Fix bug with SUB and add --lines CL arg

hera/main.py

@@ -7,6 +7,7 @@
     hera (-v | --version)
 
 Options:
+    --lines=<n>      Only execute the first n lines of the program.
     --no-dump-state  Do not print the state of the virtual machine after execution.
     --no-color       Do not print colored output.
     -h, --help       Show this message.
@@ -67,10 +68,16 @@ def main(argv=None, vm=None):
     if arguments["preprocess"]:
         preprocess_program(program)
     else:
-        execute_program(program, no_dump_state=arguments["--no-dump-state"], vm=vm)
+        lines_to_exec = int(arguments["--lines"]) if arguments["--lines"] else None
+        execute_program(
+            program,
+            lines_to_exec=lines_to_exec,
+            no_dump_state=arguments["--no-dump-state"],
+            vm=vm,
+        )
 
 
-def execute_program(program, *, no_dump_state=False, vm=None):
+def execute_program(program, *, lines_to_exec=None, no_dump_state=False, vm=None):
     """Execute the program with the given options, most of which correspond to
     command-line arguments.
 
@@ -86,7 +93,7 @@ def execute_program(program, *, no_dump_state=False, vm=None):
         program = parse(program)
         typecheck(program)
         program = preprocess(program)
-        vm.exec_many(program)
+        vm.exec_many(program, lines=lines_to_exec)
     except HERAError as e:
         if e.line:
             if e.column:

hera/vm.py

@@ -121,12 +121,13 @@ def exec_one(self, inst):
         else:
             handler(*inst.args)
 
-    def exec_many(self, program):
+    def exec_many(self, program, *, lines=None):
         """Execute a program (i.e., a list of instructions), resetting the
         machine's state beforehand.
         """
+        terminate_at = min(lines, len(program)) if lines is not None else len(program)
         self.reset()
-        while self.pc < len(program):
+        while self.pc < terminate_at:
             opc = self.pc
             try:
                 self.exec_one(program[self.pc])
@@ -212,7 +213,9 @@ def exec_sub(self, left, right):
         result = to_u16((left - right - borrow) & 0xFFFF)
 
         self.flag_carry = left >= right
-        self.flag_overflow = from_u16(result) != from_u16(left) - from_u16(right)
+        self.flag_overflow = (
+            from_u16(result) != from_u16(left) - from_u16(right) - borrow
+        )
 
         return result
 

test/test_op/test_arithmetic_logic.py

@@ -280,6 +280,13 @@ def test_sub_overflow_from_borrow(vm):
     assert vm.flag_overflow
 
 
+def test_sub_overflow_takes_borrow_into_account(vm):
+    vm.registers[1] = 10
+    vm.registers[2] = 11
+    vm.exec_sub("R0", "R1", "R2")
+    assert not vm.flag_overflow
+
+
 def test_sub_sets_carry_for_equal_operands(vm):
     vm.flag_carry = True
     vm.registers[1] = 12