Verilog - Operator, operand, expression and control - Ando KI Spring 2009

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 2 ) Contents Expression Operand Operator Bitwise and reduction operators Concatenation and replication example Sign example Shift example Expression bit-length Bit-length example Conditional statements if-else/if-else-if case Looping statements forever example repeat example while example for example control & conditional constructs if-else/if-else-if case looping repeat for while forever

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 3 ) Verilog expression Verilog expression combines operands with operators to produce a result. Scalar expression results a scalar (single- bit) result. The least significant bit of the result is used when the expression results in multi-bit. expression ::= primary | unary_operator { attribute_instance } primary | expression binary_operator { attribute_instance } expression | conditional_expression primary ::= number | hierarchical_identifier [ { [ expression ] } [ range_expression ] ] | concatenation | multiple_concatenation | function_call | system_function_call | ( mintypmax_expression ) | string

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 4 ) Verilog operand signed and unsigned constant An integer with no base specification shall be a signed value in 2s complement form. 12 and -12 An integer with an unsigned base specification shall be interpreted as unsigned value. d12 -d12 == -32d12 == FFFFFFF4 Sign base should be used for signed integer. -sd12 or –SD12 Verilog operand Constant number including real Net Variables of type reg, integer, time, real, and realtime Net bit-select Bit-select of type reg, integer, and time Net part-select Part-select of type reg, integer, and time Array element (not whole array) A call to a user-defined function or system-function that returns any of the above

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 5 ) Verilog operator (1/2) operatormeaningInteger expressReal expression unary + unary -Unary operatorsYes {} {{}}Concatenation, replicationYesNo + - * / **ArithmeticYes %ModulusYesNo > >= < <=RelationalYes !Logical negationYes && ||Logical and/orYes == !=Logical equality/inequalityYes === !==Case equality/inequalityYesNo ~ & | ^Bit-wise negation/and/or/xor/YesNo ~^ or ^~Bit-wise equivalenceYesNo & ~& | ~| ^ ~^ ^~Reduction and/nand/or/nor/xor/xnorYesNo >Logical left/right shiftYesNo >>Arithmetic left/right shiftYesNo ? :ConditionalYes orEvent orYes

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 6 ) Verilog operator (2/2) Logical operators: &&, ||, ! Result in one bit value Bitwise operators (infix): &, |, ~, ^, ~^, ^~ Operation on bit by bit basis Reduction operators (prefix): &, |, ^, ~&, ~|, ~^ Result in one bit value Logical shift operators: >>, << Result in the same size, always fills zero Concatenation operators: {, } Replication: {n{X}} Relational operators: >, =, <= Result in one bit value Logical equality operators: ==, != Result in either true or false Case equality operators: ===, !== Exact match including X/x and Z/z Conditional operators: ? : Like 2-to-1 mux Arithmetic/math operators: +, -, *, /, % If any operand is x the result is x. Unary: +, - operatorPrecedence unary + unary -Highest precedence ** * / % % + - (binary) > >> >= == != === !== & ~& ^ ^~ ~^ | ~| && || ? :Lowest precedence

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 7 ) Bit-wise and reduction operators Bit-wise binary AND Bit-wise binary OR Bit-wise binary XOR Bit-wise binary XNOR Bit-wise unary negation Unary reduction example

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 8 ) Concatenation and replication example module top; reg a; reg [1:0] b; reg [3:0] c; reg [6:0] d; reg [9:0] e; initial begin a = 1'b1; b = 2'b01; c = {4{a}}; // replication of 1b1 d = {3{b}}; // replication of 2b01 e = {b, c, b}; // concatenation $display("a=%b b=%b c=%b d=%b e=%b", a, b, c, d, e); end endmodule DIY code/verilog/expression/replicate example. # a=1 b=01 c=1111 d= e=

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 9 ) Sign example (1/2) module top; integer intA, intB, intC, intD; reg [15:0] regA, regB, regC; reg signed [15:0] regSA, regSB; initial begin intA = -12 / 3; // The result is -4. intB = -'d 12 / 3; // The result is intC = -'sd 12 / 3; // The result is -4. intD = -4'sd 12 / 3; // -4'sd12 is the negative of the 4-bit // quantity 1100, which is -4. -(-4) = 4. // The result is 1. $display("intA : -12 / 3 => %d 0x%h", intA, intA); $display("intB : -'d 12 / 3 => %d 0x%h", intB, intB); $display("intC : -'sd 12 / 3 => %d 0x%h", intC, intC); $display("intD : -4'sd 12 / 3 => %d 0x%h", intD, intD); // DIY code/verilog/expression/sign example.

