = 10
    a + b  // Last expression becomes the block's value
}

assert!(result == 15)

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