Add PPUDATA register

NES/CPU+IO.swift

@@ -106,6 +106,9 @@ internal extension Address {
 
     /// The address of the PPU's PPUADDR register in the CPU's address space.
     static let PPUADDRAddress: Address = 0x2006
+
+    /// The address of the PPU's PPUDATA register in the CPU's address space.
+    static let PPUDATAAddress: Address = 0x2007
 }
 
 /// Maps CPU memory addresses to PPU registers.
@@ -118,6 +121,10 @@ private extension PPU {
             return PPUSTATUS
         case Address.OAMDATAAddress:
             return OAMDATA
+        case Address.PPUDATAAddress:
+            defer { didReadPPUDATA() }
+
+            return bufferedPPUDATA
         default:
             return register
         }
@@ -145,6 +152,10 @@ private extension PPU {
             defer { didWritePPUADDR() }
 
             PPUADDR = value
+        case Address.PPUDATAAddress:
+            defer { didWritePPUDATA() }
+
+            PPUDATA = value
         default:
             fatalError("Attempt to write illegal PPU register address \(format(address)).")
         }

NES/PPU.swift

@@ -33,6 +33,34 @@ internal final class PPU {
     /// The PPU address register.
     var PPUADDR: UInt8 = 0
 
+    /// The PPU data port.
+    ///
+    /// This property proxies the PPU's VRAM at the address held by VRAMAddress.
+    var PPUDATA: UInt8 {
+        get {
+            return read(VRAMAddress)
+        }
+        set {
+            write(VRAMAddress, newValue)
+        }
+    }
+
+    /// This value holds buffered reads from PPUDATA.
+    var VRAMBuffer: UInt8 = 0
+
+    /// The buffered PPU data port.
+    var bufferedPPUDATA: UInt8 {
+        var data = PPUDATA
+
+        if VRAMAddress < 0x3F00 {
+            swap(&data, &VRAMBuffer)
+        } else {
+            VRAMBuffer = read(VRAMAddress &- 0x1000)
+        }
+
+        return data
+    }
+
     /// Holds the last value written to any of the above registers.
     ///
     /// Setting this will also affect the five lowest bits of PPUSTATUS.
@@ -111,4 +139,14 @@ internal final class PPU {
 
         secondWrite = !secondWrite
     }
+
+    /// Must be called after the CPU has read PPUDATA.
+    func didReadPPUDATA() {
+        VRAMAddress += VRAMAddressIncrement
+    }
+
+    /// Must be called after the CPU has written PPUDATA.
+    func didWritePPUDATA() {
+        VRAMAddress += VRAMAddressIncrement
+    }
 }

Specs/PPUSpec.swift

@@ -206,5 +206,80 @@ class PPUSpec: QuickSpec {
                 expect(PPU.VRAMAddress).to(equal(0x3DF0))
             }
         }
+
+        describe("Having the CPU read PPUDATA") {
+            beforeEach {
+                PPU.VRAMAddress = 0x0000
+                PPU.VRAMBuffer = 0xFF
+
+                PPU.write(PPU.VRAMAddress, 0x12)
+            }
+
+            it("should return the buffered value first") {
+                expect(CPU.read(.PPUDATAAddress)).to(equal(0xFF))
+            }
+
+            it("should update the buffer") {
+                CPU.read(.PPUDATAAddress)
+
+                expect(PPU.VRAMBuffer).to(equal(0x12))
+            }
+
+            it("should return the value at the current VRAM address on the second read") {
+                CPU.read(.PPUDATAAddress)
+
+                expect(CPU.read(.PPUDATAAddress)).to(equal(0x12))
+            }
+
+            describe("if the VRAM increment flag is not set") {
+                beforeEach {
+                    PPU.PPUCTRL[2] = false
+                }
+
+                it("should increment the VRAM address by 1") {
+                    CPU.read(.PPUDATAAddress)
+
+                    expect(PPU.VRAMAddress).to(equal(0x0001))
+                }
+            }
+
+            describe("if the VRAM increment flag is set") {
+                beforeEach {
+                    PPU.PPUCTRL[2] = true
+                }
+
+                it("should increment the VRAM address by 32") {
+                    CPU.read(.PPUDATAAddress)
+
+                    expect(PPU.VRAMAddress).to(equal(0x0020))
+                }
+            }
+        }
+
+        describe("Having the CPU write to PPUDATA") {
+            describe("if the VRAM increment flag is not set") {
+                beforeEach {
+                    PPU.PPUCTRL[2] = false
+                }
+
+                it("should increment the VRAM address by 1") {
+                    CPU.write(.PPUDATAAddress, 0x12)
+
+                    expect(PPU.VRAMAddress).to(equal(0x0001))
+                }
+            }
+
+            describe("if the VRAM increment flag is set") {
+                beforeEach {
+                    PPU.PPUCTRL[2] = true
+                }
+
+                it("should increment the VRAM address by 32") {
+                    CPU.write(.PPUDATAAddress, 0x12)
+
+                    expect(PPU.VRAMAddress).to(equal(0x0020))
+                }
+            }
+        }
     }
 }