fix unsigned subtraction

src/processor/implementation.rs

@@ -149,7 +149,11 @@ impl Processor {
     }
 
     #[allow(dead_code)]
-    pub fn run(&mut self, breakpoints: bool, count: Option<isize>) -> Result<usize, EmulationError> {
+    pub fn run(
+        &mut self,
+        breakpoints: bool,
+        count: Option<isize>,
+    ) -> Result<usize, EmulationError> {
         let instruction_count = self.runtime_counter;
         let end = if let Instruction::NoOperation = self.rom[self.last_instruction_address()] {
             if self.last_instruction_address() == 0 {
@@ -160,18 +164,20 @@ impl Processor {
             self.last_instruction_address()
         };
         let runtime_end = match count {
-            Some(v) => if v >= 0 {
-                self.runtime_counter + v as usize
-            } else {
-                let count= if self.runtime_counter > 0 {
-                    self.runtime_counter as isize + v
+            Some(v) => {
+                if v >= 0 {
+                    self.runtime_counter + v as usize
                 } else {
-                    0
-                };
-                self.reset();
-                count as usize
-            },
-            None => usize::MAX
+                    let count = if self.runtime_counter > 0 {
+                        self.runtime_counter as isize + v
+                    } else {
+                        0
+                    };
+                    self.reset();
+                    count as usize
+                }
+            }
+            None => usize::MAX,
         };
         while self.program_counter <= end && self.runtime_counter < runtime_end {
             if !self.tick()? {
@@ -181,7 +187,7 @@ impl Processor {
                 break;
             }
         }
-        Ok(self.runtime_counter - instruction_count)
+        Ok((self.runtime_counter as isize - instruction_count as isize) as usize)
     }
 
     #[allow(dead_code)]