Add initial register values to Processor::default

src/register.rs

@@ -10,6 +10,14 @@ pub enum Error {
 }
 
 const INITIAL_PROGRAM_COUNTER_VALUE: u16 = 0x100;
+const INITIAL_STACK_POINTER_VALUE: u16 = 0xFFFE;
+const INITIAL_A_REGISTER_VALUE: u8 = 0x01;
+const INITIAL_B_REGISTER_VALUE: u8 = 0xFF;
+const INITIAL_C_REGISTER_VALUE: u8 = 0x13;
+const INITIAL_D_REGISTER_VALUE: u8 = 0x00;
+const INITIAL_E_REGISTER_VALUE: u8 = 0xC1;
+const INITIAL_H_REGISTER_VALUE: u8 = 0x84;
+const INITIAL_L_REGISTER_VALUE: u8 = 0x03;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum Flag {
@@ -90,19 +98,19 @@ impl Registers {
 
 impl Default for Registers {
     fn default() -> Self {
-        // TODO: verify that these 0 values are correct. I feel like they must
-        // be, but I haven't read enough of the manual
         Registers {
-            a: 0,
-            b: 0,
-            c: 0,
-            d: 0,
-            e: 0,
-            h: 0,
-            l: 0,
+            a: INITIAL_A_REGISTER_VALUE,
+            b: INITIAL_B_REGISTER_VALUE,
+            c: INITIAL_C_REGISTER_VALUE,
+            d: INITIAL_D_REGISTER_VALUE,
+            e: INITIAL_E_REGISTER_VALUE,
+            h: INITIAL_H_REGISTER_VALUE,
+            l: INITIAL_L_REGISTER_VALUE,
             interrupts_enabled: false,
-            stack_pointer: 0,
+            stack_pointer: INITIAL_STACK_POINTER_VALUE,
             program_counter: INITIAL_PROGRAM_COUNTER_VALUE,
+            // TODO: this is correct on the "early" DMG0 but the more widely available DMG does have some nuance
+            // https://gbdev.io/pandocs/Power_Up_Sequence.html?highlight=FFFE#dmg_c
             flags: 0,
         }
     }