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Unconstrained array parameter
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Unconstrained array parameter

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Unconstrained Array Parameters

 

In  an  earlier  chapter  we  described  unconstrained  array  types,  in  which  the  index range  of  the  array  was  left  unspecified  using  the  “box”  (“<>”)  notation For  such  a type, we constrain the index bounds when we create an object, such as a variable or

a signal.  Another use of an unconstrained array type is as the type of a formal param- eter to procedure This use allows us to write a procedure in a general way, so that

it can operate on array values of any size or with any range of index values.   When we  call  the  procedure  and  provide  a  constrained  array  as  the  actual  parameter,  the index bounds of the actual array are used as the bounds of the formal array parameter.

 

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EXAMPLE

 

Figure 5-6 is a procedure that finds the index of the first bit set to ‘1’ in a bit vector.   The formal parameter  v is of type  bit_vector, which is an unconstrained array type Note that in writing this procedure, we do not explicitly refer to the index bounds of the formal parameter v, since they are not known.   Instead, we use the 'range attribute.

 

FIGURE 5-6

 

procedure find_first_set ( v : in bit_vector;

found : out boolean;

first_set_index : out natural ) is

begin

for index in v'range loop if v(index) = '1' then found := true;

first_set_index := index;

return;

end if; end loop; found := false;

end procedure find_first_set;

 

A procedure to find the first set bit in a bit vector.

 

When the procedure is executed, the formal parameters stand for the actual parameters provided by the caller So if we call this procedure as follows:

 

variable int_req : bit_vector (7 downto 0);

variable top_priority : natural;

variable int_pending : boolean;

…

find_first_set ( int_req, int_pending, top_priority );

v'range  returns  the  range 7 downto 0,  which is  used to  ensure that the loop pa- rameter index iterates over the correct index values for v If we make a different call:

 

variable free_block_map : bit_vector(0 to block_count–1);

variable first_free_block : natural;

variable free_block_found : boolean;

…

find_first_set ( free_block_map, free_block_found, first_free_block );

 

v'range returns the index range of the array free_block_map, since that is the actual

parameter corresponding to v.

 

 

 

 

 

 

 

 

 

 

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