Answer to question 4, practice questions for exam 2

expr -> expr₁ + expr₂: expr.code = expr₁.code + expr₂.code + single(ADD)
expr -> expr₁ * expr₂: expr.code = expr₁.code + expr₂.code + single(MULT)
expr -> NUM: expr.code = doub(PUSH_INT, NUM.val)
expr -> VAR: expr.code = doub(PUSH_VAR, get_var_offset(VAR.name))