ModelSim SE User's Manual - Electrical and Computer Engineering

UM-76 4 - VHDL simulation
ModelSim SE User’s Manual
Files and packages
Each package header and body should be compiled with the same language version.
Common problems in this area involve files as parameters and the size of type
CHARACTER. For example, consider a package header and body with a procedure that
has a file parameter:
procedure proc1 ( out_file : out std.textio.text) ...
If you compile the package header with VHDL-87 and the body with VHDL-93 or
VHDL-2002, you will get an error message such as:
"** Error: mixed_package_b.vhd(4): Parameter kinds do not conform between
declarations in package header and body: 'out_file'."
Direction of concatenation
To solve some technical problems, the rules for direction and bounds of concatenation
were changed from VHDL-87 to VHDL-93. You won't see any difference in simple
variable/signal assignments such as:
v1 := a & b;
But if you (1) have a function that takes an unconstrained array as a parameter, (2) pass
a concatenation expression as a formal argument to this parameter, and (3) the body of
the function makes assumptions about the direction or bounds of the parameter, then you
will get unexpected results. This may be a problem in environments that assume all arrays
have "downto" direction.
xnor
"xnor" is a reserved word in VHDL-93. If you declare an xnor function in VHDL-87
(without quotes) and compile it under VHDL-2002, you will get an error message like
the following:
** Error: xnor.vhd(3): near "xnor": expecting: STRING IDENTIFIER
'FOREIGN attribute
In VHDL-93 package STANDARD declares an attribute 'FOREIGN. If you declare your
own attribute with that name in another package, then ModelSim issues a warning such
as the following:
-- Compiling package foopack
** Warning: foreign.vhd(9): (vcom-1140) VHDL-1993 added a definition of the
attribute foreign to package std.standard. The attribute is also defined in
package 'standard'. Using the definition from package 'standard'.
Size of CHARACTER type
In VHDL-87 type CHARACTER has 128 values; in VHDL-93 it has 256 values. Code
which depends on this size will behave incorrectly. This situation occurs most commonly
in test suites that check VHDL functionality. It's unlikely to occur in practical designs. A
typical instance is the replacement of warning message:
by
"range nul downto del is null"
"range nul downto 'ÿ' is null" -- range is nul downto y(umlaut)