From d1cf15d5bdaae0c05f676e08f117fb849191d9de Mon Sep 17 00:00:00 2001
From: Stephanie Gredell <s.raide@gmail.com>
Date: Sat, 18 Jan 2025 13:57:01 -0800
Subject: [PATCH] starting the cpu portion

---
 cpu.rs  | 78 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 main.rs |  2 +-
 2 files changed, 79 insertions(+), 1 deletion(-)
 create mode 100644 cpu.rs

diff --git a/cpu.rs b/cpu.rs
new file mode 100644
index 0000000..7e8088e
--- /dev/null
+++ b/cpu.rs
@@ -0,0 +1,78 @@
+// This is all new to me so it's heavily commented so we can understand what is happening.
+
+pub struct CPU {
+    // the accumulator register is a specific register  used for arithmetic and logic operations
+    // the cpu instruction loads a value into the accumulator register and then updates certain
+    // flags in the processor status register to relect the operation of the result
+    pub register_a: u8,
+    // THe Process Status Register is a collection of individual bits (flags) that represent the
+    // current state of the CPU. Each bit has purpose such as if a calculation resulted in zero or
+    // if the result is negative
+    //
+    // Zero flag:
+    // - Bit 1 of the status register
+    // - Set if register_a == 0, cleared otherwise
+    //
+    // Negative flag (N) - indicates whether the result of the most recent operation is negative
+    // - Bit 7 of the status register (most significant bit is Bit 7 because it's zero based)
+    // - Set if the most significant bit of register_a is 1, cleared otherwise
+    pub status: u8,
+    // track our current position in the program
+    pub program_counter: u16,
+}
+
+impl CPU {
+    pub fn new() -> Self {
+        CPU {
+            register_a: 0,
+            status: 0,
+            program_counter: 0,
+        }
+    }
+
+    // The interpret method takes in mutalbe reference to self as we know we will need to modify
+    // register_a during execution
+    //
+    // - Fetch next instruction from instruction memory
+    // - Decode instruction
+    // - Execute the instruction
+    // - Repeat
+    pub fn interpret(&mut self, program: Vec<u8>) {
+        self.program_counter = 0;
+
+        // We need an infinite loop to continuously fetch instructions from the program array. We
+        // use the program_counter to keep track fo the current instruction.
+        loop {
+            // set the opscode to the current byte in the program at the address indicated by
+            // program counter
+            let opscode = program[self.program_counter as usize];
+            // we increment program counterto point to the next byte
+            self.program_counter += 1;
+
+            match opscode {
+                // this will implement LDA (0xA9) opcode. 0xA9 is the LDA Immediate instruction in
+                // the 6502 CPU.
+                0xA9 => {
+                    // fetch the next byte in program. This byte is the immediate value to load
+                    // into the accumulator (register_a)
+                    let param = program[self.program_counter as usize];
+                    // Increment program counter to point to the next instruction after the
+                    // parameter
+                    self.program_counter += 1;
+                    // store the fetch paramenter in register_a
+                    self.register_a = param;
+
+                    // now we wil update the status register
+                    if self.register_a == 0 {
+                        // 0b000_0010 represents a number where only the second bit (bit 1) is set
+                        // to 1 and all other bits are 0
+                        self.status = self.status | 0b000_0010;
+                    } else {
+                        // 0b1111_1101 represents a number where only bit 1 is 0 and the rest are 1
+                        self.status = self.status & 0b1111_1101;
+                    }
+                }
+            }
+        }
+    }
+}
diff --git a/main.rs b/main.rs
index 8806a0f..e101b81 100644
--- a/main.rs
+++ b/main.rs
@@ -4,7 +4,7 @@ use std::env;
 use std::net::SocketAddr;
 use tokio::net::{TcpListener, TcpStream};
 use tokio_tungstenite::{accept_async, tungstenite::protocol::Message};
-
+mod cpu;
 // this file is overly commented because I'm learning Rust while I program. I want just want to
 // thnk aloud.
 #[tokio::main]