Discussion Session 5

Debuggin Python Program (pdb)

simulate.py

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#####################################################################
#
# CAS CS 320, Fall 2014
# machine.py
#

def simulate(s):
    instructions = s if type(s) == list else s.split("\n")
    instructions = [l.strip().split(" ") for l in instructions]
    mem = {0: 0, 1: 0, 2: 0, 3: 0, 4: 0, 5: -1, 6: 0}
    control = 0
    outputs = []
    while control < len(instructions):
        # Update the memory address for control.
        mem[6] = control 
        
        # Retrieve the current instruction.
        inst = instructions[control]
        
        # Handle the instruction.
        if inst[0] == 'label':
            pass
        if inst[0] == 'goto':
            control = instructions.index(['label', inst[1]]) #25
            continue
        if inst[0] == 'branch' and mem[int(inst[2])]:
            control = instructions.index(['label', inst[1]]) #28
            continue
        if inst[0] == 'jump':
            control = mem[int(inst[1])] #31
            continue
        if inst[0] == 'set':
            mem[int(inst[1])] = int(inst[2]) #34
        if inst[0] == 'copy':
            mem[mem[4]] = mem[mem[3]] #36
        if inst[0] == 'add':
            mem[0] = mem[1] + mem[2] #38

        # Push the output address's content to the output.
        if mem[5] > -1:
            outputs.append(mem[5]) #42
            mem[5] = -1

        # Move control to the next instruction.
        control = control + 1

    print("memory: "+str(mem))
    return outputs

# Examples of useful helper functions from lecture.    
def copy(frm, to):
   return [\
      'set 3 ' + str(frm),\
      'set 4 ' + str(to),\
      'copy'\
   ]

if __name__ == "__main__":
    inss = [\
        "set 1 100", \
        "set 2 200", \
        "add"] + \
        copy(0, 5)
    ret = simulate(inss)
    print("ret is ", ret)

Debug Python program from command line.

python3 -m pdb simulate.py

Common commands for using pdb

  • n(ext)
  • c(ontinue)
  • b(reak)
  • q(uit)
  • cl(ear)

An easy-to-read blog about pdb module in Python: http://pythonconquerstheuniverse.wordpress.com/category/python-debugger/

Generating program of machine language

  • mul(ax, ay, aret)
  • make_procedure(name, inss, stack)
  • make_call(name, stack)