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What is the meaning of the following warning (raised by Quartus)?

Warning (10445): VHDL Subtype or Type Declaration warning at someFile.vhd(32): subtype or type has null range


The offending line of code is:

-- Drive unused low
q( N - 1 downto X ) <= ( others => '0' );  -- drive unused low

N and X are integer constants used successfully elsewhere in the code (without raising warnings). q is declared as q : out std_logic_vector( N - 1 downto 0 )


The same error is also raised for this line of code (in another file):

-- Upper nibble
q <= zero( N - 1 downto 8 ) & q0( 7 downto 4 ) & zero( 3 downto 0 );

N is an integer constant used successfully elsewhere in the code. q is declared as q : out std_logic_vector( N - 1 downto 0 ). q0 is declared as signal q0 : std_logic_vector( N - 1 downto 0 ). zero is declared as constant zero : std_logic_vector( N - 1 downto 0 ) := ( others => '0' )


It seems ( others => '0' ) is the common point for both, but I don't know how this relates to the null range warning.

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The range: N - 1 downto X is evaluating to a range with 0 or negative size. Consider the range: 0 downto 4, it has a size that is less than 0 since the number before the downto is less than the number after it.

The problem here is using a range that has a size less than 1 makes it a null range which has some valid uses in vhdl. However, since this is potentially a range error Quartus gives the warning you are seeing here.

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  • \$\begingroup\$ Thank you! The code was initially written for 16+ bits (N=16). Using it for 8 bits (N=8) caused the subtle error. N - 1 downto 8 became 7 downto 8 \$\endgroup\$ – Jet Blue Jun 1 '18 at 1:35
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    \$\begingroup\$ VHDL allows you to have NULL ranges and NULL slices. That's why it is a warning and no error. There are good use cases using NULL ranges. But in most cases it's an error in the calculation of slice boundaries. \$\endgroup\$ – Paebbels Jun 1 '18 at 5:46

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