new register impl & stepping execution

.gitignore

@@ -1,2 +1,3 @@
 /target
 /test-roms
+/bootrom

src/main.rs

@@ -1,7 +1,10 @@
 #![feature(exclusive_range_pattern)]
 
 use clap::Parser;
-use std::fs;
+use std::{
+    fs,
+    io::{self, Read, Write},
+};
 
 /// Simple program to greet a person
 #[derive(Parser, Debug)]
@@ -10,13 +13,26 @@ struct Args {
     /// ROM path
     #[arg(short, long)]
     rom: String,
+
+    /// BootROM path
+    #[arg(short, long)]
+    bootrom: String,
 }
 
-type Register = u16;
-type Instruction = u8;
 type Address = u16;
 type ROM = Vec<u8>;
 
+#[derive(Clone, Copy)]
+struct Inner {
+    left: u8,
+    right: u8,
+}
+
+union Register {
+    as_u8s: Inner,
+    as_u16: u16,
+}
+
 struct Memory {
     rom: ROM,
     vram: [u8; 8192],
@@ -48,7 +64,7 @@ impl Memory {
                 return self.ram[(address - 0xC000) as usize];
             }
             0xE000..0xFE00 => {
-                panic!("ram mirror")
+                return self.ram[(address - 0xE000) as usize];
             }
             0xFE00..0xFEA0 => {
                 panic!("sprite attrib memory");
@@ -72,7 +88,6 @@ impl Memory {
     }
 }
 
-#[derive(Debug)]
 struct State {
     af: Register,
     bc: Register,
@@ -86,12 +101,12 @@ impl Default for State {
     fn default() -> Self {
         // default post-bootrom values
         Self {
-            af: 0x01B0,
-            bc: 0x0013,
-            de: 0x00D8,
-            hl: 0x014D,
-            sp: 0xFFFE,
-            pc: 0x0100,
+            af: Register { as_u16: 0x01B0 },
+            bc: Register { as_u16: 0x0013 },
+            de: Register { as_u16: 0x00D8 },
+            hl: Register { as_u16: 0x014D },
+            sp: Register { as_u16: 0xFFFE },
+            pc: Register { as_u16: 0x0100 },
         }
     }
 }
@@ -102,32 +117,53 @@ struct CPU {
 }
 
 impl CPU {
-    fn exec_next(&mut self) {
+    fn exec_next(&mut self) -> u8 {
         let opcode = self.next_opcode();
-        let p1 = self.next_opcode();
-        let p2 = self.next_opcode();
+        println!("opcode: {:#X}", opcode);
+
         match opcode {
             0x0 => {
                 // noop
             }
             0x01 => {
-                self.state.bc = u8s_to_u16(&p1, &p2);
+                self.state.bc.as_u8s.left = self.next_opcode();
+                self.state.bc.as_u8s.right = self.next_opcode();
+            }
+            0x11 => {
+                self.state.de.as_u8s.left = self.next_opcode();
+                self.state.de.as_u8s.right = self.next_opcode();
+            }
+            0x21 => {
+                self.state.de.as_u8s.left = self.next_opcode();
+                self.state.de.as_u8s.right = self.next_opcode();
+            }
+            0x2C => {
+                unsafe {
+                    self.state.hl.as_u8s.right += 1;
+                };
             }
             0x66 => {
-                self.state.hl = u8s_to_u16(&self.memory.get(self.state.hl), &p2);
+                unsafe {
+                    self.state.hl.as_u8s.left = self.memory.get(self.state.hl.as_u16);
+                };
             }
             0xC3 => {
-                self.state.pc = u8s_to_u16(&p1, &p2);
+                let (a, b) = (self.next_opcode(), self.next_opcode());
+                self.state.pc.as_u8s.left = a;
+                self.state.pc.as_u8s.right = b;
             }
             _ => {
                 panic!("unimplemented opcode: {:#X}", opcode);
             }
         };
+        return opcode;
     }
     fn next_opcode(&mut self) -> u8 {
-        let opcode = self.memory.get(self.state.pc);
-        self.state.pc += 0x1;
-        return opcode;
+        unsafe {
+            let opcode = self.memory.get(self.state.pc.as_u16);
+            self.state.pc.as_u16 += 0x1;
+            return opcode;
+        };
     }
 }
 
@@ -135,21 +171,17 @@ fn main() {
     let args = Args::parse();
 
     let rom: ROM = fs::read(args.rom).expect("Could not load ROM");
+    let bootrom: ROM = fs::read(args.bootrom).expect("Could not load BootROM");
     let mut cpu = CPU {
         memory: Memory::init(rom),
         state: State::default(),
     };
     loop {
         cpu.exec_next();
+        pause();
     }
 }
 
-fn u8s_to_u16(p1: &u8, p2: &u8) -> u16 {
-    ((*p1 as u16) << 8) | *p2 as u16
-}
-
-fn u16_to_u8s(p: &u16) -> (u8, u8) {
-    let p1 = *p as u8;
-    let p2 = (*p >> 8) as u8;
-    (p1, p2)
+fn pause() {
+    io::stdin().read_line(&mut String::new()).unwrap();
 }