Subprograms

1
Subprograms
ECE 448 Lecture 14
Attributes of Arrays Array Types
2
Unconstrained Array Types
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Predefined Unconstrained Array Types

• Predefined
• bit_vector array of bits
• string array of characters
• Defined in the ieee.std_logic_1164 package
• std_logic_vector array of std_logic_vectors

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Predefined Unconstrained Array Types

• subtype byte is bit_vector(7 downto 0)
• .
• variable channel_busy bit_vector(1 to 4)
• .
• constant ready_message string ready
• .
• signal memory_bus std_logic_vector (31 downto 0)

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User-defined Unconstrained Array Types

• type sample is array (natural range ltgt) of
  integer
• .
• variable long_sample is sample(0 to 255)
• .
• constant look_up_table_1 sample
• (127, -45, 63, 23, 76)
• .

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Attributes of Arrays and Array Types
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Array Attributes

• Aleft(N) left bound of index range of dimension
  N of A
• Aright(N) right bound of index range of
  dimension N of A
• Alow(N) lower bound of index range of dimension
  N of A
• Ahigh(N) upper bound of index range of dimension
  N of A
• Arange(N) index range of dimension N of A
• Areverse_range(N) index range of dimension N of
  A
• Alength(N) length of index range of dimension N
  of A
• Aascending(N) true if index range of dimension
  N of A
• is an ascending range,
  false otherwise

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Array Attributes - Examples

• type A is array (1 to 4, 31 downto 0)
• Aleft(1) 1
• Aright(2) 0
• Alow(1) 1
• Ahigh(2) 31
• Arange(1) 1 to 4
• Alength(2) 32
• Aascending(2) false

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Subprograms
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Subprograms

• Include
• functions and procedures
• Commonly used pieces of code
• Can be placed in a library, and then reused and
  shared among various projects
• Abstract operations that are repeatedly performed
• Type conversions
• Use only sequential statements, the same as
  processes

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Typical locations of subprograms
PACKAGE PACKAGE BODY
LIBRARY
global
ENTITY
FUNCTION / PROCEDURE
local for all architectures of a given entity
ARCHITECTURE Declarative part
local for a given architecture
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Functions
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Functions basic features

• Functions
• always return a single value as a result
• are called using formal and actual parameters the
  same way as components
• never modify parameters passed to them
• parameters can only be constants (including
  generics) and signals (including ports)
• variables are not allowed the default is a
  CONSTANT
• when passing parameters, no range specification
  should be included (for example no RANGE for
  INTEGERS, or TO/DOWNTO for STD_LOGIC_VECTOR)
• are always used in some expression, and not
  called on their own

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Function syntax

• FUNCTION function_name (ltparameter_listgt)
• RETURN data_type IS
• declarations
• BEGIN
• (sequential statements)
• END function_name

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Function parameters - example

• FUNCTION f1
• (a, b INTEGER SIGNAL c STD_LOGIC_VECTOR)
• RETURN BOOLEAN IS
• BEGIN
• (sequantial statements)
• END f1

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Function calls - examples

• x lt conv_integer(a)
• IF x gt maximum(a, b) THEN ....
• WHILE minimum(a, b) LOOP
• .......

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Function example (1)

• library IEEE
• use IEEE.std_logic_1164.all
• ENTITY powerOfFour IS
• PORT(
• X IN INTEGER
• Y OUT INTEGER
• )
• END powerOfFour

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Function example (2)

• ARCHITECTURE behavioral OF powerOfFour IS
• FUNCTION Pow ( SIGNAL NINTEGER Exp
  INTEGER) RETURN INTEGER IS
• VARIABLE Result INTEGER 1
• BEGIN
• FOR i IN 1 TO Exp LOOP
• Result Result N
• END LOOP
• RETURN( Result )
• END Pow
• BEGIN
• Y lt Pow(X, 4)
• END behavioral

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Package containing a function (1)

• LIBRARY IEEE
• USE IEEE.std_logic_1164.all
• PACKAGE specialFunctions IS
• FUNCTION Pow( SIGNAL N INTEGER Exp
  INTEGER) RETURN INTEGER
• END specialFunctions

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Package containing a function (2)

• PACKAGE BODY specialFunctions IS
• FUNCTION Pow(SIGNAL N INTEGER Exp INTEGER)
• RETURN INTEGER IS
• VARIABLE Result INTEGER 1
• BEGIN
• FOR i IN 1 TO Exp LOOP
• Result Result N
• END LOOP
• RETURN( Result )
• END Pow
• END specialFunctions

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Type conversion function (1)

• LIBRARY ieee
• USE ieee.std_logic_1164.all
• --------------------------------------------------
  -----------------------------------------------
• PACKAGE my_package IS
• FUNCTION conv_integer (SIGNAL vector
  STD_LOGIC_VECTOR)
• RETURN INTEGER
• END my_package
• --------------------------------------------------
  -----------------------------------------------

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Type conversion function (2)

