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[/] [ion/] [trunk/] [vhdl/] [mips_mult.vhdl] - Rev 104
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-------------------------------------------------------------------------------- -- mips_mult.vhdl -- multiplier from Plasma project, slightly modified. -- -- The original file from Plasma has been adapted to the Ion core. Changes are -- tagged with '@ion'. There are a few notes at the end of the file with the -- rationale for the changes -- useful only if any trouble shows up later. -- The structure has not changed, only a few implementation details. -------------------------------------------------------------------------------- --------------------------------------------------------------------- -- TITLE: Multiplication and Division Unit -- AUTHORS: Steve Rhoads (rhoadss@yahoo.com) -- DATE CREATED: 1/31/01 -- FILENAME: mult.vhd -- PROJECT: Plasma CPU core -- COPYRIGHT: Software placed into the public domain by the author. -- Software 'as is' without warranty. Author liable for nothing. -- DESCRIPTION: -- Implements the multiplication and division unit in 32 clocks. -- -- To reduce space, compile your code using the flag "-mno-mul" which -- will use software base routines in math.c if USE_SW_MULT is defined. -- Then remove references to the entity mult in mlite_cpu.vhd. -- -- MULTIPLICATION -- long64 answer = 0 -- for(i = 0; i < 32; ++i) -- { -- answer = (answer >> 1) + (((b&1)?a:0) << 31); -- b = b >> 1; -- } -- -- DIVISION -- long upper=a, lower=0; -- a = b << 31; -- for(i = 0; i < 32; ++i) -- { -- lower = lower << 1; -- if(upper >= a && a && b < 2) -- { -- upper = upper - a; -- lower |= 1; -- } -- a = ((b&2) << 30) | (a >> 1); -- b = b >> 1; -- } --------------------------------------------------------------------- library ieee; use ieee.std_logic_1164.all; use ieee.std_logic_unsigned.all; use IEEE.std_logic_arith.all; use work.mips_pkg.all; entity mips_mult is generic(mult_type : string := "DEFAULT"); port(clk : in std_logic; reset_in : in std_logic; a, b : in std_logic_vector(31 downto 0); mult_func : in t_mult_function; c_mult : out std_logic_vector(31 downto 0); pause_out : out std_logic); end; --entity mult architecture logic of mips_mult is constant MODE_MULT : std_logic := '1'; constant MODE_DIV : std_logic := '0'; signal mode_reg : std_logic; signal negate_reg : std_logic; signal sign_reg : std_logic; signal sign2_reg : std_logic; signal count_reg : std_logic_vector(5 downto 0); signal aa_reg : std_logic_vector(31 downto 0); signal bb_reg : std_logic_vector(31 downto 0); signal upper_reg : std_logic_vector(31 downto 0); signal lower_reg : std_logic_vector(31 downto 0); signal a_neg : std_logic_vector(31 downto 0); signal b_neg : std_logic_vector(31 downto 0); signal sum : std_logic_vector(32 downto 0); signal sum_a : std_logic_vector(32 downto 0); signal sum_b : std_logic_vector(32 downto 0); begin -- @ion Output mux no longer uses function bv_negate. Removing one input that -- is no longer needed, even if constant, may help in some FPGA architectures -- too. -- See @note2 -- Result c_mult <= lower_reg when mult_func = MULT_READ_LO and negate_reg = '0' else not(lower_reg) + 1 when mult_func = MULT_READ_LO and --bv_negate(lower_reg) when mult_func = MULT_READ_LO and negate_reg = '1' else upper_reg; -- when mult_func = MULT_READ_HI else --ZERO; -- @ion Stall pipeline while operation completes even if output is not needed -- immediately. -- See @note3 pause_out <= '1' when (count_reg(5 downto 0) /= "000000") else '0'; --and --(mult_func = MULT_READ_LO or mult_func = MULT_READ_HI) else '0'; -- ABS and remainder signals a_neg <= not(a) + 1; --bv_negate(a); -- @ion @note2 b_neg <= not(b) + 1; --bv_negate(b); -- @ion @note2 -- @ion Replaced function bv_adder with straight vector code --sum <= bv_adder(upper_reg, aa_reg, mode_reg); sum_a <= ('0' & upper_reg); -- No sign extension: MSB of sum is special sum_b <= ('0' & aa_reg); with mode_reg select sum <= sum_a + sum_b when '1', sum_a - sum_b when others; --multiplication/division unit mult_proc: