If a variable is assigned in always_ff declaration, it becomes register variable. Register variable will be mapped to flip-flop in synthesis phase.

always_ff has mandatory clock variable, optional reset variable, and {} block. Clock and reset are placed in (). The specified clock and reset should have clock / reset type and the witdh of them should be 1bit.

if_reset is a special keyword which can be used in always_ff. It means reset condition of the register variable. If if_reset is used, always_ff must have reset variable. if_reset can be conceal reset porality and synchronisity. The actual porality and synchronisity can be configured through [build] section of Veryl.toml.

module ModuleA (
    i_clk: input clock,
    i_rst: input reset,
) {
    var a: logic<10>;
    var b: logic<10>;

    always_ff (i_clk) {
        a = 1;

    always_ff (i_clk, i_rst) {
        if_reset {
            b = 0;
        } else {
            b = 1;