memory.py

import array
from kbhit import check_key
from getch import getch

_MEMORY_SIZE = 2 ** 16
_main_memory = array.array('H', [0 for i in range(_MEMORY_SIZE)])


class MMR:
    """Memory-mapped registers """
    KBSR = 0xFE00  # keyboard status
    KBDR = 0xFE02  # keyboard data


def load_image(image):
    global _main_memory
    # load the origin, which indicates the starting address of the
    # program in memory.
    origin = int.from_bytes(image.read(2), byteorder='big')
    _main_memory = array.array("H", [0] * origin)
    # load actual program instructions in the origin address.
    max_read = _MEMORY_SIZE - origin
    _main_memory.frombytes(image.read(max_read))
    # swap loaded bytes, as LC-3 is big-endian.
    _main_memory.byteswap()
    # fill the rest of the memory with zeros.
    _main_memory.fromlist([0] * (_MEMORY_SIZE - len(_main_memory)))


def mem_write(address, value):
    _main_memory[address % _MEMORY_SIZE] = value % _MEMORY_SIZE


def mem_read(address):
    address = address % _MEMORY_SIZE
    if address == MMR.KBSR:
        if check_key():
            mem_write(MMR.KBSR, (1 << 15))
            mem_write(MMR.KBDR, ord(getch()))
    else:
        mem_write(MMR.KBSR, 0)

    return _main_memory[address]


class Registers:
    R0 = 0
    R1 = 1
    R2 = 2
    R3 = 3
    R4 = 4
    R5 = 5
    R6 = 6
    R7 = 7
    PC = 8
    COND = 9


_REGISTERS_COUNT = 10
_R = array.array('H', [0 for i in range(_REGISTERS_COUNT)])


def reg_write(which, value):
    _R[which] = value % _MEMORY_SIZE


def reg_read(which):
    return _R[which]