Implement mem device for bootrom overlay

memory/bootrom_overlay.cpp

@@ -0,0 +1,50 @@
+#include "memory/bootrom_overlay.h"
+
+BootRom::BootRom(Mem_device* lowmem, Mem_device* highmem, Mem_device* backend)
+  : lowmem(lowmem), highmem(highmem), backend(backend), enabled(true)
+{}
+
+void BootRom::enable()
+{
+  enabled = true;
+}
+void BootRom::disable()
+{
+  enabled = false;
+}
+
+void BootRom::write8(u16 addr, u8 data)
+{
+  if(enabled && (lowrange.contains(addr) || highrange.contains(addr)))
+    return;
+
+  backend->write8(addr, data);
+}
+
+u8 BootRom::read8(u16 addr)
+{
+  if(enabled && lowrange.contains(addr))
+    return lowmem->read8(addr);
+  else if(enabled && highrange.contains(addr))
+    return highmem->read8(addr);
+  else
+    return backend->read8(addr);
+}
+
+void BootRom::write16(u16 addr, u16 data)
+{
+  if(enabled && (lowrange.contains(addr) || highrange.contains(addr)))
+    return;
+
+  backend->write16(addr, data);
+}
+
+u16 BootRom::read16(u16 addr)
+{
+  if(enabled && lowrange.contains(addr))
+    return lowmem->read16(addr);
+  else if(enabled && highrange.contains(addr))
+    return highmem->read16(addr);
+  else
+    return backend->read16(addr);
+}

memory/bootrom_overlay.h

@@ -0,0 +1,28 @@
+#pragma once
+#include <memory/mem_device.h>
+
+class BootRom : Mem_device
+{
+private:
+  Mem_device* lowmem;
+  Mem_device* highmem;
+
+  Mem_device* backend;
+
+  const Range lowrange(0x0000, 0x00FF);
+  const Range highrange(0x0200, 0x08FF);
+
+  bool enabled;
+public:
+  BootRom(Mem_device* lowmem, Mem_device* highmem, Mem_device* backend);
+
+  void enable();
+  void disable();
+
+  virtual void write8(u16 addr, u8 data);
+  virtual u8 read8(u16 addr);
+
+  virtual void write16(u16 addr, u16 data);
+  virtual u16 read16(u16 addr);
+
+}

memory/mem_device.h

@@ -2,6 +2,23 @@
 
 #include "types.h"
 
+class Range {
+private:
+  u16 start;
+  u16 end;
+
+public:
+  constexpr Range(u16 start, u16 end)
+    : start(start), end(end)
+  {};
+
+  inline
+  bool contains(u16 addr)
+  {
+    return addr >= start && addr <= end;
+  }
+};
+
 class Mem_device {
 public:
   virtual void write8(u16 addr, u8 data) = 0;