Subversion Repositories nanoblaze

--##############################################################################

--

--  branchStack

--      Stack of branch addresses

--

--      This is where the program counter is pushed upon a subroutine call

--      and popped on return.

--

--      The stack is coded in a way to be mapped into a Block RAM.

--

--------------------------------------------------------------------------------

--

--  Versions / Authors

--      1.0 Francois Corthay    first implementation

--

--  Provided under GNU LGPL licence: <http://www.gnu.org/copyleft/lesser.html>

--

--  by the electronics group of "HES-SO//Valais Wallis", in Switzerland:

--  <http://www.hevs.ch/en/rad-instituts/institut-systemes-industriels/>.

--

--------------------------------------------------------------------------------

--

--  Hierarchy

--      Used by "nanoblaze/nanoProcessor".

--

--##############################################################################

LIBRARY ieee;

  USE ieee.std_logic_1164.all;

  USE ieee.numeric_std.all;

ENTITY branchStack IS

  GENERIC(

    programCounterBitNb : positive := 10;

    stackPointerBitNb   : positive := 5

  );

  PORT(

    reset             : IN  std_ulogic;

    clock             : IN  std_ulogic;

    progCounter       : IN  unsigned(programCounterBitNb-1 DOWNTO 0);

    prevPC            : IN  std_ulogic;

    storePC           : IN  std_ulogic;

    storedProgCounter : OUT unsigned(programCounterBitNb-1 DOWNTO 0)

  );

END branchStack ;

--==============================================================================

ARCHITECTURE RTL OF branchStack IS

  subtype progCounterType is unsigned(progCounter'range);

  type progCounterArrayType is array (0 to 2**stackPointerBitNb) of progCounterType;

  signal progCounterArray : progCounterArrayType;

  signal writePointer : unsigned(stackPointerBitNb-1 downto 0);

  signal readPointer  : unsigned(stackPointerBitNb-1 downto 0);

BEGIN

  ------------------------------------------------------------------------------

                                                               -- stack pointers

  updateStackPointer: process(reset, clock)

  begin

    if reset = '1' then

      writePointer <= (others => '0');

    elsif rising_edge(clock) then

      if storePC = '1' then

        writePointer <= writePointer + 1;

      elsif prevPC = '1' then

        writePointer <= writePointer - 1;

      end if;

    end if;

  end process updateStackPointer;

  readPointer <= writePointer - 1;

  ------------------------------------------------------------------------------

                                                       -- program counters stack

  updateStack: process(reset, clock)

  begin

    if rising_edge(clock) then

      if storePc = '1' then

        progCounterArray(to_integer(writePointer)) <= progCounter;

      end if;

      storedProgCounter <= progCounterArray(to_integer(readPointer));

    end if;

  end process updateStack;

END ARCHITECTURE RTL;