[openMSX-commits] SF.net SVN: openmsx:[12447] openmsx/trunk/src

[<m97...@us...>]

Revision: 12447
          http://openmsx.svn.sourceforge.net/openmsx/?rev=12447&view=rev
Author:   m9710797
Date:     2012-03-24 16:36:19 +0000 (Sat, 24 Mar 2012)
Log Message:
-----------
compute CRC16 of short sequences at compile time

Modified Paths:
--------------
    openmsx/trunk/src/fdc/RawTrack.cc
    openmsx/trunk/src/fdc/TC8566AF.cc
    openmsx/trunk/src/fdc/WD2793.cc
    openmsx/trunk/src/utils/CRC16.hh

Modified: openmsx/trunk/src/fdc/RawTrack.cc
===================================================================
--- openmsx/trunk/src/fdc/RawTrack.cc	2012-03-24 16:35:59 UTC (rev 12446)
+++ openmsx/trunk/src/fdc/RawTrack.cc	2012-03-24 16:36:19 UTC (rev 12447)
@@ -35,12 +35,11 @@
 bool RawTrack::decodeSector(int idx, Sector& sector) const
 {
 	// read (and check) address mark
-	CRC16 addrCrc;
 	// assume addr mark starts with three A1 bytes (should be
 	// located right before the current 'idx' position)
-	for (int i = 0; i < 3; ++i) {
-		addrCrc.update(0xA1);
-	}
+	CRC16 addrCrc;
+	addrCrc.init<0xA1, 0xA1, 0xA1>();
+
 	if (read(idx++) != 0xFE) return false;
 	addrCrc.update(0xFE);
 	int addrIdx = idx;

Modified: openmsx/trunk/src/fdc/TC8566AF.cc
===================================================================
--- openmsx/trunk/src/fdc/TC8566AF.cc	2012-03-24 16:35:59 UTC (rev 12446)
+++ openmsx/trunk/src/fdc/TC8566AF.cc	2012-03-24 16:36:19 UTC (rev 12447)
@@ -502,7 +502,7 @@
 			}
 			// Initialize crc
 			// TODO 0xFB vs 0xF8 depends on deleted vs normal data
-			crc.init(0xE295); // A1 A1 A1 FB
+			crc.init<0xA1, 0xA1, 0xA1, 0xFB>();
 
 			// first byte is available when it's rotated below the
 			// drive-head

Modified: openmsx/trunk/src/fdc/WD2793.cc
===================================================================
--- openmsx/trunk/src/fdc/WD2793.cc	2012-03-24 16:35:59 UTC (rev 12446)
+++ openmsx/trunk/src/fdc/WD2793.cc	2012-03-24 16:36:19 UTC (rev 12447)
@@ -253,7 +253,7 @@
 				// 3rd A1 byte. Though what we do instead is on
 				// each A1 byte initialize the value as if
 				// there were already 2 A1 bytes written.
-				crc.init(0x968B);
+				crc.init<0xA1, 0xA1>();
 			} else if (value == 0xF6) {
 				// write C2 with missing clock transitions
 				write = 0xC2;
@@ -606,7 +606,7 @@
 		//  - write A1 A1 A1 FB   (or F8)
 		// But ATM we reuse the previous location of the data block
 	}
-	crc.init(0xE295); // A1 A1 A1 FB
+	crc.init<0xA1, 0xA1, 0xA1, 0xFB>();
 
 	// wait till sector is actually rotated under head
 	schedule(FSM_TYPE2_ROTATED, next);

Modified: openmsx/trunk/src/utils/CRC16.hh
===================================================================
--- openmsx/trunk/src/utils/CRC16.hh	2012-03-24 16:35:59 UTC (rev 12446)
+++ openmsx/trunk/src/utils/CRC16.hh	2012-03-24 16:36:19 UTC (rev 12447)
@@ -28,6 +28,40 @@
 		crc = initialCRC;
 	}
 
+	/** (Re)initialize with a short initial sequence.
+	  * The initial value is guaranteed to be computed at compile time.
+	  */
+	template<byte V1> void init()
+	{
+		static const word T0 = 0xffff;
+		static const word T1 = CT_CRC16<T0, V1>::value;
+		init(T1);
+	}
+	template<byte V1, byte V2> void init()
+	{
+		static const word T0 = 0xffff;
+		static const word T1 = CT_CRC16<T0, V1>::value;
+		static const word T2 = CT_CRC16<T1, V2>::value;
+		init(T2);
+	}
+	template<byte V1, byte V2, byte V3> void init()
+	{
+		static const word T0 = 0xffff;
+		static const word T1 = CT_CRC16<T0, V1>::value;
+		static const word T2 = CT_CRC16<T1, V2>::value;
+		static const word T3 = CT_CRC16<T2, V3>::value;
+		init(T3);
+	}
+	template<byte V1, byte V2, byte V3, byte V4> void init()
+	{
+		static const word T0 = 0xffff;
+		static const word T1 = CT_CRC16<T0, V1>::value;
+		static const word T2 = CT_CRC16<T1, V2>::value;
+		static const word T3 = CT_CRC16<T2, V3>::value;
+		static const word T4 = CT_CRC16<T3, V4>::value;
+		init(T4);
+	}
+
 	/** Update CRC with one byte
 	 */
 	void update(byte value)
@@ -45,6 +79,20 @@
 private:
 	word crc;
 	static const word CRC16Table[256];
+
+	// The Stuff below is template magic to perform the following
+	// computation at compile-time:
+	//    for (int i = 8; i < 16; ++i) {
+	//        crc = (crc << 1) ^ ((((crc ^ (data << i)) & 0x8000) ? 0x1021 : 0));
+	//    }
+	template<word C, word V, int B> struct CT_H {
+		static const word D = word(C << 1) ^ (((C ^ V) & 0x8000) ? 0x1021 : 0);
+		static const word value = CT_H<D, word(V << 1), B - 1>::value;
+	};
+	template<word C, word V> struct CT_H<C, V, 0> {
+		static const word value = C;
+	};
+	template<word IN, byte VAL> struct CT_CRC16 : CT_H<IN, VAL << 8, 8> {};
 };
 
 } // namespace openmsx

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