Question

interface simple_bus (input bit clk); // Define the interface
    logic req, gnt;
    logic [7:0] addr, data;
    logic [1:0] mode;
    logic start, rdy;

    task masterRead(input logic [7:0] raddr); // masterRead method
        // ...
    endtask: masterRead

    task slaveRead; // slaveRead method
        // ...
    endtask: slaveRead
endinterface: simple_bus

module memMod(interface a); // Uses any interface
    logic avail;

    always @(posedge a.clk) // the clk signal from the interface
        a.gnt <= a.req & avail // the gnt and req signals in the interface

    always @(a.start)
        a.slaveRead;
endmodule

module cpuMod(interface b);
    enum {read, write} instr;
    logic [7:0] raddr;

    always @(posedge b.clk)
        if (instr == read)

    b.masterRead(raddr); // call the Interface method
    ...
endmodule

module top;
    logic clk = 0;
    simple_bus sb_intf(clk); // Instantiate the interface
    memMod mem(sb_intf);
    cpuMod cpu(sb_intf);
endmodule

A function prototype specifies the types and directions of the arguments and the return value of a function which is defined elsewhere. Similarly, a task prototype specifies the types and directions of the arguments of a task which is defined elsewhere. In a modport, the import and export constructs can either use task or function prototypes or use just the identifiers. The only exception is when a modport is used to import a function or task from another module, in which case a full prototype shall be used.

The number and types of arguments in a prototype must match the argument types in the function or task declaration. The rules for type equivalency are described in Section 5.8.1, Equivalent types.