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SystemVerilog constructs always_comb and always_ff.

always_comb : This construct uniquely differentiates Verilog always block for just combinational logic. So use only blocking assignments within this blocks. All statements are executed one after another.  Example next

 

 

module casetest ( …. );

//input/output ports

…....

always_comb

begin

casex (write_addess)

3'b00?:  w_ena_memory_block1: = 1’b1;

3'b01?:  w_ena_memory_block2  = 1'b1;

3'b1??:  w_ena_memory_block3  = 1'b1;

default:  w_ena_memory_block3  = 1'b1;

endcase

........

end

endmodule

always_ff: This construct uniquely differentiates Verilog always block for just synchronous logic. So use only non-blocking assignments within this blocks. All statements are executed in parallel in presence of clock.  So when these constructs are used, flip-flops are inferred for all preset_state to next_state assignments. Example Next.

module test ( …. );

.. //input/output ports

…....

........

always_ff @ (posedge clk, negedge reset_n)

begin

if(!reset_n)

next_state <= ’0;

else

next_state <= present_state;

End

…....

........

endmodule

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In system verilog two different syntax for always blocks can be used to limit the logical usage within code structure. The two new syntax’s are always_ff and always_comb. These constructs allow in simple differentiation of  combinational logic from sequential code segments.