initial commit

1 files changed, 145 insertions, 0 deletions

diff --git a/main.cc b/main.cc
new file mode 100644
index 0000000..3df70d8
--- /dev/null
+++ b/main.cc
@@ -0,0 +1,145 @@
+import <cstdio>;
+import <cstdint>;
+import <cstdlib>;
+
+using Byte = unsigned char;
+using Word = uint16_t;
+
+struct Mem {
+  static constexpr uint32_t MAX_MEM = 1024 * 64;
+  Byte Data[MAX_MEM];
+
+  auto Initialise() -> void {
+    for (uint32_t i{}; i < MAX_MEM; ++i) {
+      Data[i] = 0;
+    }
+  }
+
+  // Read 1 byte
+  auto operator[](uint32_t Address) const -> Byte { return Data[Address]; }
+
+  auto operator[](uint32_t Address) -> Byte { return Data[Address]; }
+
+  // write 2 bytes
+  auto WriteWord(Word Value, uint32_t Address, uint32_t &Cycles) -> void {
+    Data[Address] = Value & 0xFF;
+    Data[Address + 1] = (Value >> 8);
+    Cycles -= 2;
+  }
+};
+
+struct CPU {
+
+  Word PC{}; // Program counter
+  Word SP{}; // Stack pointer
+
+  Byte A{}, X{}, Y{}; // registers
+
+  // Status flags
+  Byte C : 1;
+  Byte Z : 1;
+  Byte I : 1;
+  Byte D : 1;
+  Byte B : 1;
+  Byte V : 1;
+  Byte N : 1;
+
+  auto Reset(Mem &mem) -> void {
+    PC = 0xFFFC;
+    SP = 0x0100;
+    C = 0;
+    Z = 0;
+    I = 0;
+    D = 0;
+    B = 0;
+    V = 0;
+    N = 0;
+    A = 0;
+    X = 0;
+    Y = 0;
+
+    mem.Initialise();
+  }
+
+  auto FetchByte(uint32_t &Cycles, Mem &mem) -> Byte {
+    Byte Data{mem[PC]};
+    ++PC;
+    --Cycles;
+    return Data;
+  }
+
+  auto FetchWord(uint32_t &Cycles, Mem &mem) -> Word {
+    // 6502 is little endian
+    Word Data{mem[PC]};
+    ++PC;
+
+    Data |= (mem[PC] << 8);
+    ++PC;
+
+    Cycles += 2;
+
+    return Data;
+  }
+
+  auto ReadByte(uint32_t &Cycles, Byte Address, Mem &mem) -> Byte {
+    Byte Data{mem[Address]};
+    --Cycles;
+    return Data;
+  }
+
+  // opcodes
+  static constexpr Byte INS_LDA_IM{0xA9}, INS_LDA_ZP{0xA5}, INS_LDA_ZPX{0xB5},
+      INS_JSR{0x20};
+
+  auto LDASetStatus() -> void {
+    Z = (A == 0);
+    N = (A & 0b10'000'000) > 0;
+  }
+
+  auto Execute(uint32_t Cycles, Mem &mem) -> void {
+    while (Cycles > 0) {
+      Byte Ins{FetchByte(Cycles, mem)};
+      switch (Ins) {
+      case INS_LDA_IM: {
+        Byte Value{FetchByte(Cycles, mem)};
+        A = Value;
+        LDASetStatus();
+        break;
+      }
+      case INS_LDA_ZP: {
+        Byte ZeroPageAddress{FetchByte(Cycles, mem)};
+        A = ReadByte(Cycles, ZeroPageAddress, mem);
+        LDASetStatus();
+        break;
+      }
+      case INS_LDA_ZPX: {
+        Byte ZeroPageAddress{FetchByte(Cycles, mem)};
+        ZeroPageAddress += X;
+        --Cycles;
+        A = ReadByte(Cycles, ZeroPageAddress, mem);
+        LDASetStatus();
+        break;
+      }
+      case INS_JSR: {
+        Word SubAddr{FetchWord(Cycles, mem)};
+        mem[SP] = PC - 1;
+        mem.WriteWord(PC - 1, SP, Cycles);
+        PC = SubAddr;
+        --Cycles;
+        break;
+      }
+      default: {
+        printf("Instruction not handled %d", Ins);
+      }
+      }
+    }
+  }
+};
+
+int main() {
+  Mem mem{};
+  CPU cpu{};
+  cpu.Reset(mem);
+  cpu.Execute(2, mem);
+  return 0;
+}