io(placeholder): add naive io structure

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

include/io.hh

@@ -0,0 +1,20 @@
+#pragma once
+#include <cstdint>
+
+// NOLINTBEGIN(cppcoreguidelines-avoid-c-arrays)
+
+namespace matar {
+class IoRegisters {
+  public:
+    uint8_t read(uint32_t) const;
+    void write(uint32_t, uint8_t);
+
+  private:
+    struct {
+        bool post_boot_flag           = false;
+        bool interrupt_master_enabler = false;
+    } system;
+};
+}
+
+// NOLINTEND(cppcoreguidelines-avoid-c-arrays)

include/memory.hh

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "header.hh"
+#include "io.hh"
 #include <array>
 #include <cstddef>
 #include <cstdint>
@@ -51,6 +52,7 @@ class Memory {
 
 #undef MEMORY_REGION
 
+    IoRegisters io;
     std::unordered_map<uint32_t, uint8_t> invalid_mem;
     std::vector<uint8_t> rom;
     Header header;

src/io.cc

@@ -0,0 +1,40 @@
+#include "io.hh"
+#include "util/log.hh"
+
+namespace matar {
+#define ADDR static constexpr uint32_t
+
+ADDR POSTFLG = 0x4000300;
+ADDR IME     = 0x4000208;
+
+#undef ADDR
+
+uint8_t
+IoRegisters::read(uint32_t address) const {
+    switch (address) {
+        case POSTFLG:
+            return system.post_boot_flag;
+        case IME:
+            return system.interrupt_master_enabler;
+        default:
+            glogger.warn("Unused IO address read at 0x{:08X}", address);
+    }
+
+    return 0xFF;
+}
+
+void
+IoRegisters::write(uint32_t address, uint8_t byte) {
+    switch (address) {
+        case POSTFLG:
+            system.post_boot_flag = byte & 1;
+            break;
+        case IME:
+            system.interrupt_master_enabler = byte & 1;
+            break;
+        default:
+            glogger.warn("Unused IO address written at 0x{:08X}", address);
+    }
+    return;
+}
+}

src/memory.cc

@@ -41,6 +41,10 @@ Memory::read(uint32_t address) const {
     MATCHES(BIOS, bios)
     MATCHES(BOARD_WRAM, board_wram)
     MATCHES(CHIP_WRAM, chip_wram)
+
+    if (address >= 0x04000000 && address <= 0x040003FE)
+        return io.read(address);
+
     MATCHES(PALETTE_RAM, palette_ram)
     MATCHES(VRAM, vram)
     MATCHES(OAM_OBJ_ATTR, oam_obj_attr)
@@ -64,6 +68,12 @@ Memory::write(uint32_t address, uint8_t byte) {
 
     MATCHES(BOARD_WRAM, board_wram)
     MATCHES(CHIP_WRAM, chip_wram)
+
+    if (address >= 0x04000000 && address <= 0x040003FE) {
+        io.write(address, byte);
+        return;
+    }
+
     MATCHES(PALETTE_RAM, palette_ram)
     MATCHES(VRAM, vram)
     MATCHES(OAM_OBJ_ATTR, oam_obj_attr)

src/meson.build

@@ -1,6 +1,7 @@
 lib_sources = files(
   'memory.cc',
-  'bus.cc'
+  'bus.cc',
+  'io.cc'
 )
 
 subdir('util')