zpu_idecode.py

#
# This module contains the instruction decode.  This decodes the instruction
# to an enumeration for the execution unit.
#
 
from myhdl import *
 
from zpu_pkg import *
 
def zpu_idecode(
    pc_r,       #  Program Counter (instruction memory address)
    i_data,     #  Instruction data
    opcode,     #  Decode opcode
    BYTE_BITS=3 # 
    ):
    """
 
    """
 
    topcode  = Signal(intbv(0)[8:])
    d_opcode = decode_t.DEC_NOP
 
    # Each instruction is only 8bits but the instruction ROM data
    # width can be 32, 16, or 8
    @always_comb
    def i_sel():
        a = pc_r[BYTE_BITS:]
 
        if a == 0:
            topcode.next = i_data[32:24]
        elif a == 1:
            topcode.next = i_data[24:16]
        elif a == 2:
            topcode.next = i_data[16:8]
        else:  # a == 3
            topcode.next = i_data[8:0]
 
 
    @always_comb
    def decode_control():
 
        opcode.next = d_opcode
 
        if topcode[7] == OPCODE_IM:
            d_opcode = decode_t.DEC_IM
        elif topcode[8:5] == OPCODE_STORESP:
            d_opcode = decode_t.DEC_LOAD_SP
        elif topcode[8:5] == OPCODE_LOADSP:
            d_opcode = decode_t.DEC_LOAD_SP
        elif topcode[8:5] == OPCODE_EMULATE:
            d_opcode = decode_t.DEC_EMULATE
        elif topcode[8:5] == OPCODE_ADDSP:
            d_opcode = decode_t.DEC_ADD_SP
        else:
            if topcode[4:0] == OPCODE_BREAK:
                d_opcode = decode_t.DEC_BREAK
            elif topcode[4:0] == OPCODE_PUSHSP:
                d_opcode = decode_t.DEC_PUSH_SP
            elif topcode[4:0] == OPCODE_POPPC:
                d_opcode = decode_t.DEC_POP_PC
            elif topcode[4:0] == OPCODE_ADD:
                d_opcode = decode_t.DEC_ADD
            elif topcode[4:0] == OPCODE_OR:
                d_opcode = decode_t.DEC_OR
            elif topcode[4:0] == OPCODE_AND:
                d_opcode = decode_t.DEC_AND
            elif topcode[4:0] == OPCODE_LOAD:
                d_opcode = decode_t.DEC_LOAD
            elif topcode[4:0] == OPCODE_NOT:
                d_opcode = decode_t.DEC_NOT
            elif topcode[4:0] == OPCODE_FLIP:
                d_opcode = decode_t.DEC_FLIP
            elif topcode[4:0] == OPCODE_STORE:
                d_opcode = decode_t.DEC_STORE
            elif topcode[4:0] == OPCODE_POPSP:
                d_opcode = decode_t.DEC_POP_SP
            else:
                d_opcode = decode_t.DEC_NOP
 
 
 
    return instances()
 
 
if __name__ == '__main__':
 
    pc_r   = Signal(intbv(0)[2:])     # Input, emulated 
    i_data = Signal(intbv(0)[32:])    # Instruction data
    opcode = Signal(decode_t.DEC_NOP) # Decoded Opcode, Enum
 
    dut = zpu_idecode(pc_r, i_data, opcode)