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 10 ) Sign example (2/2) intA = -4'd12; // intA will be unsigned FFFFFFF4 regA = intA / 3; // expression result is -4, // intA is an integer data type, regA is regB = -4'd12; // regB is intB = regB / 3; // expression result is 21841, // regB is a reg data type intC = -4'd12 / 3; // expression result is // -4'd12 is effectively a 32-bit reg data type regC = -12 / 3; // expression result is -4, -12 is effectively // an integer data type. regC is regSA = -12 / 3; // expression result is -4. regSA is a signed reg regSB = -4'sd12 / 3; // expression result is 1. -4'sd12 is actually 4. $display("intA : -4'd12 => %d 0x%h", intA, intA); $display("regA : intA / 3 => %d 0x%h", regA, regA); $display("regB : -4'd12 => %d 0x%h", regB, regB); $display("intB : regB / 3 => %d 0x%h", intB, intB); $display("intC : -4'd12 / 3 => %d 0x%h", intC, intC); $display("regC : -12 / 3 => %d 0x%h", regC, regC); $display("regSA : -12 / 3 => %d 0x%h", regSA, regSA); $display("regSB : -4'sd12 / 3 => %d 0x%h", regSB, regSB); end endmodule DIY code/verilog/expression/sign example. # intA : -12 / 3 => -4 0xfffffffc # intB : -'d 12 / 3 => x # intC : -'sd 12 / 3 => -4 0xfffffffc # intD : -4'sd 12 / 3 => 1 0x # intA : -4'd12 => -12 0xfffffff4 # regA : intA / 3 => xfffc # regB : -4'd12 => xfff4 # intB : regB / 3 => x # intC : -4'd12 / 3 => x # regC : -12 / 3 => xfffc # regSA : -12 / 3 => -4 0xfffc # regSB : -4'sd12 / 3 => 1 0x0001

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 11 ) Shift example module top; reg [3:0] valueL, resultL, resultLS; reg signed [3:0] valueA, resultA, resultAS; initial begin valueL = 4'b1000; resultL = (valueL >> 2); resultLS = (valueL >>> 2); // be careful valueA = 4'b1000; resultA = (valueA >> 2); // be careful resultAS = (valueA >>> 2); // $display("valueL : %d 0x%h", valueL, valueL); $display("resultL : (valueL >> 2) ==> %d 0x%h", resultL, resultL); $display("resultLS : (valueL >>> 2) ==> %d 0x%h", resultLS, resultLS); $display("valueA : %d 0x%h", valueA, valueA); $display("resultA : (valueA >> 2) ==> %d 0x%h", resultA, resultA); $display("resultAS : (valueA >>> 2) ==> %d 0x%h", resultAS, resultAS); end endmodule DIY code/verilog/expression/shift example. # valueL : 8 0x8 # resultL : (valueL >> 2) ==> 2 0x2 # resultLS : (valueL >>> 2) ==> 2 0x2 # valueA : -8 0x8 # resultA : (valueA >> 2) ==> 2 0x2 # resultAS : (valueA >>> 2) ==> -2 0xe

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 12 ) Expression bit-length The Verilog uses the bit length of the operands to determine how many bits to use while evaluating an expression. reg [15:0] a, b; // 16-bit regs reg [15:0] sumA; // 16-bit reg reg [16:0] sumB; // 17-bit reg sumA = a + b; // expression evaluates using 16 bits // it can lose carry overflow. sumB = a + b; // expression evaluates using 17 bits sumA = (a+b)>>1; // will not work properly

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 13 ) Bit-length example module top; reg [3:0] a, b, sumA; reg [4:0] sumB; initial begin b = 10; for (a=0; a<10; a=a+1) begin sumA = a + b; $display("sumA (%d) = a (%d) + b (%d)", sumA, a, b); b = b + 1; end b = 10; for (a=0; a<10; a=a+1) begin sumB = a + b; $display("sumB (%d) = a (%d) + b (%d)", sumB, a, b); b = b + 1; end b = 10; for (a=0; a<10; a=a+1) begin sumA = (a + b)>>1; // incorrect result sumB = (a + b)>>1; // correct result $display("sumA (%d) = (a (%d) + b (%d)) >> 1; %d", sumA, a, b, sumB); b = b + 1; end endmodule DIY code/verilog/expression/bitlength example.

