compression.md

@@ -6,8 +6,8 @@ struct {
   uint8_t magic[3]; // has to be {0x4C, 0x4F, 0x42} ("LOB") for Ambermoon
   struct {
     uint8_t method;     // has to be 6 for Ambermoon
-    uint8_t rawSize[3]; // big-endian size of uncompressed data
-    uint8_t cmpSize[4]; // big-endian size of compressed data
+    uint8_t rawSize[3]; // big-endian size of uncompressed data (even)
+    uint8_t cmpSize[4]; // big-endian size of compressed data (even)
     uint8_t cmpData[cmpSize];
   }[count];
 };
@@ -98,36 +98,41 @@ Example stream
 ==============
 
 The 22 character string "ababababababababababab"
-can be compressed (including header) to 19 bytes:
+can be compressed (including header) to 20 bytes:
 
 	01 4C 4F 42     magic
 	06              method
 	00 00 16        rawSize = 22
-	00 00 00 07     cmpSize = 7
+	00 00 00 08     cmpSize = 8
 	D8              flags   = 11011___
 	61              literal = 'a'
 	62              literal = 'b'
 	0F 02           len/off = 18/-2
 	61              literal = 'a'
 	62              literal = 'b'
+	__              alignment
 
 Since the header has 12 octets, the first code has to be a literal,
 and at least one length-distance pair is required to encode more
 raw data than required for the compression header and data stream,
-the minimum compressible raw data are 17 identical bytes:
+the minimum compressible raw data are 18 identical bytes:
 
 	01 4C 4F 42     magic
 	06              method
-	00 00 11        rawSize = 17
+	00 00 12        rawSize = 18
 	00 00 00 04     cmpSize = 4
 	80              flags   = 10______
 	__              literal = __
-	0D 01           len/off = 16/-1
+	0E 01           len/off = 17/-1
 
 
 Notes
 =====
 
+The Ambermoon in-game decompression routine moves the data in long words
+to the end of the in-place buffer. Therefore, cmpSize _and_ rawSize both
+have to be even, else an address error is raised on/by MC68000 CPUs.
+ 
 It is strongly recommended that the size of compressed data in the LOB header
 exactly matches file/item size minus the JH/LOB header(s), since it is used
 in the original decoder to move the data to the end of the buffer before