VHDL Programming

The keyword POSTPONEDis specified before the PROCESSkeyword to

specify a postponed process.

Pure and Impure Functions

Functions in VHDL87 were very restrictively defined. The input mode of

all input arguments were constant, and only input arguments were

allowed. The function could have no side effects such as modifying a value

outside the function. The only information returned from the function was

through the return value. In VHDL93 this type of function is known as a

pure function. VHDL93 also contains impure functionswhich can mod-

ify data outside their own scope. These functions must be explicitly

declared as being impure as shown here:

FILE bit_file : TEXT OPEN READ_OPEN IS “ram_data”;

IMPURE FUNCTION get_val RETURN BIT IS

VARIABLE myline : LINE;

VARIABLE result : BIT;

BEGIN

READLINE( bit_file, myline );

READ ( myline, result );

RETURN result;

END get_val;

This function is used to read a set of bits from a file. Function get_val

is declared impure so that it has access to data outside the function. The file

bit_fileis opened externally to function get_valbut since the function

is impure, access to file bit_fileis possible.

Functions in VHDL87 do not have access to data outside of the function

so this function would not work. In VHDL87 the file would have to be

declared within the function declaration section, and implicitly opened and

closed from within the function. In VHDL93 the file can be opened external

to the function and an impure functioncan access the file.

Pulse Reject

In VHDL87 there were two types of delay categories, inertial and

transport.Chapter 2,“Behavioral Modeling,”talks about the differences

between them. The VHDL87 inertial delay will reject pulses smaller than

460 Appendix D: VHDL93 Updates