• PACKAGE BODY my_package IS
• FUNCTION conv_integer (SIGNAL vector
  STD_LOGIC_VECTOR)
• RETURN INTEGER
• VARIABLE result INTEGER RANGE 0 TO
  2vectorLENGTH - 1
• VARIABLE carry STD_LOGIC
• BEGIN
• IF(vector(vectorHIGH)1 THEN result1
• ELSE result 0
• FOR i IN (vectorHIGH-1) DOWNTO
  (vectorLOW) LOOP
• result result2
• IF (vector(i) 1 THEN result
  result1
• END IF
• RETURN result
• END conv_integer
• END my_package

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Type conversion function (3)

• LIBRARY ieee
• USE ieee.std_logic_1164.all
• USE work.my_package.all
• --------------------------------------------------
  -----------------------------------------------
• ENTITY conv_int2 IS
• PORT ( a IN STD_LOGIC_VECTOR (0 TO 3)
• y OUT INTEGER RANGE 0 TO 15)
• END conv_int2
• --------------------------------------------------
  -----------------------------------------------
• ARCHITECTURE my_arch OF conv_int2 IS
• BEGIN
• y lt conv_integer(a)
• END my_arch

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Procedures
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Procedures basic features

• Procedures
• do not return a value
• are called using formal and actual parameters the
  same way as components
• may modify parameters passed to them
• each parameter must have a mode IN, OUT, INOUT
• parameters can be constants (including generics),
  signals (including ports), and variablesthe
  default for inputs (mode in) is a constant, the
  default for outputs (modes out and inout) is a
  variable
• when passing parameters, range specification
  should be included (for example RANGE for
  INTEGERS, and TO/DOWNTO for STD_LOGIC_VECTOR)
• Procedure calls are statements on their own

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Procedure syntax

• PROCEDURE procedure_name (ltparameter_listgt) IS
• declarations
• BEGIN
• (sequential statements)
• END function_name

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Procedure parameters - example

• PROCEDURE my_procedure
• ( a IN BIT
• SIGNAL b, c IN BIT
• SIGNAL x OUT BIT_VECTOR(7 DOWNTO 0)
• SIGNAL y INOUT INTEGER RANGE 0 TO 99) IS
• BEGIN
• (sequantial statements)
• END my_procedure

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Procedure calls - examples

• compute_min_max(in1, in2, in3, out1, out2)
• divide(dividend, divisor, quotient, remainder)
• IF (a gt b) THEN
• compute_min_max(in1, in2, in3, out1, out2)
• .......

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Procedure example (1)

• LIBRARY ieee
• USE ieee.std_logic_1164.all
• ENTITY min_max IS
• GENERIC (limit INTEGER 255)
• PORT (
• ena IN BIT
• inp1, inp2 IN INTEGER RANGE 0 TO limit
• min_out, max_out OUT INTEGER RANGE 0 TO
  limit)
• )
• END min_max

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Procedure example (2)

• ARCHITECTURE my_architecture OF min_max IS
• PROCEDURE sort (SIGNAL in1, in2 IN INTEGER
  RANGE 0 TO limit
• SIGNAL min, max
  OUT INTEGER RANGE 0 TO limit) IS
• BEGIN
• IF (in1 gt in2) THEN
• max lt in1
• min lt in2
• ELSE
• max lt in2
• min lt in1
• END IF
• END sort
• BEGIN
• PROCESS (ena)
• BEGIN
• IF(ena1) THEN
• sort (inp1, inp2, min_out,
  max_out)

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Operators
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Operator as a function (1)

• LIBRARY ieee
• USE ieee.std_logic_1164.al
• --------------------------------------------------
  -----------------------------------------------
• PACKAGE my_package IS
• FUNCTION "" (a, b STD_LOGIC_VECTOR)
• RETURN STD_LOGIC_VECTOR
• END my_package
• --------------------------------------------------
  -----------------------------------------------

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Operator as a function (2)

• PACKAGE BODY my_package IS
• FUNCTION "" (a, b STD_LOGIC_VECTOR)
• RETURN STD_LOGIC_VECTOR
• VARIABLE result STD_LOGIC_VECTOR
• VARIABLE carry STD_LOGIC
• BEGIN
• carry 0
• FOR i IN aREVERSE_RANGE LOOP
• result(i) a(i) XOR b(i) XOR carry
• carry (a(i) AND b(i)) OR (a(i) AND
  carry) OR (b(i) AND carry))
• END LOOP
• RETURN result
• END ""
• END my_package

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Operator Overloading
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Operator overloading

• Operator overloading allows different argument
  types for a given operation (function)
• The VHDL tools resolve which of these functions
  to select based on the types of the inputs
• This selection is transparent to the user as long
  as the function has been defined for the given
  argument types.

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Different declarations for the same operator -
Example

• Declarations in the package ieee.std_logic_unsigne
  d
• function ( L std_logic_vector
  Rstd_logic_vector) return std_logic_vector
• function ( L std_logic_vector R
  integer) return std_logic_vector
• function ( L std_logic_vector
  Rstd_logic) return std_logic_vector

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Different declarations for the same operator -
Example

• signal count std_logic_vector(7 downto 0)
• You can use
• count lt count 0000_0001
• or
• count lt count 1
• or
• count lt count 1