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Task and Functions

Jagannadha Naidu K

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Task

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Task Properties Declared within a module

Referenced only by a behavior within the module

Task can be used with delay, timing or event control constructs. Tasks can have zero or more arguments.

Supports input, output and inout arguments.

Passes the values through output and inout arguments to the

task call.

A task can call other tasks and functions as well.

Tasks are declared with keyword task and endtask.

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Task Format

task ;

input declarations;

output declarations;

inout declarations;

begin

end

endtask

tasktask log_callog_cal;;

parameterparameter DEL = 30;DEL = 30;

outputoutput [7:0]and_out,or_out,xor_out;[7:0]and_out,or_out,xor_out;

inputinput [7:0]in1, in2;[7:0]in1, in2;

beginbegin

#DEL#DEL and_outand_out = in1 & in2;= in1 & in2;or_outor_out = in1 | in2;= in1 | in2;

xor_outxor_out = in1 ^ in2;= in1 ^ in2;

endend

endtaskendtask

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Example

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Re-entrant TaskProblem

Task is called concurrently from two places inthe code, these task calls will operate on the

same task variables

Solution Keyword automatic is added in front of the

task keyword to make the tasks re-entrant

All items declared inside automatic tasks areallocated dynamically for each invocation

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Example

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Cont

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Functions

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Function properties

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Function propertiesFunction properties

Functions should not contain delay information.

Functions must contain at least one input.

Supports only input arguments.

A function can call other function only. Can return only one value.

Functions are declared with keyword function and

endfunction.

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Function Formatfunction

;

input declarations;

begin

end

endfunction

functionfunction par_calpar_cal;;

inputinput [15:0]dat_in;[15:0]dat_in;

beginbegin

par_calpar_cal = ^= ^dat_indat_in;;

endend

endfunctionendfunction

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Function Example

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Function Calling

A function is called or enabled by a function enablecall that specifies the argument values passed to thefunction.

A function call is a part of an expression i.e. anoperand within an expression.

Function returns the computed result in the name ofthe function itself

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Function Calling

The list of input arguments must match the orderof input declarations in the function definition.

Arguments are passed by value, not by reference.

A function can be called more than onceconcurrently with each call having its own control.

Variables declared within a function is static.

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Function calling exampleFunction calling example

functionfunction par_calpar_cal;;

inputinput [15:0]dat_in;[15:0]dat_in;

beginbegin

par_calpar_cal = ^= ^dat_indat_in;;

endend

endfunctionendfunction

wire [15:0]data;

reg parity_value;

always @(enable)

begin

if(enable == 1b1)

parity_value = par_cal(data);

end

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Automatic (Recursive ) Functions

The keyword automatic can be used to declare a

recursive (automatic) function where all functiondeclarations are allocated dynamically for each

recursive calls

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Task and Functions Summary

functions

can enable other function, nota task.

execute in 0 simulation time.

do not support delay, event ortiming controls.

must have at least one input

argument, can have more thanone input.

return single value , they cannot have output or inoutarguments.

tasks

can enable other functions andtasks.

execute in non 0 simulationtime.

can have delays, event ortiming controls.

can have zero or morearguments of type input,output or inout.

do not return any value, butcan pass multiple values thro

output and inout args.