def __write_cmp(jump_type):
"""Pop 2 items off the stack, compare them, push the result back on"""
# pop an item off the stack
# D = M
# pop another item off the stack
# D = D - M
# @TRUE_0
# D; JEQ
# (FALSE_0)
# D = 0
# @END_0
# 0; jump_type
# (TRUE_0)
# D = 1
# (END_0)
# push()
if jump_type not in ['EQ', 'GT', 'LT', 'GE', 'LE', 'NE']:
raise Exception("Unexpected Jump Type: {0}".format(jump_type))
true_label = unique("TRUE")
false_label = unique("FALSE")
end_label = unique("END")
return (pop() +
['D=M'] +
pop() +
['D=M-D',
at(true_label),
'D;J{0}'.format(jump_type),
'({0})'.format(false_label),
'D=0',
at(end_label),
'0;JMP',
'({0})'.format(true_label),
'D=-1', # -1 == 11111...11 in 2's complement
'({0})'.format(end_label)] +
push())
def write_pop(segment, i, ftitle=None):
"""Pop a value from the stack and store it at position i in segment"""
# local, argument, this, and that are implemented the same way
standard_segs = {
"local": "LCL",
"argument": "ARG",
"this": "THIS",
"that": "THAT",
}
if segment in standard_segs:
segname = standard_segs[segment]
return ([
at(segname), # addr=segname+i
"D=M",
at(i),
"D=D+A",
at('addr'),
"M=D"
] + pop() + [
"D=M",
at('addr'), # *addr=*SP
"A=M",
"M=D"
])
elif segment == "temp":
return ([
at(5), # temp base address is 5
"D=A",
at(i),
"D=D+A",
at('addr'),
"M=D"
] + pop() + [
"D=M",
at('addr'), # *addr=*SP
"A=M",
"M=D"
])
elif segment == "static":
label = "{0}.{1}".format(ftitle, i)
return pop() + ["D=M", at(label), "M=D"]
elif segment == "pointer":
if int(i) not in [0, 1]:
raise Exception('Invalid i for pop pointer: ' + str(i))
target = "THAT" if int(i) else "THIS"
return [
"@SP", # SP--
"M=M-1",
"A=M", # THIS/THAT = *SP
"D=M",
at(target),
"M=D"
]
raise Exception("{0} segment type not implemented yet".format(segment))
def write_return():
# endFrame = LCL
# retAddr = *(endFrame-5)
# *ARG = pop(), sets return value to arg[0]
# SP = ARG+1
# endFrame -=1, THAT = *(endFrame)
# endFrame -=1, THIS = *(endFrame)
# endFrame -=1, ARG = *(endFrame)
# endFrame -=1, LCL = *(endFrame)
# goto retAddr
# ---- implementation ----
# endFrame = LCL
output = [at("LCL"), "D=M", at("endFrame"), "M=D"]
# retAddr = *(endFrame-5)
output += __write_save_retAddr()
# *ARG = pop()
output += (["// *ARG = pop()"] + pop() + ["D=M", "@ARG", "A=M", "M=D"])
# SP = ARG+1
output += ["// SP = ARG+1", "@ARG", "D=M+1", "@SP", "M=D"]
# endFrame -=1, segm_var = *(endFrame)
output += __write_restore("THAT")
output += __write_restore("THIS")
output += __write_restore("ARG")
output += __write_restore("LCL")
# goto retAddr
output += ["// goto retAddr", "@retAddr", "A=M", "0;JMP"]
return output
def __write_bitwise(op):
"""Pop 2 items off the stack, do a bitwise operation, push result back on
this is only used for operations which do not need a jump"""
if op not in ['|', '&', '+', '-']:
raise Exception("Unexpected Bitwise Operation: {0}".format(op))
return pop() + ['D=M'] + pop() + ['D=M{0}D'.format(op)] + push()
def write_not():
"""Pop 1 item off the stack, negate it, push the result back on"""
return pop() + ['D=!M'] + push()
def write_if_goto(label, func):
return pop() + ["D=M", at(func + '$' + label), "D; JNE"]
def __write_save_retAddr():
"""Saves the return address in a local variable. """
return (["// retAddr = *(endFrame - 5)",
at("endFrame"), "D=M"] + push() + [at(5), "D=A", "// push"] +
push() + ["// write_sub"] + write_sub() + ["// pop"] + pop() +
["A=M", "D=M", at("retAddr"), "M=D"])