Special index types of VHDL arrays

Actually, in VHDL your array don't have to have integer indexes.

1 2	type ByteIndex is (B0,B1,B2,B3); type LaneType is array (ByteIndex) of std_logic_vector(7 downto 0);

I used this fact when I had to use multidimensional arrays to prevent mistakes when you mix up dimensions of your arrays.
For example, let's suppose you have a bus, consisting of 4 lanes and each of the lanes contains 4 bytes.

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type LaneType is array (0 to 3) of std_logic_vector(7 downto 0); type LaneBusType is array(0 to 3) of LaneType; ..... for Lane in 0 to 3 loop L(Lane)(0)(i) <= not L(Lane)(0)(i); --OK -- L(0)(Lane)(i) <= not L(0)(Lane)(i); -- error, but absolutely legal end loop;

In contrast, if you use non-integer indexes:

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type LaneIndex is (L0,L1,L2,L3); type ByteIndex is (B0,B1,B2,B3); type LaneType is array(ByteIndex) of std_logic_vector(7 downto 0); type LaneBusType is array(LaneIndex) of LaneType; signal clk : std_logic; signal L : LaneBusType := (others => (others => (others => '0'))); begin process is begin clk <= '0'; wait for 5 ns; clk <= '1'; wait for 5 ns; end process; process(clk) is variable i : natural := 0; begin if rising_edge(clk) then for Lane in LaneIndex loop L(Lane)(B0)(i) <= not L(Lane)(B0)(i); --OK -- L(B0)(Lane)(i) <= not L(B0)(Lane)(i); -- type error near b0 ; current type byteindex; -- expected type laneindex end loop; i := (i+1) mod 8; end if; end process;

The method helps you to avoid mistakes by controlling types of indexes. On the other hand, it adds some hassle. It is up to you when to use it.