add LDA, STA, TAX, INX, BRK

Sources/CPU.swift

@@ -0,0 +1,182 @@
+
+
+enum AddressingMode {
+   case Immediate
+   case ZeroPage
+   case ZeroPage_X
+   case ZeroPage_Y
+   case Absolute
+   case Absolute_X
+   case Absolute_Y
+   case Indirect_X
+   case Indirect_Y
+   case NoneAddressing
+}
+
+class CPU {
+    var register_a: UInt8 = 0
+    var register_x: UInt8 = 0
+    var register_y: UInt8 = 0
+
+    var status: UInt8 = 0
+    var programCounter: UInt16 = 0
+    private var memory = [UInt8](repeating: 0, count: 0xFFFF)
+
+
+    func getOpperandAddress(_ mode: AddressingMode) -> UInt16 {
+        switch mode {
+        case .Immediate:
+            return programCounter
+        case .ZeroPage:
+            return UInt16(memRead(programCounter))
+        case .Absolute:
+            return memReadU16(programCounter)
+        case .ZeroPage_X:
+            let pos = memRead(programCounter)
+            let addr = pos &+ register_x
+            return UInt16(addr)
+        case .ZeroPage_Y:
+            let pos = memRead(programCounter)
+            let addr = pos &+ register_y
+            return UInt16(addr)
+        case .Absolute_X:
+            let base = memReadU16(programCounter)
+            return base &+ UInt16(register_x)
+        case .Absolute_Y:
+            let base = memReadU16(programCounter)
+            return base &+ UInt16(register_y)
+        case .Indirect_X:
+            let base = memRead(programCounter)
+
+            let ptr = UInt8(base) &+ register_x
+            let lo = memRead(UInt16(ptr))
+            let hi = memRead(UInt16(ptr &+ 1))
+            return UInt16(hi) << 8 | UInt16(lo)
+        case .Indirect_Y:
+            let base = memRead(programCounter)
+
+            let lo = memRead(UInt16(base))
+            let hi = memRead(UInt16(base &+ 1))
+            let deref_base = UInt16(hi) << 8 | UInt16(lo)
+            let deref = deref_base &+ UInt16(register_y)
+            return deref
+        case .NoneAddressing:
+            fatalError("mode \(mode) is not implemented")
+        }
+    }
+
+    func memRead(_ addr: UInt16) -> UInt8 {
+        memory[Int(addr)]
+    }
+
+    func memReadU16(_ addr: UInt16) -> UInt16 {
+        let lo = UInt16(memRead(addr))
+        let hi = UInt16(memRead(addr + 1))
+        return (hi << 8) | lo
+    }
+
+    func memWriteU16(addr: UInt16, data: UInt16) {
+        let hi = UInt8(data >> 8)
+        let lo = UInt8(data & 0xff)
+        self.memWrite(addr: addr, data: lo)
+        self.memWrite(addr: addr + 1, data: hi)
+    }
+
+    func memWrite(addr: UInt16, data: UInt8) {
+        memory[Int(addr)] = data
+    }
+
+    func reset() {
+        register_a = 0
+        register_x = 0
+        register_y = 0
+        status = 0
+
+        programCounter = self.memReadU16(0xFFFC)
+    }
+
+    func loadAndRun(_ program: [UInt8]) {
+        load(program)
+        reset()
+        run()
+    }
+
+    func load(_ program: [UInt8]) {
+        memory[0x8000 ..< (0x8000 + program.count)] = program[0..<program.count]
+        memWriteU16(addr: 0xFFFC, data: 0x8000)
+    }
+
+    func run() {
+        let opcodes = OPCODES_MAP
+
+        while true {
+            let code = memRead(programCounter)
+            programCounter += 1
+
+            let programCounterState = programCounter
+            guard let opcode = opcodes[code] else {fatalError("OpCode \(code) not recgonized!")}
+
+            switch code {
+            /// LDA
+            case 0xa9, 0xa5, 0xb5, 0xad, 0xbd, 0xb9, 0xa1, 0xb1:
+                lda(opcode.mode)
+            /// STA
+            case 0x85, 0x95, 0x8d, 0x9d, 0x99, 0x81, 0x91:
+                sta(opcode.mode)
+            /// TAX
+            case 0xAA:
+                tax()
+            /// INX
+            case 0xE8:
+                inx()
+            /// BRK
+            case 0x00:
+                return
+            default: fatalError("TODO!")
+            }
+
+            if programCounterState == programCounter {
+                programCounter += UInt16(opcode.len - 1)
+            }
+        }
+    }
+
+    func updateZeroAndNegativeFlags(_ result: UInt8) {
+        if result == 0 {
+            status = status | 0b0000_0010
+        } else {
+            status = status & 0b1111_1101
+        }
+
+        if result & 0b1000_0000 != 0 {
+            status = status | 0b1000_0000
+        } else {
+            status = status & 0b0111_1111
+        }
+    }
+
+    func lda(_ mode: AddressingMode) {
+        let addr = getOpperandAddress(mode)
+        let value = memRead(addr)
+
+        register_a = value
+        updateZeroAndNegativeFlags(register_a)
+    }
+
+    func tax() {
+        register_x = register_a
+        updateZeroAndNegativeFlags(register_x)
+    }
+
+    func inx() {
+        register_x = register_x &+ 1
+        updateZeroAndNegativeFlags(register_x)
+    }
+
+    func sta(_ mode: AddressingMode) {
+        let addr = getOpperandAddress(mode)
+        memWrite(addr: addr, data: register_a)
+    }
+
+
+}

