comporg-assembler

comporg-assembler.py

# A Hack assembler in 5 Minutes
import re

# Set up hash tables (dictionaries, in Python) to capture all of
# the instruction transformations. This is a "lookup table," of sorts.
# When we encounter an "A", it will always be transformed to
# "110000", which we will get by looking up the key "A" in the dictionary
# called "comp".
#
# I like my hash tables to line up all pretty-like when I can pull it off.

comp = {'A'   : '110000',
        'D'   : '001100',
        'D+A' : '000010'}
     
# Do the same for the destinations
dest = {'D' : '010',
        'M' : '001'}

# We will need a table for the jumps.

# Open the file. It might be good to read it into an
# array; at least, for small (lines < 100K) programs.
f = open('program.hack', 'r')

# Convert a number into a 15 bit binary number.
# From this Stack Overflow post:
# http://goo.gl/ihXfp
def as_binary (n):
  return '{0:015b}'.format(int(n))

# Convert a textual line of Hack assembly into a binary string.
# There are probably be better ways.
def convertInstruction (line):
  # We'll return this at the end.
  binary_string = "ERROR: %s" % line
  
  search_result = re.search("@(\d+)", line)
  if search_result != None:
    binary_string = "0" + as_binary(search_result.group(1))
    
  search_result = re.search('^(.*?)=(.*?)$', line)
  if search_result != None:
    lhs = search_result.group(1)
    rhs = search_result.group(2)
    # print "L %s R %s" % (lhs, rhs)
    if re.search(rhs, 'M'):
      a = 1
    else:
      a = 0      
    j ="000"
  
    binary_string = "111%s%s%s%s" % (a, comp[rhs], dest[lhs], j)
  
  # Our 'if' clauses should have set the value of "binary_string"
  # before this point. If not, we will return the line that caused
  # the error for our conversion. That will help us debug our assembler.
  return binary_string

# Instead of doing everything in the loop, I
# put the original contents into a function.
# It does the same thing, but this allows us to extend our
# processing within the loop with new functions.
# For example, you need to build a "lookup table"
# for variables when you see things like:
# 
# @varRef
#
# and turn those into actual RAM locations. That must happen before
# we can write out binary.
for line in f:
  zeros_and_ones = convertInstruction(line)
  print zeros_and_ones
  

program.hack

@3
D=A
@5
D=D+A
@0
M=D