Improve efficiency of some instructions

When a value is to be read and kept on the stack, instead of popping and
immediately pushing it we now use a more efficient `get` stack method.

The pop_u16 stack method has also been changed to a variation that
assembles to fewer CPU instructions.

1 files changed, 14 insertions, 2 deletions

diff --git a/src/stack.rs b/src/stack.rs
index 2a0a514..14e1799 100644
--- a/src/stack.rs
+++ b/src/stack.rs
@@ -28,8 +28,20 @@ impl Stack {
     }
 
     pub fn pop_u16(&mut self) -> u16 {
-        let byte_low = self.pop_u8();
-        let byte_high = self.pop_u8();
+        self.sp = self.sp.wrapping_sub(1);
+        let byte_low = self.mem[self.sp as usize];
+        self.sp = self.sp.wrapping_sub(1);
+        let byte_high = self.mem[self.sp as usize];
+        u16::from_be_bytes([byte_high, byte_low])
+    }
+
+    pub fn get_u8(&mut self) -> u8 {
+        self.mem[self.sp.wrapping_sub(1) as usize]
+    }
+
+    pub fn get_u16(&mut self) -> u16 {
+        let byte_low = self.mem[self.sp.wrapping_sub(1) as usize];
+        let byte_high = self.mem[self.sp.wrapping_sub(2) as usize];
         u16::from_be_bytes([byte_high, byte_low])
     }