Sources/main.swift

@@ -0,0 +1,7 @@
+// The Swift Programming Language
+// https://docs.swift.org/swift-book
+
+let cpu = CPU()
+let program: [UInt8] = [0xa9, 0xc0, 0xaa, 0xe8, 0x00]
+cpu.loadAndRun(program)
+print(cpu.status)

Sources/opcodes.swift

@@ -0,0 +1,38 @@
+struct OpCode {
+    let code: UInt8
+    let mnemonic: String
+    let len: UInt8
+    let cycles: UInt8
+    let mode: AddressingMode
+}
+
+let CPU_OP_CODES: [OpCode] = [
+    OpCode(code: 0x00, mnemonic: "BRK", len: 1, cycles: 7, mode: .NoneAddressing),
+    OpCode(code: 0xaa, mnemonic: "TAX", len: 1, cycles: 2, mode: .NoneAddressing),
+    OpCode(code: 0xe8, mnemonic: "INX", len: 1, cycles: 2, mode: .NoneAddressing),
+
+    OpCode(code: 0xa9, mnemonic: "LDA", len: 2, cycles: 2, mode: .Immediate),
+    OpCode(code: 0xa5, mnemonic: "LDA", len: 2, cycles: 3, mode: .ZeroPage),
+    OpCode(code: 0xb5, mnemonic: "LDA", len: 2, cycles: 4, mode: .ZeroPage_X),
+    OpCode(code: 0xad, mnemonic: "LDA", len: 3, cycles: 4, mode: .Absolute),
+    OpCode(code: 0xbd, mnemonic: "LDA", len: 3, cycles: 4 /* +1 if page crossed */, mode: .Absolute_X),
+    OpCode(code: 0xb9, mnemonic: "LDA", len: 3, cycles: 4 /* +1 if page crossed */, mode: .Absolute_Y),
+    OpCode(code: 0xa1, mnemonic: "LDA", len: 2, cycles: 6, mode: .Indirect_X),
+    OpCode(code: 0xb1, mnemonic: "LDA", len: 2, cycles: 5 /* +1 if page crossed */, mode: .Indirect_Y),
+
+    OpCode(code: 0xa5, mnemonic: "STA", len: 2, cycles: 3, mode: .ZeroPage),
+    OpCode(code: 0x95, mnemonic: "STA", len: 2, cycles: 4, mode: .ZeroPage_X),
+    OpCode(code: 0x8d, mnemonic: "STA", len: 3, cycles: 4, mode: .Absolute),
+    OpCode(code: 0x9d, mnemonic: "STA", len: 3, cycles: 5, mode: .Absolute_X),
+    OpCode(code: 0x99, mnemonic: "STA", len: 3, cycles: 5, mode: .Absolute_Y),
+    OpCode(code: 0x81, mnemonic: "STA", len: 2, cycles: 6, mode: .Indirect_X),
+    OpCode(code: 0x91, mnemonic: "STA", len: 2, cycles: 6, mode: .Indirect_Y),
+]
+
+let OPCODES_MAP: [UInt8: OpCode] = {
+    var map: [UInt8:OpCode] = [:]
+    for cpuop in CPU_OP_CODES {
+        map[cpuop.code] = cpuop
+    }
+    return map
+}()