--ROM-based waveform generatorPACKAGE rompac ISCONSTANT rom_width : POSITIVE := 3;CONSTANT addr_high : POSITIVE := 12;SUBTYPE rom_word IS BIT_VECTOR(0 TO rom_width);TYPE rom_table IS ARRAY(0 TO addr_high) OF rom_word;CONSTANT rom : rom_table :=("1100","1100","0100","0000","0110","0101","0111","1100","0100","0000","0110","0101","0111");END rompac;--WAVEFORM GENERATOR USING A ROM LOOK-UP TABLE 15-6-92--THE ROM IS A CONSTANT DECLARED WITHIN THE PACKAGE rompac.--USE work.rompac.ALL;ENTITY romwaves ISPORT(clock : IN BIT; reset : IN BOOLEAN;waves : OUT rom_word);END romwaves;ARCHITECTURE behaviour OF romwaves ISSIGNAL step : NATURAL;BEGIN--address counter for rom look-up tablestep_counter:PROCESSBEGINWAIT UNTIL clock'EVENT AND clock = '1';IF reset THEN --check for reset conditionstep <= 0;ELSIF step = addr_high THEN --check for last wave valuestep <= addr_high;ELSEstep <= step + 1; --get next wave valueEND IF;END PROCESS;--output value from rom look-up tablewaves <= rom(step);END behaviour;