PPU: Implement registers

Sources/Data/Bus.swift

@@ -0,0 +1,84 @@
+class Bus {
+    var cpuVram: [UInt8] = .init(repeating: 0, count: 2048)
+    var prgRom: [UInt8]
+    var ppu: NesPPU
+
+    fileprivate let RAM : UInt16 = 0x0000
+    fileprivate let RAM_MIRRORS_END: UInt16 = 0x1FFF
+    fileprivate let PPU_REGISTERS: UInt16 = 0x2000
+    fileprivate let PPU_REGISTERS_MIRRORS_END: UInt16 = 0x3FFF
+    fileprivate let ROM_ADDRESS_START: UInt16 = 0x8000
+    fileprivate let ROM_ADDRESS_END: UInt16 = 0xFFFF
+
+    init(_ rom: Rom) {
+        ppu = NesPPU(rom.character, rom.screenMirror)
+        self.prgRom = rom.program
+    }
+}
+
+
+extension Bus: Memory {
+    func memRead(_ addr: UInt16) -> UInt8 {
+        switch addr {
+        case RAM...RAM_MIRRORS_END:
+            let mirrorDownAddr = addr & 0b00000111_11111111
+            return self.cpuVram[Int(mirrorDownAddr)]
+        case 0x2000, 0x2001, 0x2003, 0x2005, 0x2006, 0x4014:
+            fatalError("Attempt to read from write-only PPU address \(addr)")
+        case 0x2002:
+            return ppu.readStatus()
+        case 0x2004:
+            return ppu.readOamData()
+        case 0x2007:
+            return ppu.readData()
+        case 0x2008...PPU_REGISTERS_MIRRORS_END:
+            let mirrorDownAddr = addr & 0b00100000_00000111;
+            return self.memRead(mirrorDownAddr)
+        case ROM_ADDRESS_START...ROM_ADDRESS_END:
+            return readProgramRom(addr)
+        default:
+            print("Ignoring mem access at \(addr)")
+            return 0
+        }
+    }
+
+    func memWrite(_ addr: UInt16, data: UInt8) {
+        switch addr {
+        case RAM...RAM_MIRRORS_END:
+            let mirrorDownAddr = addr & 0b11111111111
+            self.cpuVram[Int(mirrorDownAddr)] = data
+        case 0x2000:
+            ppu.writeToCtrl(data)
+        case 0x2001:
+            ppu.writeToMask(data)
+        case 0x2002:
+            fatalError("Attempt to write to PPU status register")
+        case 0x2003:
+            ppu.writeToOamAddr(data)
+        case 0x2004:
+            ppu.writeToOamData(data)
+        case 0x2005:
+            ppu.writeToScroll(data)
+        case 0x2006:
+            ppu.writeToPPUAddr(data)
+        case 0x2007:
+            ppu.writeToData(data)
+        case 0x2008...PPU_REGISTERS_MIRRORS_END:
+            let mirrorDownAddr = addr & 0b00100000_00000111
+            memWrite(mirrorDownAddr, data: data)
+        case ROM_ADDRESS_START...ROM_ADDRESS_END:
+            fatalError("Attempt to write to Cartridge ROM space: \(addr)")
+        default:
+            print("Ignorming mem-write at \(addr)")
+        }
+    }
+
+    func readProgramRom(_ addr: UInt16) -> UInt8 {
+        var addr = addr - 0x8000
+        if prgRom.count == 0x4000 && addr >= 0x4000 {
+            // rom mirroring
+            addr = addr % 0x4000
+        }
+        return prgRom[Int(addr)]
+    }
+}

Sources/Data/Rom.swift

@@ -0,0 +1,50 @@
+struct Rom {
+    fileprivate let PRG_ROM_PAGE_SIZE = 16384
+    fileprivate let CHR_ROM_PAGE_SIZE = 8192
+
+    var program: [UInt8]
+    var character: [UInt8]
+    var mapper: UInt8
+    var screenMirror: Mirroring
+
+    init(_ raw: [UInt8]) throws {
+        guard raw[0..<4] == [0x4E, 0x45, 0x53, 0x1A] else { throw HeaderParseError.notINES("File is not in iNES file format.") }
+        mapper = (raw[7] & 0b1111_0000) | (raw[6] >> 4)
+
+        let inesVer = (raw[7] >> 2) & 0b11
+        guard inesVer == 0 else { throw HeaderParseError.iNes2("iNES2.0 format not supported.") }
+
+        let fourScreen = raw[6] & 0b1000 != 0
+        let vertMirroring = raw[6] & 0b1 != 0
+        screenMirror = switch (fourScreen, vertMirroring) {
+        case (true, _):
+            Mirroring.fourScreen
+        case (false, true):
+            Mirroring.vertical
+        case (false, false):
+            Mirroring.horizontal
+        }
+
+        let programSize = Int(raw[4]) * PRG_ROM_PAGE_SIZE
+        let characterSize = Int(raw[5]) * CHR_ROM_PAGE_SIZE
+
+        let skipTrainer = raw[6] & 0b100 != 0
+
+        let programStart = 16 + (skipTrainer ? 512 : 0)
+        let characterStart = programStart + programSize
+
+        program = Array(raw[programStart..<(programStart + programSize)])
+        character = Array(raw[characterStart..<(characterStart + characterSize)])
+    }
+}
+
+enum Mirroring {
+    case vertical
+    case horizontal
+    case fourScreen
+}
+
+enum HeaderParseError: Error {
+    case notINES(String)
+    case iNes2(String)
+}