From 3ccd2006af80471967f4be1c701ba6013dd28b4f Mon Sep 17 00:00:00 2001
From: Atreya Bain <atreyabain@gmail.com>
Date: Sun, 18 Feb 2024 09:46:10 +0530
Subject: [PATCH] [pcr] Implement PC

---
 src/emu/iomem.rs           | 15 +++++++--------
 src/emu/mod.rs             |  3 ++-
 src/emu/program_counter.rs | 38 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 47 insertions(+), 9 deletions(-)
 create mode 100644 src/emu/program_counter.rs

diff --git a/src/emu/iomem.rs b/src/emu/iomem.rs
index 04a12e5..ca3e720 100644
--- a/src/emu/iomem.rs
+++ b/src/emu/iomem.rs
@@ -1,4 +1,5 @@
 use crate::emu::mmu::Memory;
+use crate::emu::program_counter::ProgramCounter;
 use crate::misc::emulator_error::EmulatorError;
 use crate::misc::endian::{read_big_endian_u16, read_big_endian_u24};
 use crate::misc::result::EmulatorResult;
@@ -7,13 +8,13 @@ use crate::misc::result::EmulatorResult;
 pub struct MemoryMappedIO {
     //FIXME use a keyboard
     keyboard_bytes: [u8; 2],
-    program_counter: [u8; 3],
+    program_counter: ProgramCounter,
     //FIXME use a device
     pixel_reg: u8,
     //FIXME use a device
     audio_sample_address_base: [u8; 2],
 }
-
+/// Represents the memory mapped segment of IO. Aggregates the mapping logic.
 impl MemoryMappedIO {
     // 2 byte keyboard bits
     pub const KEYBOARD_BIT_START: u32 = 0;
@@ -32,10 +33,10 @@ impl MemoryMappedIO {
     pub const AUDIO_SAMPLE_BASE_LEN: u32 = 2;
     const AUDIO_SAMPLE_BASE_END: u32 = Self::AUDIO_SAMPLE_BASE_START + Self::AUDIO_SAMPLE_BASE_LEN - 1;
 
-    pub fn new() -> MemoryMappedIO {
+    pub fn new(program_counter: ProgramCounter) -> MemoryMappedIO {
         MemoryMappedIO {
             keyboard_bytes: [0, 0],
-            program_counter: [0, 0, 0],
+            program_counter,
             pixel_reg: 0,
             audio_sample_address_base: [0, 0],
         }
@@ -53,10 +54,8 @@ impl Memory for MemoryMappedIO {
                 keyboard_byte
             }
             Self::PC_START_ADDR..=Self::PC_END_ADDR => {
-                let pc_index = (address - Self::PC_START_ADDR) as usize;
-                let pc_byte = self.program_counter[pc_index];
-                log::trace!("Fetching PC({}) byte segment {} -> {}",read_big_endian_u24(&self.program_counter),pc_index,pc_byte);
-                pc_byte
+                let pc_index = address - Self::PC_START_ADDR;
+                self.program_counter.try_get_byte(pc_index)?
             }
             Self::PIXEL_BASE => {
                 log::trace!("Fetching pixel base reg {}",self.pixel_reg);
diff --git a/src/emu/mod.rs b/src/emu/mod.rs
index a2169e4..6c70821 100644
--- a/src/emu/mod.rs
+++ b/src/emu/mod.rs
@@ -1,4 +1,5 @@
 pub mod cpu;
 pub mod mmu;
 pub mod ram;
-pub mod iomem;
\ No newline at end of file
+pub mod iomem;
+mod program_counter;
\ No newline at end of file
diff --git a/src/emu/program_counter.rs b/src/emu/program_counter.rs
new file mode 100644
index 0000000..4af8327
--- /dev/null
+++ b/src/emu/program_counter.rs
@@ -0,0 +1,38 @@
+use crate::emu::mmu::Memory;
+use crate::misc::emulator_error::DeviceType::PC;
+use crate::misc::emulator_error::EmulatorError;
+use crate::misc::endian::{read_big_endian_u24, write_big_endian_u24};
+use crate::misc::result::EmulatorResult;
+
+
+#[derive(Debug,Default, Copy, Clone)]
+pub struct ProgramCounter {
+    /// 24bit location register
+    program_counter_register: [u8; 3],
+}
+
+impl ProgramCounter {
+    // get the current program counter as an address
+    pub fn get_pc_value(&self) -> u32 {
+        read_big_endian_u24(&self.program_counter_register)
+    }
+    // assign a value of PC to start execution
+    pub fn set_address(&mut self, address: u32) {
+        log::debug!("Setting PC as {}",address);
+        write_big_endian_u24(address, &mut self.program_counter_register)
+    }
+}
+/// Allow Using Program counter as mapped memory
+impl Memory for ProgramCounter {
+    fn try_get_byte(&self, address: u32) -> EmulatorResult<u8> {
+        log::trace!("Fetching PC({}) byte segment index {}",read_big_endian_u24(&self.program_counter_register),address);
+        self.program_counter_register.get(address as usize)
+            .map(|e| *e)
+            .ok_or(EmulatorError::UnreachableMemory(PC, address))
+    }
+    /// TODO: Set a byte of PC from the memory.
+    fn try_set_byte(&mut self, address: u32, value: u8) -> EmulatorResult<()> {
+        log::error!("Unsupported Method!");
+        todo!()
+    }
+}
\ No newline at end of file