A RISC Design: Synthesis of the MIPS Processor Core - Rapid Prototyping of Digital Systems - page 297

ARCHITECTURE behavior OF Idecode IS
TYPE register_file IS ARRAY ( 0 TO 31 ) OF STD_LOGIC_VECTOR ( 31 DOWNTO 0 );

SIGNAL register_array: register_file;

SIGNAL write_register_address : STD_LOGIC_VECTOR ( 4 DOWNTO 0 );

SIGNAL write_data : STD_LOGIC_VECTOR ( 31 DOWNTO 0 );

SIGNAL read_register_1_address : STD_LOGIC_VECTOR ( 4 DOWNTO 0 );

SIGNAL read_register_2_address : STD_LOGIC_VECTOR ( 4 DOWNTO 0 );

SIGNAL write_register_address_1 : STD_LOGIC_VECTOR ( 4 DOWNTO 0 );

SIGNAL write_register_address_0 : STD_LOGIC_VECTOR ( 4 DOWNTO 0 );

SIGNAL Instruction_immediate_value : STD_LOGIC_VECTOR ( 15 DOWNTO 0 );

BEGIN
read_register_1_address <= Instruction( 25 DOWNTO 21 );
read_register_2_address <= Instruction( 20 DOWNTO 16 );
write_register_address_1 <= Instruction( 15 DOWNTO 11 );
write_register_address_0 <= Instruction( 20 DOWNTO 16 );
Instruction_immediate_value <= Instruction( 15 DOWNTO 0 );
-- Read Register 1 Operation
read_data_1 <= register_array( CONV_INTEGER( read_register_1_address) );
-- Read Register 2 Operation
read_data_2 <= register_array( CONV_INTEGER( read_register_2_address) );
-- Mux for Register Write Address
write_register_address <= write_register_address_1
WHEN RegDst = '1' ELSE write_register_address_0;
-- Mux to bypass data memory for Rformat instructions
write_data <= ALU_result( 31 DOWNTO 0 )
WHEN ( MemtoReg = '0' ) ELSE read_data;
-- Sign Extend 16-bits to 32-bits
Sign_extend <= X"0000" & Instruction_immediate_value

WHEN Instruction_immediate_value(15) = '0'

ELSE X"FFFF" & Instruction_immediate_value;

PROCESS
BEGIN
WAIT UNTIL clock 'EVENT AND clock = '1';
IF reset = '1' THEN
-- Initial register values on reset are register = reg#
-- use loop to automatically generate reset logic
-- for all registers
FOR i IN 0 TO 31 LOOP
register_array(i) <= CONV_STD_LOGIC_VECTOR( i, 32 );
END LOOP ;
-- Write back to register - don't write to register 0
ELSIF RegWrite = '1' AND write_register_address /= 0 THEN
register_array( CONV_INTEGER( write_register_address)) <= write_data;
END IF ;
END PROCESS ;
END behavior;