Verilog execution order

Question

I'm just starting to learn Verilog on my own after taking a course on VHDL. I'm having a trouble understanding the order in which behavioral statements are executed. Here is the code in question.

//This files is an experiment into the order in which verilog executes it's statements

module MainCircuit(clk, start);

    parameter cycles = 8;
    input clk;
    input start;
    //input [15:0] data;

    integer i;

    always @(posedge clk)
    begin
        if(start)
        begin
            i=0;

            repeat(cycles)
            begin
                @(posedge clk) $display("%d\ti = %d", $time, i);
                i = i + 1;
            end
        end
    end

endmodule

module tester;

    reg clk;
    wire start;

    assign start = 1'b1;
    initial clk = 1'b0;

    MainCircuit myMain(clk, start);
    initial repeat(40)
    begin
        #5 clk = 1'b1;
        #5 clk = 1'b0;
    end

endmodule

and here is the output

                  15    i =           0
                  25    i =           1
                  35    i =           2
                  45    i =           3
                  55    i =           4
                  65    i =           5
                  75    i =           6
                  85    i =           7
                 105    i =           0
                 115    i =           1
                 125    i =           2
                 135    i =           3
                 145    i =           4
                 155    i =           5
                 165    i =           6
                 175    i =           7
                 195    i =           0
                 205    i =           1
                 215    i =           2
                 225    i =           3
                 235    i =           4
                 245    i =           5
                 255    i =           6
                 265    i =           7
                 285    i =           0
                 295    i =           1
                 305    i =           2
                 315    i =           3
                 325    i =           4
                 335    i =           5
                 345    i =           6
                 355    i =           7
                 375    i =           0
                 385    i =           1
                 395    i =           2

I don't understand why i isn't being reset to zero at each positive clock edge. Does myMain remember where it is in execution and continue from there each time clock is called? If so, where is it stopping? And how would all this be synthesized?

Two other minor question:

I tried to write start <= 1'b01; instead of assign start = 1'b01; in the second module but it wouldn't work. Why not?

the second is what's with the weird spacing in the output?

Thank you for your time.

Answer 1

1. always @(posedge clk) doesn't automatically execute at every clock edge. An always block can only restart once it has reached the end of it's current execution (you can't simultaneously have two threads executing a block). Your always block doesn't reach end until after 8 clocks because it is stuck in your (repeat cycles) loop. Only after 8 posedge clocks can it finish the loop, hit the end of the always block, and then restart on the next posedge.

2. start <= 1'b1 is a nonblocking assignment and can only be executed inside an always block.

3. The $display is probably reserving enough space to print the max value of a 32bit decimal. If you don't want all the extra spaces try %0d instead of %d.