-- Booth Multiplier

-- This file contains all the entity-architectures for a complete

-- k-bit x k-bit Booth multiplier.

-- the design makes use of the new shift operators available in the VHDL-93 std

-- this design passes the Synplify synthesis check

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

--top level design unit

library IEEE;

use IEEE.Std_logic_1164.all;

ENTITY booth_multiplier IS

     GENERIC(k : POSITIVE := 7); --input number word length less one

     PORT(multiplicand, multiplier : IN BIT_VECTOR(k DOWNTO 0);

        clock : IN BIT; product : INOUT BIT_VECTOR((2*k + 1) DOWNTO 0));

END booth_multiplier;

ARCHITECTURE structural OF booth_multiplier IS

SIGNAL mdreg, adderout, carries, augend, tcbuffout : BIT_VECTOR(k DOWNTO 0);

SIGNAL mrreg : BIT_VECTOR((k + 1) DOWNTO 0);

SIGNAL adder_ovfl : BIT;

SIGNAL comp ,clr_mr ,load_mr ,shift_mr ,clr_md ,load_md ,clr_pp ,load_pp ,shift_pp : BIT;

SIGNAL boostate : NATURAL RANGE 0 TO 2*(k + 1);

BEGIN

PROCESS --main clocked process containing all sequential elements

BEGIN

        WAIT UNTIL (clock'EVENT AND clock = '1');

        --register to hold multiplicand during multiplication

        IF clr_md = '1' THEN

                mdreg <= (OTHERS => '0');

        ELSIF load_md = '1' THEN

                mdreg <= multiplicand;

        ELSE

                mdreg <= mdreg;

        END IF;

        --register/shifter to product pair of bits used to control adder

        IF clr_mr = '1' THEN

                mrreg <= (OTHERS => '0');

        ELSIF load_mr = '1' THEN

                mrreg((k + 1) DOWNTO 1) <= multiplier;

                mrreg(0) <= '0';

        ELSIF shift_mr = '1' THEN

                mrreg <= mrreg SRL 1;

        ELSE

                mrreg <= mrreg;

        END IF;

        --register/shifter accumulates partial product values

        IF clr_pp = '1' THEN

                product <= (OTHERS => '0');

        ELSIF load_pp = '1' THEN

                product((2*k + 1) DOWNTO (k + 1)) <= adderout; --add to top half

                product(k DOWNTO 0) <= product(k DOWNTO 0);  --refresh bootm half

        ELSIF shift_pp = '1' THEN

                product <= product SRA 1; --shift right with sign extend

        ELSE

                product <= product;

        END IF;

END PROCESS;

--adder adds/subtracts partial product to multiplicand

augend <= product((2*k+1) DOWNTO (k+1));

addgen : FOR i IN adderout'RANGE

        GENERATE

                lsadder : IF i = 0 GENERATE

                        adderout(i) <= tcbuffout(i) XOR augend(i) XOR comp;

                        carries(i) <= (tcbuffout(i) AND augend(i)) OR

                                      (tcbuffout(i) AND comp) OR

                                      (comp AND augend(i));

                        END GENERATE;

                otheradder : IF i /= 0 GENERATE

                        adderout(i) <= tcbuffout(i) XOR augend(i) XOR carries(i-1);

                        carries(i) <= (tcbuffout(i) AND augend(i)) OR

                                      (tcbuffout(i) AND carries(i-1)) OR

                                      (carries(i-1) AND augend(i));

                        END GENERATE;

        END GENERATE;

        --twos comp overflow bit

        adder_ovfl <= carries(k-1) XOR carries(k);

--true/complement buffer to generate two's comp of mdreg

tcbuffout <= NOT mdreg WHEN (comp = '1') ELSE mdreg;

--booth multiplier state counter

PROCESS BEGIN 

        WAIT UNTIL (clock'EVENT AND clock = '1');

        IF boostate < 2*(k + 1) THEN boostate <= boostate + 1;

        ELSE boostate <= 0;

        END IF;

END PROCESS;

--assign control signal values based on state

PROCESS(boostate)

BEGIN

        --assign defaults, all registers refresh

        comp <= '0';

        clr_mr <= '0';

        load_mr <= '0';

        shift_mr <= '0';

        clr_md <= '0';

        load_md <= '0';

        clr_pp <= '0';

        load_pp <= '0';

        shift_pp <= '0';

        IF boostate = 0 THEN

                load_mr <= '1';

                load_md <= '1';

                clr_pp <= '1';

        ELSIF boostate MOD 2 = 0 THEN   --boostate = 2,4,6,8 ....

                shift_mr <= '1';

                shift_pp <= '1';

        ELSE    --boostate = 1,3,5,7......

                IF mrreg(0) = mrreg(1) THEN

                        NULL; --refresh pp

                ELSE

                        load_pp <= '1'; --update product       

                END IF;

                comp <= mrreg(1);       --subract if mrreg(1 DOWNTO 0) ="10"

        END IF;

END PROCESS;

END structural;