Procedural Blocks: begin-end and fork-join

Procedural Blocks: begin-end and fork-join

begin-end and fork-join are used to combine a group of statements in a single block. 

General syntax with begin-end is as follows:

type_of_block @(sensitivity_list) begin: group_name local_variable_declarations; statements; end

          

type_of_block may be initial or always. sensitivity_list is optional and possible only in always block. You are knowing about initial and always block in the previous chapter.

           Sensitivity list is used to model sequential or combinational logic. To make it more clear, observe these examples:

module comb(a, b, c); input c; input b; output a; reg a; always @( c or b) begin a = c & b; end endmodule

module seq(a, b, c); output a; input b; input c; reg a; always @( posedge c) begin a <= b; end endmodule

Output of module comb

Output of the module seq

            Inside an initial or always block, we can group statements using begin--end or fork--join. 

            These are some examples to understand how begin--end and fork--join executes.
 

module forkjoin(clk, a, b); input clk; output a; output b; reg a, b; initial begin a = 0; b = 0; end always @(posedge clk) fork #2 a = 1; #1 b = 1; join endmodule	module forkjoin1(clk, a, b); input clk; output a; output b; reg a, b; initial begin a = 0; b = 0; end always @(posedge clk) fork #2 a = 1; #1 b = a; join endmodule
Output of forkjoin ^	Output of forkjoin1 ^
module beginend(clk, a,b);     input clk;     output a;     output b;     reg a, b;     initial     begin        a = 0;        b = 0;     end     always @(posedgeclk)     begin         #2 a = 1;         #1 b = a;     end endmodule	Output of beginend ^

From these examples, you can understand the execution.

We can nest begin--end and fork--join. That is, inside a begin--end we can have fork--join and inside fork--join we can have begin--end.  Consider these codes to find out how nested begin--end and fork--join works.

module nesting1(clk, a, b, c, d, e, f); input clk; output a, b, c, d, e, f; reg a, b, c, d, e, f; initial begin a = 0; b = 0; c = 0; d = 0; e = 0; f = 0; end always @(posedge clk) fork #2 a = 1; #2 b = 1; begin #2 c = 1; #2 d = 1; #2 e = 1; end #2 f = 1; join endmodule

and here is the output: You can notice that a, b, c and f became high 2 ns after the clock, d 2ns after c and e 2ns after d.

module nesting2(clk, a, b, c, d, e, f);     input clk;     output a, b, c, d, e, f;     reg a, b, c, d, e, f;     initial     begin        a = 0;        b = 0;        c = 0;        d = 0;        e = 0;        f = 0;     end     always @(posedge clk)     begin         #2 a = 1;         #2 b = 1;         fork             #2 c = 1;             #2 d = 1;             #2 e = 1;         join         #2 f = 1;     end endmodule

Output wave of the above code is here. Notice that a, b and c became 1 with 2ns delay in-between, d and e became 1 together with c, after that f with 2 ns delay.

Program control will come out of the fork--join block when all the statements finishes executing. in the case of code bellow, control will come out of block after 30ns.

fork
   #20 c = 1;
   #30 d = 1;
   #10 e = 1;
join