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 14 ) Bit-length example # sumA (10) = a ( 0) + b (10) # sumA (12) = a ( 1) + b (11) # sumA (14) = a ( 2) + b (12) # sumA ( 0) = a ( 3) + b (13) # sumA ( 2) = a ( 4) + b (14) # sumA ( 4) = a ( 5) + b (15) # sumA ( 6) = a ( 6) + b ( 0) # sumA ( 8) = a ( 7) + b ( 1) # sumA (10) = a ( 8) + b ( 2) # sumA (12) = a ( 9) + b ( 3) # sumB (10) = a ( 0) + b (10) # sumB (12) = a ( 1) + b (11) # sumB (14) = a ( 2) + b (12) # sumB (16) = a ( 3) + b (13) # sumB (18) = a ( 4) + b (14) # sumB (20) = a ( 5) + b (15) # sumB ( 6) = a ( 6) + b ( 0) # sumB ( 8) = a ( 7) + b ( 1) # sumB (10) = a ( 8) + b ( 2) # sumB (12) = a ( 9) + b ( 3) # sumA ( 5) = (a ( 0) + b (10)) >> 1; 5 # sumA ( 6) = (a ( 1) + b (11)) >> 1; 6 # sumA ( 7) = (a ( 2) + b (12)) >> 1; 7 # sumA ( 0) = (a ( 3) + b (13)) >> 1; 8 # sumA ( 1) = (a ( 4) + b (14)) >> 1; 9 # sumA ( 2) = (a ( 5) + b (15)) >> 1; 10 # sumA ( 3) = (a ( 6) + b ( 0)) >> 1; 3 # sumA ( 4) = (a ( 7) + b ( 1)) >> 1; 4 # sumA ( 5) = (a ( 8) + b ( 2)) >> 1; 5 # sumA ( 6) = (a ( 9) + b ( 3)) >> 1; 6 DIY Overflow occurs

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 15 ) Conditional statement: if-else/if-else-if The conditional statement (if-else or if- else-if statement) is used to make a decision about whether a statement is executed. If the expression evaluates to true (that is, has a nonzero known value), the first statement shall be executed. If it evaluates to false (that is, has a zero value or the value is x or z), the first statement shall not execute. If there is an else statement and expression is false, the else statement shall be executed. conditional_statement ::= if_else_statement | if_else_if_statement if_else_statement ::= if ( expression ) statement [ else statement ] if_else_if_statement ::= if ( expression ) statement { else if ( expression ) statement } [ else statement ] Statement will be a single statement or a block of statement that is a group of states enclosed by begin and end.

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 16 ) Case statement The case statement is a multiway decision statement that tests whether an expression matches one of a number of other expressions and branches accordingly. The default statement shall be optional. In a case expression comparison, the comparison only succeeds when each bit matches exactly with respect to the values 0, 1, x, and z. The bit length of all the expressions shall be equal so that exact bitwise matching can be performed. Dont care cases casez treats z or ? symbols in the expression or case items as dont-cares. casex treats z or x or ? symbols as dont- cares. case_statement ::= case0_statement | casez_statement | casex_statement case0_statement ::= case ( expression ) case_item { case_item } endcase casez_statement ::= casez ( expression ) case_item { case_item } endcase casex_statement ::= casex ( expression ) case_item { case_item } endcase case_item ::= expression {, expression } : statement | default [ : ] statement

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 17 ) Looping statements forever Continuously executes a statement. repeat Executes a statement a fixed number of times. If the expression evaluates to unknown or high impedance, it shall be treated as zero, and no statement shall be executed. while Executes a statement until an expression becomes false. If the expression starts out false, the statement shall not be executed at all. Looping statements provide a means of controlling the execution of a statement zero, one, or more times. for Controls execution of its associated statement(s) by a three-step process, as follows: a) Executes an assignment normally used to initialize a variable that controls the number of loops executed. b) Evaluates an expression. If the result is zero, the for loop shall exit. If it is not zero, the for loop shall execute its associated statement(s) and then perform step c). If the expression evaluates to an unknown or high-impedance value, it shall be treated as zero. c) Executes an assignment normally used to modify the value of the loop-control variable, then repeats step b). loop_statement ::= forever statement | repeat ( expression ) statement | while ( expression ) statement | for ( variable_assignment ; expression ; variable_assignment ) statement

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 18 ) Forever example module top; reg clkA, clkB; initial begin clkA = 0; forever #5 clkA = ~clkA; end initial begin clkB = 1; forever clkB = #5 ~clkB; end initial begin $dumpfile("wave.vcd"); $dumpvars(1); #2000 $finish; end endmodule DIY code/verilog/expression/forever example. forever statement;

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 19 ) Repeat example module top; integer count; initial begin count = 10; repeat (count) begin $display("%d count down", count); count = count - 1; end endmodule DIY code/verilog/expression/repeat example. The loop will execute 10 times regardless of whether the value of count changes after entry to the loop. repeat (expression) statement;

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 20 ) While example module top; integer count; initial begin count = 10; while (count>0) begin $display("%d count down", count); count = count - 1; end endmodule DIY code/verilog/expression/while example. while (expression) statement;

Copyright © by Ando KiIntroduction to Verilog-HDL expression ( 21 ) For example module top; integer count; initial begin count = 10; for (count=10; count>0; count=count-1) begin $display("%d count down", count); end endmodule DIY code/verilog/expression/for example. for (initial; condition; step) statement;