VHDL standard layout & syntax for "header" file -

ide: quartus 15

i'm new vhdl programming there nuances not used (translating c++). whilst have found resources programming "source" files, i've struggled find "header" files.

in short, standard layout / syntax of vhdl "header" file?

to keep things simple, use cases i'm interested in declaring subtype , function references use between "source" files.

i found following code snippet here has helped little, i'm still not sure of differences between package , package body are. i'm unsure of "work" comes from.

"header":

    package defs         constant major_version: integer := 0;         constant minor_version: integer := 22;         constant maxreg: integer := 52;         type regs_type array (0 maxreg) of std_logic_vector(15 downto 0);         function opndrn(inp: std_logic) return std_logic;     end package defs;      package body defs         function opndrn(inp: std_logic) return std_logic         begin             case inp                 when '0' => return '0';                 when others => return 'z';             end case;             end;     end package body defs; 

"source":

    library work;     use work.defs.all; 

any , appreciated.

thankfully, there no header files in vhdl.

what have there package, isn't ever "include"d in anything.

unlike header file, package seperately compiled - library, can "use" doing.

unlike header file, package creates own namespace. use work.defs.all; imports entire namespace, using namespace defs in c++. it's better practice write use work.defs; allows selectively use namespace, , refer defs.major_version in source, (a) keeping global namespace uncluttered, , (b) documenting major_version defined.

unlike header file, package encourages proper separation of interface , implementation. source can access things exported package, i.e. declared in package not package body. allows information hiding, opaque types , better abstractions.

for example can declare major_version in package hide actual value in body (see "deferred constants")

it's legal both package , package body (interface , implementation) in same file, if want enforce separation of interface , implementation, package body separate file.

then can modify package body (implementation) , recompile : nothing uses package needs recompilation (unless changed interface.

any customer of package has @ package file, not package body.

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in short, doing looks pretty ok.

but can go further in information hiding.

and 1 thing beware of creating 1 all-purpose god package : far better separate bus constants, functions etc "bus" package, register types (and maybe enumeration of names) in "registers" package, instructions in "instructions" package etc.