1 /*
2  * sys-emu - A system emulator for tutorials
3  * Copyright (C) 2018 - 2019 osdevelopment-info
4  *
5  * This program is free software: you can redistribute it and/or modify
6  * it under the terms of the GNU Affero General Public License as published by
7  * the Free Software Foundation, either version 3 of the License, or
8  * (at your option) any later version.
9  *
10  * This program is distributed in the hope that it will be useful,
11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
13  * GNU Affero General Public License for more details.
14  *
15  * You should have received a copy of the GNU Affero General Public License
16  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
17  */
18 package info.osdevelopment.sysemu.memory
19 
20 import info.osdevelopment.sysemu.support.Utilities._
21 import java.nio.ByteBuffer
22 import java.nio.channels.SeekableByteChannel
23 import scala.util.Try
24 
25 /**
26   * A read-only memory backed by a `SeekableByteChannel`, e.g. a file. This
27   * implementation reads part of the file into a cache and tries to serve the data from this cache.
28   * @param data the data of the read-only memory
29   */
30 class ChannelReadOnlyMemory(val data: SeekableByteChannel) extends ReadOnlyMemory {
31 
32   /** The number of the block that is cached. */
33   private var cachedBlock = -1L
34   /** The cache of the read-only memory. */
35   private val romCache = ByteBuffer.allocate(256.Ki.asInstanceOf[Int])
36   /** The number of bytes read from the channel during the last caching operation. */
37   private var cachedBytes = -1
38 
39   /**
40     * Returns the size of the memory.
41     * @return the size of the memory
42     */
43   override def size: Long = data.size
44 
45   /**
46     * The read method which caches a block of data and returns the value to be read from the cache.
47     * @param address the address to read
48     * @return a `Success` with the byte read at the given address or a `Failure`
49     */
50   override protected def doRead(address: Long): Try[Byte] = {
51     val block = address / 256.Ki
52     Try {
53       // In the cache is the wrong block
54       if (block != cachedBlock) {
55         data.position(block * 256.Ki)
56         romCache.clear
57         cachedBytes = data.read(romCache)
58         cachedBlock = block
59       }
60       val indexInCache = (address % 256.Ki).asInstanceOf[Int]
61       if (indexInCache > cachedBytes) throw new IllegalAddressException("Address outside memory")
62       romCache.get(indexInCache)
63     }
64   }
65 
66 }