process(clk, reset_in, a, b, mult_func, a_neg, b_neg, sum, sign_reg, mode_reg, negate_reg, count_reg, aa_reg, bb_reg, upper_reg, lower_reg) variable count : std_logic_vector(2 downto 0); begin count := "001"; -- @ion Old asynchronous reset converted to synchronous, for consistency -- (Code indenting mangled by the new 'if' level) --if reset_in = '1' then if rising_edge(clk) then if reset_in = '1' then mode_reg <= '0'; negate_reg <= '0'; sign_reg <= '0'; sign2_reg <= '0'; count_reg <= "000000"; aa_reg <= ZERO; bb_reg <= ZERO; upper_reg <= ZERO; lower_reg <= ZERO; --elsif rising_edge(clk) then else case mult_func is when MULT_WRITE_LO => lower_reg <= a; negate_reg <= '0'; when MULT_WRITE_HI => upper_reg <= a; negate_reg <= '0'; when MULT_MULT => mode_reg <= MODE_MULT; aa_reg <= a; bb_reg <= b; upper_reg <= ZERO; count_reg <= "100000"; negate_reg <= '0'; sign_reg <= '0'; sign2_reg <= '0'; when MULT_SIGNED_MULT => mode_reg <= MODE_MULT; if b(31) = '0' then aa_reg <= a; bb_reg <= b; sign_reg <= a(31); else aa_reg <= a_neg; bb_reg <= b_neg; sign_reg <= a_neg(31); end if; sign2_reg <= '0'; upper_reg <= ZERO; count_reg <= "100000"; negate_reg <= '0'; when MULT_DIVIDE => mode_reg <= MODE_DIV; aa_reg <= b(0) & ZERO(30 downto 0); bb_reg <= b; upper_reg <= a; count_reg <= "100000"; negate_reg <= '0'; when MULT_SIGNED_DIVIDE => mode_reg <= MODE_DIV; if b(31) = '0' then aa_reg(31) <= b(0); bb_reg <= b; else aa_reg(31) <= b_neg(0); bb_reg <= b_neg; end if; if a(31) = '0' then upper_reg <= a; else upper_reg <= a_neg; end if; aa_reg(30 downto 0) <= ZERO(30 downto 0); count_reg <= "100000"; negate_reg <= a(31) xor b(31); when others => if count_reg /= "000000" then if mode_reg = MODE_MULT then -- Multiplication if bb_reg(0) = '1' then upper_reg <= (sign_reg xor sum(32)) & sum(31 downto 1); lower_reg <= sum(0) & lower_reg(31 downto 1); sign2_reg <= sign2_reg or sign_reg; sign_reg <= '0'; bb_reg <= '0' & bb_reg(31 downto 1); -- The following six lines are optional for speedup --elsif bb_reg(3 downto 0) = "0000" and sign2_reg = '0' and -- count_reg(5 downto 2) /= "0000" then -- upper_reg <= "0000" & upper_reg(31 downto 4); -- lower_reg <= upper_reg(3 downto 0) & lower_reg(31 downto 4); -- count := "100"; -- bb_reg <= "0000" & bb_reg(31 downto 4); else upper_reg <= sign2_reg & upper_reg(31 downto 1); lower_reg <= upper_reg(0) & lower_reg(31 downto 1); bb_reg <= '0' & bb_reg(31 downto 1); end if; else -- Division if sum(32) = '0' and aa_reg /= ZERO and bb_reg(31 downto 1) = ZERO(31 downto 1) then upper_reg <= sum(31 downto 0); lower_reg(0) <= '1'; else lower_reg(0) <= '0'; end if; aa_reg <= bb_reg(1) & aa_reg(31 downto 1); lower_reg(31 downto 1) <= lower_reg(30 downto 0); bb_reg <= '0' & bb_reg(31 downto 1); end if; count_reg <= count_reg - count; end if; --count end case; end if; end if; end process; end; --architecture logic -------------------------------------------------------------------------------- -- @note1 : bv_adder function removed -- This function was a slightly modified adder/substractor coded in a bitwise -- manner that made it hard for synth tools to recognize it as such. At least -- that's what I think. Replacing it with straigth code results in smaller and -- faster logic (about 23% faster). -- -- @note2 : bv_negate function removed -- This function computed a 2's complement bitwise. Removed on the same grounds -- as @note1 but with no apparent improvement in synthesis results. -- -- @note3 : pause_out active until operation complete -- The original Plasma module allowed the pipeline and the multiplier to run -- concurrently until the multiplier result was needed, and only then the -- pipeline was stalled if the mul/div operation had not finished yet. -- We want to make sure we can abort a mul/div so for the time being we stall -- until the operation is complete. -- I *think* that's what the libraries and the toolchain assume anyway. -- Note that if we later want to change this, the parent module will need -- changes too (logic for p1_muldiv_running). --------------------------------------------------------------------------------
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