vhdl for data forwarding in a pipeline machine

Discussion in 'VHDL' started by Eqbal Z, Oct 9, 2003.

  1. Eqbal Z

    Eqbal Z Guest

    Hi,

    I am very new to vhdl and I am trying to modify this pipeline (5
    stage) architecture to include data forwarding. Here is the code that
    I use. For some reason it causes the decode to stall forever after a
    few instructions. IRTemp is instruction that goes into execute, IRx
    goes into memory and IRm goes into write back and IRin is the one that
    comes into the decode segment (for which I have included a part). I
    have added a 4 signal mux to the architecture.
    If anyone can help me with this I would appreciate it.

    Thanks
    -----------------code segment start----------------
    process (IRtemp,adin,bdin,IRin,IRx,IRm,xstall,write_flag,finvalid,bren)
    constant REGOUT : std_logic_vector(1 downto 0) := "00";
    constant ALUOUT : std_logic_vector(1 downto 0) := "01";
    constant RESULT : std_logic_vector(1 downto 0) := "10";
    constant DOUT : std_logic_vector(1 downto 0) := "11";
    begin
    if finvalid = '0' then -- fetch out is valid
    dstalltemp <= xstall; -- set dstall to xstall (assume we will
    stall if x is stalled)
    -- delayed branching segement
    if (IRin(15 downto 14) = "00" and IRin(11) = '0') then
    invalid <= bren; -- delete instruction after branch from pipe if
    not a delayed branch
    end if;
    -- data forwarding segment
    if (IRin(15 downto 14) = "01" or IRin(15 downto 14) = "10") then
    -- if this = ALU or MEM

    if ((IRtemp(13 downto 11) = IRin(10 downto 8)) and --
    EXECUTE DEST
    ((IRtemp(15 downto 14) = "01" and IRtemp(5) = '0') or --
    and memory read
    (IRtemp(15 downto 14) = "10") or --
    or ALU
    (IRtemp(15 downto 14) = "11"))) then --
    or set
    muxASel <= RESULT; --
    adin <= result; ??? when to chose result and when aluout?

    elsif ((IRx(13 downto 11) = IRin(10 downto 8)) and --
    MEMORY DEST
    ((IRx(15 downto 14) = "01" and IRx(5) = '0') or --
    memory read
    (IRx(15 downto 14) = "10") or --
    or ALU
    (IRx(15 downto 14) = "11"))) then --
    or set
    muxASel <= DOUT; --
    adin <= dout; ???

    elsif ((IRm(13 downto 11) = IRin(10 downto 8)) and --
    WRITE DEST
    ((IRm(15 downto 14) = "01" and IRm(5) = '0') or --
    memory read
    (IRm(15 downto 14) = "10") or --
    or ALU
    (IRm(15 downto 14) = "11"))) then --
    or set
    muxASel <= REGOUT;
    else
    dstalltemp <= xstall;
    end if;

    if (IRin(3) = '1') then --and -- check second register for
    hazards

    if ((IRtemp(13 downto 11) = IRin(2 downto 0)) and
    -- EXECUTE DEST
    ((IRtemp(15 downto 14) = "01" and IRtemp(5) = '0') or
    -- memory read
    (IRtemp(15 downto 14) = "10") or
    -- or ALU
    (IRtemp(15 downto 14) = "11"))) then
    -- or set
    muxBSel <= RESULT;
    elsif ((IRx(13 downto 11) = IRin(2 downto 0)) and
    -- MEMORY DEST
    ((IRx(15 downto 14) = "01" and IRx(5) = '0') or
    -- memory read
    (IRx(15 downto 14) = "10") or
    -- or ALU
    (IRx(15 downto 14) = "11"))) then
    -- or set
    muxBSel <= DOUT;
    elsif ((IRm(13 downto 11) = IRin(2 downto 0)) and
    -- WRITE BACK
    ((IRm(15 downto 14) = "01" and IRm(5) = '0') or
    -- memory read
    (IRm(15 downto 14) = "10") or
    -- or ALU
    (IRm(15 downto 14) = "11"))) then
    -- or set
    muxBSel <= REGOUT;

    else
    dstalltemp <= xstall;
    end if;
    else
    dstalltemp <= xstall;
    end if;

    if (IRin(15 downto 14) = "01" and IRin(5) = '1') then --if this
    = MEM(write) (destination = val to write)
    if (((IRtemp(13 downto 11) = IRin(13 downto 11)) and
    ((IRtemp(15 downto 14) = "01" and IRtemp(5) = '0') or
    (IRtemp(15 downto 14) = "10") or
    (IRtemp(15 downto 14) = "11"))) or
    ((IRx(13 downto 11) = IRin(13 downto 11)) and
    ((IRx(15 downto 14) = "01" and IRx(5) = '0') or
    (IRx(15 downto 14) = "10") or
    (IRx(15 downto 14) = "11"))) or
    ((IRm(13 downto 11) = IRin(13 downto 11)) and
    ((IRm(15 downto 14) = "01" and IRm(5) = '0') or
    (IRm(15 downto 14) = "10") or
    (IRm(15 downto 14) = "11")))) then
    -- if out dest = this dest and out is doing a
    MEM(read), ALU, SET op or
    -- execute dest = this dest and execute is doing a
    MEM(read), ALU, SET op or
    -- memory dest = this dest and memory is doing a
    MEM(read), ALU, SET op
    dstalltemp <= '1';
    invalid <= '1';
    elsif write_flag = '0' then
    dstalltemp <= '1';
    else
    dstalltemp <= xstall;
    end if;
    else
    dstalltemp <= xstall;
    end if;
    else
    dstalltemp <= xstall;
    end if;
    else
    dstalltemp <= xstall;
    invalid <= '1';
    end if;
    bsel <= IRin(2 downto 0);
    if write_flag = '1' then -- insert 2nd write cycle
    asel <= IRin(13 downto 11); -- data to be written
    else
    asel <= IRin(10 downto 8); --fetch register data
    end if;
    end process;
    ------code segment end-------------
     
    Eqbal Z, Oct 9, 2003
    #1
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  2. Eqbal Z

    MM Guest

    For starters, you don't seem to have any default values for your muxes'
    outputs:
    What should happen to muxASel when all of these conditions are not
    satisfied? You may want to add a default value right after the begin
    statement, before all if's ...

    /Mikhail
     
    MM, Oct 9, 2003
    #2
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