From 1b9024373897a1c939a2af5fcef4d7f3cd9d4b1c Mon Sep 17 00:00:00 2001
From: Sam M W <you@example.com>
Date: Thu, 13 Apr 2023 10:23:55 +0100
Subject: [PATCH] implementation of RTS

---
 src/cpu.rs | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/src/cpu.rs b/src/cpu.rs
index a2a0d0c..3cec77d 100644
--- a/src/cpu.rs
+++ b/src/cpu.rs
@@ -409,11 +409,13 @@ impl<M: Bus> CPU<M> {
             }
             (Instruction::PLA, OpInput::UseImplied) => {
                 // Pull accumulator
+                self.pull_from_stack();
                 let val: u8 = self.pull_from_stack();
                 self.registers.accumulator = val as i8;
             }
             (Instruction::PLP, OpInput::UseImplied) => {
                 // Pull status
+                self.pull_from_stack();
                 let val: u8 = self.pull_from_stack();
                 // The `truncate` here won't do anything because we have a
                 // constant for the single unused flags bit. This probably
@@ -443,6 +445,12 @@ impl<M: Bus> CPU<M> {
                 CPU::<M>::rotate_right_with_flags(&mut operand, &mut self.registers.status);
                 self.memory.set_byte(addr, operand);
             }
+            (Instruction::RTS, OpInput::UseImplied) => {
+                self.pull_from_stack();
+                let pcl: u8 = self.pull_from_stack();
+                let pch: u8 = self.fetch_from_stack();
+                self.registers.program_counter = (((pch as u16) << 8) | pcl as u16).wrapping_add(1);
+            }
 
             (Instruction::SBC, OpInput::UseImmediate(val)) => {
                 debug!("subtract with carry immediate: {}", val);
@@ -921,6 +929,12 @@ impl<M: Bus> CPU<M> {
         self.registers.stack_pointer.increment();
         out
     }
+
+    fn fetch_from_stack(&mut self) -> u8 {
+        // gets the next value on the stack but does not update the stack pointer
+        let addr = self.registers.stack_pointer.to_u16();
+        self.memory.get_byte(addr)
+    }
 }
 
 impl<M: Bus> core::fmt::Debug for CPU<M> {